Buss 2019 - Evolutionary Psychology.pdf

Evolutionary Psychology Where did we come from? What is our connection with other life forms? What are the mechanisms of mind that define what it means to be a human being? Evolutionary psychology is a revolutionary new science, a true synthesis of modern principles of psychology and evolutionary...

Evolutionary Psychology Where did we come from? What is our connection with other life forms? What are the mechanisms of mind that define what it means to be a human being? Evolutionary psychology is a revolutionary new science, a true synthesis of modern principles of psychology and evolutionary biology. Since the publication of the award-winning first edition of Evolutionary Psychology, there has been an explosion of research within the field. In this book, David M. Buss examines human behavior from an evolutionary perspective, providing students with the conceptual tools needed to study evolutionary psychology and apply them to empirical research on the human mind. This edition contains expanded coverage of cultural evolution, with a new section on culture–gene co-evolution, additional studies discussing interbreeding between modern humans and Neanderthals, expanded discussions of evolutionary hypotheses that have been empirically disconfirmed, and much more! Evolutionary Psychology features a wealth of student-friendly pedagogy including critical-thinking questions and case study boxes designed to show how to apply evolutionary psychology to real-life situations. It is also accompanied by a thoroughly updated companion website featuring PowerPoints for each chapter, test bank questions, and links to web resources and videos. Evolutionary Psychology is an invaluable resource for undergraduates studying psychology, biology, and anthropology. David M. Buss received his Ph.D. from the University of California at Berkeley. He began his career in academics at Harvard, later moving to the University of Michigan before accepting his current position as professor of psychology at the University of Texas. His primary research interests include human sexuality, mating strategies, conflict between the sexes, homicide, stalking, and sexual victimization. The author of more than 300 scientific articles and six books, Buss has won numerous awards including the American Psychological Association (APA) Distinguished Scientific Award for Early Career Contribution to Psychology, the APA G. Stanley Hall Lectureship, the APA Distinguished Scientist Lecturer Award, and a Robert W. Hamilton Book Award for the first edition of Evolutionary Psychology: The New Science of the Mind. He is also the editor of the first comprehensive Handbook of Evolutionary Psychology (Wiley) and co-editor (with Patricia Hawley) of The Evolution of Personality and Individual Differences. In 2013, he was named one of the 30 most influential living psychologists in the world. He enjoys extensive cross-cultural research collaborations and lectures widely within the United States and abroad. His hobbies include tennis, squash, and disc golf, and he is an avid film buff. Evolutionary Psychology The New Science of the Mind Sixth Edition David M. Buss Sixth edition published 2019 by Routledge 52 Vanderbilt Avenue, New York, NY 10017 and by Routledge 2 Park Square, Milton Park, Abingdon, Oxon, OX14 4RN Routledge is an imprint of the Taylor & Francis Group, an informa business © 2019, 2015, 2012, 2008 Taylor & Francis. The right of David M. Buss to be identified as author of this work has been asserted by him in accordance with sections 77 and 78 of the Copyright, Designs and Patents Act 1988. All rights reserved. No part of this book may be reprinted or reproduced or utilised in any form or by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying and recording, or in any information storage or retrieval system, without permission in writing from the publishers. Trademark notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Library of Congress Cataloging-in-Publication Data Names: Buss, David M., author. Title: Evolutionary psychology : the new science of the mind / David M. Buss. Description: 6th Edition. | New York : Routledge, 2019. | Revised edition of the author’s Evolutionary psychology, | Includes bibliographical references. Identifiers: LCCN 2018057589 | ISBN 9781138088184 (hardback) | ISBN 9781138088610 (pbk.) | ISBN 9780429061417 (ebook) Subjects: LCSH: Evolutionary psychology—Textbooks. | Human evolution—Textbooks. Classification: LCC BF698.95.B87 2019 | DDC 155—dc23 LC record available at https://lccn.loc.gov/2018057589 ISBN: 978-1-138-08818-4 (hbk) ISBN: 978-1-138-08861-0 (pbk) ISBN: 978-0-429-06141-7 (ebk) Typeset in Sabon LT Pro by Apex CoVantage, LLC Visit the companion website: www.routledge.com/cw/Buss This book is dedicated to: Charles Darwin Francis Galton Gregor Mendel R. A. Fisher W. D. Hamilton George C. Williams John Maynard Smith Robert Trivers E. O. Wilson Richard Dawkins Donald Symons Martin Daly Margo Wilson Leda Cosmides John Tooby And to all students of evolutionary psychology, past, present, and future Contents Preface Supplements Acknowledgments Part 1: Foundations of Evolutionary Psychology 1. The Scientific Movements Leading to Evolutionary Psychology 2. The New Science of Evolutionary Psychology Part 2: Problems of Survival 3. Combating the Hostile Forces of Nature Part 3: Challenges of Sex and Mating 4. Women’s Long-Term Mating Strategies 5. Men’s Long-Term Mating Strategies 6. Short-Term Sexual Strategies Part 4: Challenges of Parenting and Kinship 7. Problems of Parenting 8. Problems of Kinship Part 5: Problems of Group Living 9. Cooperative Alliances 10. Aggression and Warfare 11. Conflict Between the Sexes 12. Status, Prestige, and Social Dominance 13. Toward a Unified Evolutionary Psychology Bibliography Credits Index Preface It is especially exciting to be an evolutionary psychologist during this time in the history of science. Most scientists operate within long-established paradigms. Evolutionary psychology, in contrast, is a revolutionary new science, a true synthesis of modern principles of psychology and evolutionary biology. By taking stock of the field at this time, I hope this book contributes in some modest measure to the fulfillment of a scientific revolution that will provide the foundation for psychology in the future. Since the publication of the award-winning first edition of Evolutionary Psychology: The New Science of the Mind, there has been an explosion of new research within the field. New journals in evolutionary psychology have been started, and the volume of evolutionary publications in mainstream psychology journals has steadily increased. New courses in evolutionary psychology are being taught in colleges and universities throughout the world. Many gaps in scientific knowledge remain, and each new discovery brings fresh questions and new domains to explore. The field of evolutionary psychology is vibrant, exciting, and brimming with empirical discoveries and theoretical innovations. Indeed, as Harvard professor Steven Pinker notes, “In the study of humans, there are major spheres of human experience—beauty, motherhood, kinship, morality, cooperation, sexuality, violence—in which evolutionary psychology provides the only coherent theory” (Pinker, 2002, p. 135). Charles Darwin must be considered the first evolutionary psychologist for this prophesy at the end of his classic treatise On the Origin of Species (1859): “In the distant future I see open fields for far more important researches. Psychology will be based on a new foundation.” More than 150 years later, after some false starts and halting steps, the science of evolutionary psychology is finally emerging. The purpose of this book is to showcase the foundations of this new science and the fascinating discoveries of its practitioners.  When I first started to conduct research in evolutionary psychology as an assistant professor at Harvard University in 1981, evolutionary speculations about humans abounded, but practically no empirical research had been conducted to back them up. Part of the problem was that scientists who were interested in evolutionary questions could not bridge the gap between the grand evolutionary theories and the actual scientific study of human behavior. Today that gap has closed considerably, because of both conceptual breakthroughs and an avalanche of hard-won empirical achievements. Many exciting questions still cry out for empirical scrutiny, of course, but the existing base of findings is currently so large that the problem I faced was how to keep this book to a reasonable length while still doing justice to the dazzling array of theoretical and empirical insights. Although it is written with undergraduates in mind, it is also designed to appeal to a wider audience of laypersons, graduate students, and professionals who seek an up-to-date overview of evolutionary psychology. I wrote the first edition of this book with another purpose as well—frankly, a revolutionary one. I wrote it so that the hundreds of professors at colleges and universities throughout the world who have been thinking and writing about evolution and human behavior would be motivated to teach formal courses in evolutionary psychology and get those courses established as part of required psychology curricula. Already evolutionary psychology is attracting the best and the brightest young minds. I hope that this book helps to accelerate the trend and in some small way contributes to the fulfillment of Darwin’s prophesy. New to This Edition In revising the book for this edition, I had two goals in mind. First, I sought to provide a major update of new discoveries. Toward this end, roughly 200 new references have been added to this edition. Second, I sought to fill in important omissions, based on an explosion of new theories and research: Expanded coverage of cultural evolution, with a new section on culture– gene co-evolution. New studies on evidence for a small amount of interbreeding between modern humans and Neanderthals. New large-scale studies on ovulation effects on women’s mate preferences. Expanded discussion of evolutionary hypotheses that have been empirically disconfirmed. Expanded discussion of the emotion of “disgust” as central to the behavioral immune system and “sexual disgust” as a specific evolved defense and links to preferred mating strategy. Discovery of a new cue to physical attractiveness—lumbar curvature. New findings on the emotion of sexual regret and gender differences therein from the highly gender-egalitarian country of Norway. Mate preferences among the semi-nomadic Himba of Namibia. Discoveries of the effects of marriage on longevity. Discussion of “mismatches” between ancestral and modern environments. Expanded discussion of theories and empirical tests of homosexuality, bisexuality, and other sexual orientations. Use of modern apps, such as Snapchat, to seek mating opportunities. New discussion of the method of evolutionary computer simulations and agent-based modeling. Effects of physical and behavioral resemblance of child to father on paternal investment. New work on food aversions in pregnant women.  Eye-tracking studies showing that dangerous predators capture our visual attention, even in the face of visual distractions. Study of 53 cultures reveals an effect of “burdensomeness to kin” on suicidal ideation and suicide. New section on accidental death. Influence tactics children and parents use on each other. Effects of genetic relatedness, such as full versus half-siblings, on kin investment. Evidence from Icelandic Vikings on causes of Viking raids and evidence that kin protect against getting killed in raids. Empirical refutation of a kin-based hypothesis about sexual infidelity. New section on need-based transfers, risk pooling, and social insurance. Use of social media and cyber-bullying in intrasexual competition. Eye-tracking studies reveal spontaneous assessment of physical formidability in others. Individual difference predictors of the sexual over-perception bias. “Dark Triad” personality traits predict sexual deception and sexual harassment. Massive study of 63,000 people reveals strong gender differences in upset over sexual versus emotional infidelity among heterosexuals, but not among those with other sexual orientations. New study of sexual jealousy among women in Mozambique. Women’s resistance tactics to male mate guarding. “Death before dishonor”—the tremendous importance of social reputation. New study of 33 non-industrial populations reveals effects of men’s status on a variety of reproductive outcomes. New research on the emotions of pride and shame in status and reputation. New research on the importance of physical formidability in modern status hierarchies. Expanded section on the evolutionary psychology of religion and the role of “Big Gods” in large-scale cooperation. I have received many inspiring letters and e-mails from teachers and students who have used previous editions of Evolutionary Psychology and hope that future readers will also share their enthusiasm. The quest for understanding the human mind is a noble undertaking. As the field of evolutionary psychology matures, we are beginning to gain answers to the mysteries that have probably intrigued humans for hundreds of thousands of years: Where did we come from? What is our connection with other life forms? And what are the mechanisms of mind that define what it means to be a human being? Supplements Please visit the companion website at www.routledge.com/cw/Buss. Acknowledgments The acknowledgments for this book must include not only colleagues who have directly commented on its contents but also those who have influenced my personal evolutionary odyssey, which has spanned more than 25 years. My interest in evolution began in an undergraduate geology class in the mid- 1970s, when I first realized that there were theories designed specifically to explain the origins of things. My first evolutionary groping was a term paper for a course in 1975 in which I speculated, drawing on now-laughable primate comparisons, that the main reason men have evolved a status-striving motive is higher status produced increased sexual opportunities. My interest in evolution and human behavior grew when I was in graduate school at the University of California at Berkeley, but I found the most fertile evolutionary soil at Harvard University, which offered me a position as Assistant Professor of Psychology. There I began teaching a course on human motivation using evolutionary principles, although the text scarcely mentioned evolution. My lectures were based on the works of Charles Darwin, W. D. Hamilton, Robert Trivers, and Don Symons. I started corresponding with Don Symons, whose 1979 book is considered by many the first modern treatise on human evolutionary psychology. I owe Don special thanks; his friendship and insightful commentary have informed practically everything that I’ve written on the subject of evolutionary psychology. Influenced by Don’s ideas, I designed my first evolutionary research project on human mating, which eventually mushroomed into a cross-cultural study of 10,047 participants from 37 cultures around the world. After word got around about my evolutionary interests, a brilliant young Harvard graduate student named Leda Cosmides rapped on my office door and introduced herself. We had the first of many discussions (actually arguments) about evolution and human behavior. Leda introduced me to her equally brilliant husband and collaborator John Tooby, and together they tried to correct some of the more egregious errors in my thinking—something they continue to do to this day. Through Leda and John, I met Irv DeVore, a prominent Harvard anthropologist who conducted “simian seminars” at his Cambridge home, and Martin Daly and Margo Wilson, who came to Harvard on sabbatical. At that point, the early to mid-1980s, Leda and John had not yet published anything on evolutionary psychology, and no one was called an evolutionary psychologist. The next pivotal event in my evolutionary quest occurred when I was elected to be a fellow at the Center for Advanced Study in the Behavioral Sciences in Palo Alto. Thanks to the encouragement of Director Gardner Lindzey, I proposed a special center project entitled “Foundations of Evolutionary Psychology.” The acceptance of this proposal led Leda Cosmides, John Tooby, Martin Daly, Margo Wilson, and me to spend 1989 and 1990 at the Center working on the foundations of evolutionary psychology, even through the earthquake that rocked the Bay area. In writing this book, I owe the greatest intellectual debt to Leda Cosmides, John Tooby, Don Symons, Martin Daly, and Margo Wilson, pioneers and founders of the emerging field of evolutionary psychology. Harvard on one coast and the Center for Advanced Study on the other provided a bounty for budding evolutionary scholars, but I must also thank two other institutions and their inhabitants. First, the University of Michigan supported the evolution and human behavior group between 1986 and 1994. I owe special thanks to Al Cain, Richard Nisbett, Richard Alexander, Robert Axelrod, Barb Smuts, Randolph Nesse, Richard Wrangham, Bobbi Low, Kim Hill, Warren Holmes, Laura Betzig, Paul Turke, Eugene Burnstein, and John Mitani for playing key roles at Michigan. Second, I thank the Department of Psychology at the University of Texas at Austin, which had the prescience to form one of the first graduate programs in evolutionary psychology in the world under the heading of Individual Differences and Evolutionary Psychology. Special thanks go to Joe Horn, Dev Singh, Del Thiessen, Lee Willerman, Peter MacNeilage, David Cohen, and the department chairs Randy Diehl, Mike Domjan, and Jamie Pennebaker for their roles at UT. I owe tremendous thanks to friends and colleagues who have contributed to the ideas in this book in one form or another: Dick Alexander, Bob Axelrod, Robin Baker, Jerry Barkow, Jay Belsky, Laura Betzig, George Bittner, Don Brown, Eugene Burnstein, Arnold Buss, Bram Buunk, Liz Cashdan, Nap Chagnon, Jim Chisholm, Helena Cronin, Michael Cunningham, Richard Dawkins, Irv DeVore, Frans de Waal, Mike Domjan, Paul Ekman, Steve Emlen, Mark Flinn, Robin Fox, Robert Frank, Steve Gangestad, Karl Grammer, W. D. Hamilton, Kim Hill, Warren Holmes, Sarah Hrdy, Bill Jankowiak, Doug Jones, Doug Kenrick, Lee Kirkpatrick, Judy Langlois, Bobbi Low, Kevin MacDonald, Neil Malamuth, Janet Mann, Linda Mealey, Geoffrey Miller, Randolph Nesse, Dick Nisbett, Steve Pinker, David Rowe, Paul Rozin, Joanna Scheib, Paul Sherman, Irwin Silverman, Jeff Simpson, Dev Singh, Barb Smuts, Michael Studd, Frank Sulloway, Del Thiessen, Nancy Thornhill, Randy Thornhill, Lionel Tiger, Bill Tooke, John Townsend, Robert Trivers, Jerry Wakefield, Lee Willerman, George Williams, D. S. Wilson, E. O. Wilson, and Richard Wrangham. I would like to thank the following reviewers for their feedback on the first edition: Clifford R. Mynatt, Bowling Green State University; Richard C. Keefe, Scottsdale College; Paul M. Bronstein, University of Michigan-Flint; Margo Wilson, McMaster University; W. Jake Jacobs, University of Arizona; and A. J. Figueredo, University of Arizona; as well as the reviewers for the second edition: John A. Johnson, Penn State, DuBois; Kevin MacDonald, California State University, Long Beach; and Todd K. Shackelford, Florida Atlantic University. Also, a special thank you to the third edition reviewers: Brad Duchaine, Harvard University; Heide Island, University of Central Arkansas; Angelina Mackewn, University of Tennessee at Martin; Roger Mellgren, University of Texas at Arlington; Amy R. Pearce, Arkansas State University; and Thomas Sawyer, North Central College. The creation of the second edition benefited from the exceptionally thoughtful comments and suggestions by and discussions with a number of friends and colleagues: Petr Bakalar, Clark Barrett, Leda Cosmides, Martin Daly, Richard Dawkins, Todd DeKay, Josh Duntley, Mark Flinn, Barry Friedman, Steve Gangestad, Joonghwan Jeon, Doug Kenrick, Martie Haselton, Bill von Hippel, Rob Kurzban, Peter MacNeilage, Geoffrey Miller, Steve Pinker, David Rakison, Kern Reeve, Paul Sherman, Valerie Stone, Larry Sugiyama, Candace Taylor, John Tooby, Glenn Weisfeld, and Margo Wilson. Josh Duntley must be singled out for sharing his encyclopedic knowledge and keen insights. I would also like to thank Carolyn Merrill of Allyn & Bacon for wise counsel, persistence, and prescience. I would like to thank the following individuals for help making additions and improvements to the third edition: Leda Cosmides, Josh Duntley, Ernst Fehr, Herbert Gintis, Anne Gordon, Ed Hagen, Martie Haselton, Joe Henrich, Joonghwan Jeon, Mark Flinn, Barry X. Kuhle, Rob Kurzban, Dan O’Connell, John Patton, Steve Pinker, David Rakison, Pete Richardson, Andy Thompson, and Wade Rowatt. I would like to thank the following individuals for insightful comments and suggestions on the fourth edition: Alice Andrews, Ayla Arslan, Sean Bocklebank, Joseph Carroll, Elizabeth Cashdan, Lee Cronk, John Edlund, Bruce Ellis, A. J. Figueredo, Aaron Goetz, Joe Henrich, Sarah Hill, Russell Jackson, Peter Karl Jonason, Jeremy Koster, Barry Kuhle, David Lewis, Frank McAndrew, David McCord, Geoffrey Miller, David Rakison, Brad Sagarin, David Schmitt, Todd Shackelford, Candace Taylor, and Gregory Webster. I would also like to thank my wonderful editor, Susan Hartman, who provided support and enthusiasm throughout several editions of this book, and my superb and meticulous production editors, Aparna Yellai and Revathi Viswanathan. The fifth edition benefitted from discussions with Alice Andrews, Harald Euler, Lee Kirkpatrick, Cristine Legare, Stacey Lynn, Ara Norenzyan, David Rakison, and Andy Thompson. Acknowledgments for the Sixth Edition Thanks go to my students past and present who have made major contributions to the field of evolutionary psychology: Laith Al-Shawaf, Kelly Asao, April Bleske, Mike Botwin, Jaime Confer, Sean Conlan, Dan Conroy- Beam, Courtney Crosby, Todd DeKay, Josh Duntley, Judith Easton, Bruce Ellis, Diana Fleischman, Cari Goetz, Aaron Goetz, Heidi Greiling, Arlette Greer, Martie Haselton, Sarah Hill, Russell Jackson, Joonghwan Jeon, Barry Kuhle, Liisa Kyl-Heku, David Lewis, Anne McGuire, Carin Perilloux, David Schmitt, Anna Sedlacek, Todd Shackelford, and Joy Wyckoff. Special thanks also to Kevin Daly, Todd DeKay, Josh Duntley, A. J. Figueredo, Barry Kuhle, Martie Haselton, Rebecca Sage, Todd Shackelford, and W. Jake Jacobs for generously providing detailed comments on the entire book. This sixth edition benefitted especially from the detailed and thoughtful suggestions by Dan Conroy-Beam (mating and agent-based modeling), Patrick Durkee (status and dominance), Cristine Legare (cultural evolution), and half a dozen anonymous reviewers. The greatest debt for this edition is to Athena Aktipis, who generously provided extensive comments on many chapters and greatly improved both the intellectual vibrancy and tone of those chapters. Part 1 Foundations of Evolutionary Psychology Two chapters introduce the foundations of evolutionary psychology. Chapter 1 traces the scientific movements leading to evolutionary psychology. First, we describe the landmarks in the history of evolutionary theory, starting with theories of evolution developed before Charles Darwin and ending with modern formulations of evolutionary theory widely accepted in the biological sciences today. Next, we examine three common misunderstandings about evolutionary theory. Finally, we trace landmarks in the field of psychology, starting with the influence Darwin had on the psychoanalytic theories of Sigmund Freud and ending with modern formulations of cognitive psychology. Chapter 2 provides the conceptual foundations of modern evolutionary psychology and introduces the scientific tools used to test evolutionary psychological hypotheses. The first section examines theories about the origins of human nature. Then we turn to a definition of the core concept of an evolved psychological mechanism and outline the properties of these mechanisms. The middle portion of Chapter 2 describes the major methods used to test evolutionary psychological hypotheses and the sources of evidence on which these tests are based. Because the remainder of the book is organized around human adaptive problems, the end of Chapter 2 focuses on the tools evolutionary psychologists use to identify adaptive problems, starting with survival and ending with the problems of group living. Chapter 1 The Scientific Movements Leading to Evolutionary Psychology Learning Objectives After studying this chapter, the reader will be able to: ◼ Identify the three essential ingredients of natural selection. ◼ Define particulate inheritance. ◼ List three common misunderstandings about evolutionary theory. ◼ Identify when Neanderthals went extinct. ◼ Explain why radical behaviorism went into scientific decline. In the distant future I see open fields for more important researches. Psychology will be based on a new foundation, that of the necessary acquirement of each mental power and capacity by gradation. —Charles Darwin (1859) As the archeologist dusted off the dirt and debris from the skeleton, she noticed something strange: The left side of the skull had a large dent, apparently from a ferocious blow, and the rib cage—also on the left side—had the head of a spear lodged in it. Back in the laboratory, scientists determined that the skeleton was that of a Neanderthal man who had died roughly 50,000 years ago, the earliest known homicide victim. His killer, judging from the damage to the skull and rib cage, bore the lethal weapon in his right hand. The fossil record of injuries to bones reveals two strikingly common patterns (Jurmain et al., 2009; Trinkaus & Zimmerman, 1982; Walker, 1995). First, the skeletons of men contain far more fractures and dents than do the skeletons of women. Second, the injuries are located mainly on the left frontal sides of the skulls and skeletons, suggesting mostly right-handed attackers. The bone record alone cannot tell us with certainty that combat among men was a central feature of human ancestral life. Nor can it tell us with certainty that men evolved to be the more physically aggressive sex. But skeletal remains provide clues that yield a fascinating piece of the puzzle of where we came from, the forces that shaped who we are, and the nature of our minds today. The huge human brain, approximately 1,350 cubic centimeters, is the most complex organic structure in the known world. Understanding the human mind/brain mechanisms in evolutionary perspective is the goal of the new scientific discipline called evolutionary psychology. Evolutionary psychology focuses on four key questions: (1) Why is the mind designed the way it is—that is, what causal processes created, fashioned, or shaped the human mind into its current form? (2) How is the human mind designed—what are its mechanisms or component parts, and how are they organized? (3) What are the functions of the component parts and their organized structure—that is, what is the mind designed to do? (4) How does input from the current environment interact with the design of the human mind to produce observable behavior? Contemplating the mysteries of the human mind is not new. Ancient Greeks such as Aristotle and Plato wrote manifestos on the subject. More recently, theories of the human mind such as the Freudian theory of psychoanalysis, the Skinnerian theory of reinforcement, and connectionism have vied for the attention of psychologists. Only within the past few decades have we acquired the conceptual tools to synthesize our understanding of the human mind under one unifying theoretical framework—that of evolutionary psychology. This discipline pulls together findings from all disciplines of the mind, including those of brain imaging; learning and memory; attention, emotion, and passion; attraction, jealousy, and sex; self-esteem, status, and self-sacrifice; parenting, persuasion, and perception; kinship, warfare, and aggression; cooperation, altruism, and helping; ethics, morality, religion, and medicine; and commitment, culture, and consciousness. This book offers an introduction to evolutionary psychology and provides a road map to this new science of the mind.  This chapter starts by tracing the major landmarks in the history of evolutionary biology that were critical to the emergence of evolutionary psychology. Then we turn to the history of the field of psychology and show the progression of accomplishments that led to the need for integrating evolutionary theory with modern psychology. Landmarks in the History of Evolutionary Thinking We begin our examination of the history of evolutionary thinking well before the contributions of Charles Darwin and then consider the various milestones in its development through the end of the 20th century. Evolution Before Darwin Evolution refers to change over time. Change in life forms was postulated by scientists to have occurred long before Darwin published his classic 1859 book On the Origin of Species (see Glass, Temekin, & Straus, 1959; and Harris, 1992, for historical treatments). Jean Baptiste Lamarck (1744–1829) was one of the first scientists to use the word biologie, which recognized the study of life as a distinct science. Lamarck believed in two major causes of species change: first, a natural tendency for each species to progress toward a higher form and, second, the inheritance of acquired characteristics. Lamarck proposed that animals must struggle to survive and this struggle causes their nerves to secrete a fluid that enlarges the organs involved in the struggle. Giraffes evolved long necks, he thought, through their attempts to eat from higher and higher leaves (recent evidence suggests that long necks may also play a role in mate competition through physical battles). Lamarck believed that the neck changes that came about from these strivings were passed down to succeeding generations of giraffes, hence the phrase “the inheritance of acquired characteristics.” Another theory of change in life forms was developed by Baron Georges Léopold Chrétien Frédérick Dagobert Cuvier (1769–1832). Cuvier proposed a theory called catastrophism, according to which species are extinguished periodically by sudden catastrophes, such as meteorites, and then replaced by different species. Biologists before Darwin also noticed the bewildering variety of species, some with astonishing structural similarities. Humans, chimpanzees, and orangutans, for example, all have exactly five digits on each hand and foot. The wings of birds are similar to the flippers of seals, perhaps suggesting that one was modified from the other (Daly & Wilson, 1983). Comparisons among these species suggested that life was not static, as some scientists and theologians had argued. Further evidence suggesting change over time also came from the fossil record. Bones from older geological strata were not the same as bones from more recent geological strata. These bones would not be different, scientists reasoned, unless there had been a change in organic structure over time. Another source of evidence came from comparing the embryological development of different species (Mayr, 1982). Biologists noticed that such development was strikingly similar in species that otherwise seemed very different from one another. An unusual loop-like pattern of arteries close to the bronchial slits characterizes the embryos of mammals, birds, and frogs. This evidence suggested, perhaps, that these species might have come from the same ancestors millions of years ago. All these pieces of evidence, present before 1859, suggested that life was not fixed or unchanging. The biologists who believed that life forms changed over time called themselves evolutionists. Another key observation had been made by evolutionists before Darwin: Many species possess characteristics that seem to have a purpose. The porcupine’s quills help it fend off predators. The turtle’s shell helps to protect its tender organs from the hostile forces of nature. The beaks of many birds are designed to aid in cracking nuts. This apparent functionality, so abundant in nature, required an explanation. Missing from the evolutionists’ accounts before Darwin, however, was a theory to explain how change might take place over time and how such seemingly purposeful structures such as the giraffe’s long neck and the porcupine’s sharp quills could have come about. A causal process to explain these biological phenomena was needed. Charles Darwin provided the theory of just such a process. Darwin’s Theory of Natural Selection Darwin’s task was more difficult than it might at first appear. He wanted not only to explain why change takes place over time in life forms but also to account for the particular ways it proceeds. He wanted to determine how new species emerge (hence the title of his book On the Origin of Species), as well as why others vanish or go extinct. Darwin wanted to explain why the component parts of animals—the long necks of giraffes, the wings of birds, and the trunks of elephants—existed in those particular forms. And he wanted to explain the apparent purposive quality of those forms, or why they seem to function to help organisms accomplish specific tasks. The answers to these puzzles can be traced to a voyage Darwin took after graduating from Cambridge University. He traveled the world as a naturalist on a ship, the Beagle, for a 5-year period, from 1831 to 1836. During this voyage, he collected dozens of samples of birds and other animals from the Galápagos Islands in the Pacific Ocean. On returning from his voyage, he discovered that the Galápagos finches, which he had presumed were all of the same species, actually varied so much that they constituted different species. Indeed, each island in the Galápagos had a distinct species of finch. Darwin determined that these different finches had a common ancestor but had become different from each other because of the local ecological conditions on each island. This geographic variation was pivotal to Darwin’s conclusion that species are not immutable but can change over time. What could account for why species change? Darwin struggled with several different theories of the origins of change but rejected all of them because they failed to explain a critical fact: the existence of adaptations. Darwin wanted to account for change, of course, but he also wanted to account for why organisms appeared so well designed for their local environments. Charles Darwin created a scientific revolution in biology with his theory of natural selection. His book On the Origin of Species (1859) is packed with theoretical arguments and detailed empirical data that he amassed over the 25 years prior to the book’s publication.  It was… evident that [these other theories] could not account for the innumerable cases in which organisms of every kind are beautifully adapted to their habits of life—for instance, a woodpecker or tree-frog to climb trees, or a seed for dispersal by hooks and plumes. I had always been much struck by such adaptations, and until these could be explained it seemed to me almost useless to endeavour to prove by indirect evidence that species have been modified. (Darwin, from his autobiography; cited in Ridley, 1996, p. 9) Darwin unearthed a key to the puzzle of adaptations in Thomas Malthus’s An Essay on the Principle of Population (published in 1798), which introduced Darwin to the notion that organisms exist in numbers far greater than can survive and reproduce. The result must be a “struggle for existence,” in which favorable variations tend to be preserved and unfavorable ones tend to die out. When this process is repeated generation after generation, the end result is the formation of new adaptation. More formally, Darwin’s answer to all these puzzles of life was the theory of natural selection and its three essential ingredients: variation, inheritance, and differential reproductive success.1 First, organisms vary in all sorts of ways, such as in wing length, trunk strength, bone mass, cell structure, fighting ability, defensive ability, and social cunning. Variation is essential for the process of evolution to operate—it provides the “raw materials” for evolution. Second, only some of these variations are inherited—that is, passed down reliably from parents to their offspring, who then pass them on to their offspring down through the generations. Other variations, such as a wing deformity caused by an environmental accident, are not inherited by offspring. Only those variations that are inherited play a role in the evolutionary process. The third critical ingredient of Darwin’s theory is selection. Organisms with some heritable variants leave more offspring because those attributes help with the tasks of survival or reproduction. In an environment in which the primary food source might be nut-bearing trees or bushes, some finches with a particular shape of beak, for example, might be better able to crack nuts and get at their meat than finches with other shapes of beaks. More finches who have beaks better shaped for nut cracking survive than those with beaks poorly shaped for nut cracking. An organism can survive for many years, however, and still not pass on its inherited qualities to future generations. To pass its inherited qualities to future generations, it must reproduce. Thus, differential reproductive success, brought about by the possession of heritable variants that increase or decrease an individual’s chances of surviving and reproducing, is the “bottom line” of evolution by natural selection. Differential reproductive success or failure is defined by reproductive success relative to others. The characteristics of organisms that reproduce more than others, therefore, get passed down to future generations at a relatively greater frequency. Because survival is usually necessary for reproduction, it took on a critical role in Darwin’s theory of natural selection. Darwin’s Theory of Sexual Selection Darwin had a wonderful scientific habit of noticing facts that seemed inconsistent with his theories. He observed several that seemed to contradict his theory of natural selection, which he sometimes referred to as the theory of “survival selection.” First, he noticed weird structures that seemed to have absolutely nothing to do with survival; the brilliant plumage of peacocks was a prime example. How could this strange luminescent structure possibly have evolved? The plumage is obviously metabolically costly to the peacock. Furthermore, it seems like an open invitation to predators. Darwin became so obsessed with this apparent anomaly that he once commented, “The sight of a feather in a peacock’s tail, whenever I gaze at it makes me sick!” (quoted in Cronin, 1991, p. 113). Darwin also observed that in some species, the sexes differed dramatically in size and structure. Why would the sexes differ so much, Darwin wondered, when both males and females confront essentially the same problems of survival, such as eating, fending off predators, and combating diseases? Darwin got sick at the sight of a peacock because, initially, the brilliant plumage seemed to have no obvious survival value and hence could not be explained by his original theory of natural selection. He eventually developed the theory of sexual selection, which could explain the peacock’s plumage, and presumably he stopped getting sick when he witnessed one. Darwin’s answer to these apparent contradictions to the theory of natural selection was to devise a second evolutionary theory: the theory of sexual selection. In contrast to the theory of natural selection, which focused on adaptations that have arisen as a consequence of successful survival, the theory of sexual selection focused on adaptations that arose as a consequence of successful mating. Darwin proposed two primary means by which sexual selection could operate. The first is intrasexual competition—competition between members of one sex, the outcomes of which contributed to mating access to the other sex. The prototype of intrasexual competition is two stags locking horns in combat. The victor gains sexual access to a female either directly or through controlling territory or resources desired by the female. The loser typically fails to mate. Whatever qualities lead to success in the same-sex contests, such as greater size, strength, or athletic ability, will be passed on to the next  generation because of the mating success of the victors. Qualities that are linked with losing fail to get passed on. So evolution—change over time— can occur simply as a consequence of intrasexual competition. Stags locking horns in combat is a form of sexual selection called intrasexual competition. The qualities that lead to success in these same-sex combats get passed on in greater numbers to succeeding generations because the victors gain increased mating access to members of the opposite sex. The second means by which sexual selection could operate is intersexual selection, or preferential mate choice. If members of one sex have some consensus about the qualities that are desired in members of the opposite sex, then individuals of the opposite sex who possess those qualities will be preferentially chosen as mates. Those who lack the desired qualities fail to get mates. In this case, evolutionary change occurs simply because the qualities that are desired in a mate increase in frequency with the passing of each generation. If females prefer to mate with males who give them gifts of food, for example, then males with qualities that lead to success in acquiring food gifts will increase in frequency over time. Darwin called the process of intersexual selection female choice because he observed that throughout the animal world, females of many species were discriminating or choosy about whom they mated with. Darwin’s theory of sexual selection succeeded in explaining the anomalies that worried him. The peacock’s tail, for example, evolved because of the process of intersexual selection: Peahens prefer to mate with males who have the most brilliant and luminescent plumage. Males are often larger than females in species in which males engage in physical combat with other males for sexual access to females—a sex difference caused by the process of intrasexual competition. The Role of Natural Selection and Sexual Selection in Evolutionary Theory Darwin’s theories of natural and sexual selection are relatively simple to describe, but many sources of confusion surround them even to this day. This section clarifies some important aspects of selection and its place in understanding evolution. First, natural selection and sexual selection are not the only causes of evolutionary change. Some changes, for example, can occur because of a process called genetic drift, which is defined as random changes in the genetic makeup of a population. Random changes come about through several processes, including mutation (a random hereditary change in the DNA), founder effects, and genetic bottlenecks. Founder effects occur when a small portion of a population establishes a new colony and the founders of the new colony are not genetically representative of the original population. Imagine, for example, that the 200 colonizers who migrate to a new island happen by chance to include an unusually large number of redheads. As the population on the island grows, say, to 2,000 people, it will contain a larger proportion of redheads than did the original population from which the colonizers came. Thus, founder effects can produce evolutionary change—in this example, an increase in genes coding for red hair. A similar random change can occur through genetic bottlenecks, which happen when a population shrinks, perhaps owing to a random catastrophe such as an earthquake. The survivors of the random catastrophe carry only a subset of the genes of the original population. In sum, although natural selection is the primary cause of evolutionary change and the only known cause of adaptations, it is not the only cause of evolutionary change. Genetic drift—through mutations, founder effects, and genetic bottlenecks—can also produce change in the genetic makeup of a population. Second, evolution by natural selection is not forward looking and is not “intentional.” The giraffe does not spy the juicy leaves stirring high in the tree and “evolve” a longer neck. Rather, those giraffes that, owing to an inherited variant, happen to have longer necks have an advantage over other giraffes in getting to those leaves. Hence they have a greater chance of surviving and thus of passing on their slightly longer necks to their offspring. Natural selection merely acts on variants that happen to exist. Evolution is not intentional and cannot look into the future and foresee distant needs. Another critical feature of selection is that it is gradual, at least when evaluated relative to the human life span. The short-necked ancestors of giraffes did not evolve long necks overnight or even over the course of a few generations. It has taken dozens, hundreds, thousands, and in some cases millions of generations for the process of selection to gradually shape the organic mechanisms we see today. Of course, some changes occur extremely slowly, others more rapidly. And there can be long periods of no change, followed by a relatively sudden change, a phenomenon known as “punctuated equilibrium” (Gould & Eldredge, 1977). But even these “rapid” changes occur in tiny increments in each generation and take dozens, hundreds, or thousands of generations to occur. Darwin’s theory of natural selection offered a powerful explanation for many baffling aspects of life. It explained the origin of new species (although Darwin failed to recognize the full importance of geographic isolation as a precursor to natural selection in the formation of new species; see Cronin, 1991). It accounted for the modification of organic structures over time. It accounted for the apparent purposive quality of the component parts of those structures—that is, they seemed “designed” to serve particular functions that contributed to survival or reproduction. Perhaps most astonishing to some (but upsetting to others), in 1859, Darwin’s natural selection united all species into one grand tree of descent in one bold stroke. For the first time in recorded history, each species was viewed as being connected with all other species through a common ancestry. Human beings and chimpanzees, for example, share more than 98 percent of each other’s DNA and shared a common ancestor roughly 6 or 7 million years ago (Wrangham & Peterson, 1996). Even more startling is the fact that many human genes turn out to have counterpart genes in a transparent worm called Caenorhabditis elegans. They are highly similar in chemical structure, suggesting that humans and this worm evolved from a distant common ancestor (Wade, 1997). In short, Darwin’s theory made it possible to locate humans in the grand tree of life, showing our place in nature and our links with all other living creatures. Darwin’s theory of natural selection created a storm of controversy. Lady Ashley, a contemporary of Darwin, remarked on hearing his theory that human beings descended from apes: “Let’s hope it’s not true; but if it is true, let’s hope that it does not become widely known.” In a famous debate at Oxford University, Bishop Wilberforce bitingly asked his rival debater Thomas Huxley whether the “ape” from which Huxley descended was on his grandmother’s or his grandfather’s side. Even biologists at the time were highly skeptical of Darwin’s theory of natural selection. One objection was that Darwinian evolution lacked a coherent theory of inheritance. Darwin himself preferred a “blending” theory of inheritance, in which offspring are mixtures of their parents, much like pink paint is a mixture of red paint and white paint. This theory of inheritance is now known to be wrong, so early critics were correct in the objection that the theory of natural selection lacked a solid theory of heredity. Another objection was that some biologists could not imagine how the early stages of the evolution of an adaptation could be useful to an organism. How could a partial wing help a bird, if a partial wing is insufficient for flight? How could a partial eye help a reptile, if a partial eye is insufficient for sight? Darwin’s theory of natural selection requires that each and every step in the gradual evolution of an adaptation be advantageous in the currency of reproduction. Thus, partial wings and eyes must yield an adaptive advantage, even before they evolve into fully developed wings and eyes. For now, it is sufficient to note that partial forms can indeed offer adaptive advantages; partial wings, for example, can keep a bird warm and aid in mobility for catching prey or avoiding predators, even if they don’t afford full flight. This objection to Darwin’s theory is therefore surmountable (Dawkins, 1986). Further, it is important to stress that just because biologists or other scientists have difficulty imagining certain forms of evolution, such as how a partial wing might be useful, that is not a good argument against such forms having evolved. This “argument from ignorance,” or as Dawkins (1982) calls it, “the argument from personal incredulity,” is not good science, however intuitively compelling it might seem. Indeed, most people find evolution by natural selection and evolutionary time scales extremely difficult to conceptualize (Rodeheffer, Daugherty, & Brase, 2011). A third objection came from religious creationists, many of whom viewed species as immutable (unchanging) and created by a deity rather than by the gradual process of evolution by selection. Furthermore, Darwin’s theory implied that the emergence of humans and other species was “blind,” resulting from the slow, unplanned, cumulative process of selection. This contrasted with the view that creationists held of humans (and other species) as part of God’s grand plan or intentional design. Darwin had anticipated this reaction and apparently delayed the publication of his theory in part because he was worried about upsetting his wife, Emma, who was deeply religious. The controversy continues to this day. Although Darwin’s theory of evolution, with some important modifications, is the unifying and nearly universally accepted theory within the biological sciences, its application to humans, which Darwin clearly envisioned, still meets some resistance. But humans are not exempt from the evolutionary process. We finally have the conceptual tools to complete Darwin’s revolution and forge an evolutionary psychology of the human species. Evolutionary psychology is able to take advantage of key theoretical insights and scientific discoveries that were not known in Darwin’s day. The first among these is the physical basis of inheritance—the gene. The Modern Synthesis: Genes and Particulate Inheritance  When Darwin published On the Origin of Species, he did not know the nature of the mechanism by which inheritance occurred. An Austrian monk named Gregor Mendel showed that inheritance was “particulate” and not blended. That is, the qualities of the parents are not blended with each other but rather are passed on intact to their offspring in distinct packets called genes. Furthermore, parents must be born with the genes they pass on; genes cannot be acquired by experience. Mendel’s discovery that inheritance is particulate, which he demonstrated by crossbreeding different strains of pea plants, remained unknown to most of the scientific community for some 30 years. Mendel had sent Darwin copies of his papers, but either they remained unread or Darwin did not recognize their significance. A gene is defined as the smallest discrete unit that is inherited by offspring intact, without being broken up or blended—this was Mendel’s critical insight. Genotypes, in contrast, refer to the entire collection of genes within an individual. Genotypes, unlike genes, are not passed down to offspring intact. Rather, in sexually reproducing species such as our own, genotypes are broken up with each generation. Each of us inherits a random half of genes from our mother’s genotype and a random half from our father’s genotype. The specific half of the genes we inherit from each parent, however, is identical to half of those possessed by that parent because they get transmitted as a discrete bundle, without modification. The unification of Darwin’s theory of evolution by natural selection with the discovery of particulate gene inheritance culminated in a movement in the 1930s and 1940s called the “Modern Synthesis” (Dobzhansky, 1937; Huxley, 1942; Mayr, 1942; Simpson, 1944). The Modern Synthesis discarded a number of misconceptions in biology, including Lamarck’s theory of inheritance of acquired characteristics and the blending theory of inheritance. It confirmed the importance of Darwin’s theory of natural selection and put it on a firmer footing with a well-articulated understanding of the nature of inheritance. The Ethology Movement  To some people, evolution is most clearly envisioned when it applies to physical structures. We can easily see how a turtle’s shell is an adaptation for protection and a bird’s wings an adaptation for flight. We recognize similarities between ourselves and chimpanzees, and so most people find it relatively easy to believe that human beings and chimps have a common ancestry. The paleontological record of skulls, although incomplete, shows enough evidence of physical evolution that most concede reveals that change has taken place over time. The evolution of behavior, however, has historically been more difficult for scientists and laypeople to imagine. Behavior, a er all, leaves no fossils, at least not directly (the skulls and skeletons with human-inflicted traumas, described at the start of this chapter, can be considered a kind of fossilized record of behavior; fossilized feces [coprolite], to take another example, can reveal much about our ancestral diet). Darwin clearly envisioned his theory of natural selection to be just as applicable to behavior, including social behavior, as to physical structures. Several lines of evidence support this view. First, all behavior requires underlying physical structures. Bipedal locomotion is a behavior, for example, and requires the physical structures of two legs and a multitude of muscles to support those legs. Second, species can be bred for certain behavioral characteristics using the principle of selection. Dogs, for example, can be bred through artificial selection for aggressiveness or passivity. These lines of evidence all point to the conclusion that behavior is not exempt from the sculpting hand of evolution. The first major discipline to form around the study of behavior from an evolutionary perspective was the field of ethology, and one of the first phenomena the ethologists documented was imprinting. Ducklings imprint on the first moving object they observe in life—forming an association during a critical period of development. Usually this object is the duck’s mother. After imprinting, the baby ducks follow the object of their imprinting wherever it goes. Imprinting is clearly a form of learning—an association is formed between the duckling and the mother that was not there before the exposure to her motion. This form of learning, however, is “preprogrammed” and clearly part of the evolved structures of the duckling’s biology. Although many have seen pictures of a line of baby ducks following their mother, the fact is that if the first object a duck sees is a human leg, it will follow that person instead. Imprinting was first noticed by 19th-century amateur biologist Douglas Spalding and later rediscovered by the biologist Oskar Heinroth. Konrad Lorenz studied imprinting extensively, showed that it occurred during a “critical period” early in life, and even showed that baby ducks would follow him rather than their mother if exposed to his leg during the critical period shortly after birth. Lorenz (1965) was one of the founders of a new branch of evolutionary biology called ethology, and imprinting in birds was a vivid phenomenon used to launch this new field. Ethology is defined as “the study of the proximate mechanisms and adaptive value of animal behavior” (Alcock, 1989, p. 548). Konrad Lorenz was one of the founders of the field of ethology. He is most well known for studying the phenomenon of imprinting, whereby ducklings will become attached to and follow the first object they see moving. In most cases, ducklings get imprinted on their mothers, not the legs of a scientist.  The ethology movement was in part a reaction to the extreme environmentalism in U.S. psychology. Ethologists were interested in four key issues, which have become known as the four “whys” of behavior advanced by one of the founders of ethology, Nikolaas Tinbergen (1951): (1) the immediate influences on behavior (e.g., the movement of the mother); (2) the developmental influences on behavior (e.g., the events during the duck’s lifetime that cause changes); (3) the function of behavior, or the “adaptive purpose” it fulfills (e.g., keeping the baby duck close to the mother, which helps it to survive); and (4) the evolutionary or phylogenetic origins of behavior (e.g., what sequence of evolutionary events led to the origins of an imprinting mechanism in the duck). Ethologists developed an array of concepts to describe what they believed to be the innate properties of animals. Fixed action patterns, for example, are the stereotypic behavioral sequences an animal follows after being triggered by a well-defined stimulus (Tinbergen, 1951). Once a fixed action pattern is triggered, the animal performs it to completion. Showing certain male ducks a plastic facsimile of a female duck, for example, will trigger a rigid sequence of courting behavior. Concepts such as fixed action patterns were useful in allowing ethologists to partition the ongoing stream of behavior into discrete units for analysis. The ethology movement went a long way toward orienting biologists to focus on the importance of adaptation. Indeed, the glimmerings of evolutionary psychology itself may be seen in the early writings of Lorenz, who wrote, “our cognitive and perceptual categories, given to us prior to individual experience, are adapted to the environment for the same reasons that the horse’s hoof is suited for the plains before the horse is born, and the fin of a fish is adapted for water before the fish hatches from its egg” (Lorenz, 1941, p. 99; translated from the original German by I. Eibl-Eibesfeldt, 1989, p. 8). Ethology also forced psychologists to reconsider the role of biology in the study of human behavior. This set the stage for an important scientific revolution, brought about by a fundamental reformulation of Darwin’s theory of natural selection. The Inclusive Fitness Revolution In the early 1960s, a young graduate student named William D. Hamilton was working on his doctoral dissertation at University College, London. Hamilton proposed a radical new revision of evolutionary theory, which he termed “inclusive fitness theory.” Legend has it that his professors failed to understand the dissertation or its significance (perhaps because it was highly mathematical), and so his work was initially rejected. When it was finally accepted and published in 1964 in the Journal of Theoretical Biology, however, Hamilton’s theory sparked a revolution that transformed the entire field of biology. Hamilton reasoned that classical fitness—the measure of an individual’s direct reproductive success in passing on genes through the production of offspring —was too narrow to describe the process of evolution by selection. He theorized that natural selection favors characteristics that cause an organism’s genes to be passed on, regardless of whether the organism produces offspring directly. Parental care—investing in one’s own children—was reinterpreted as merely a special case of caring for kin who carry copies of parents’ genes in their bodies. An organism can also increase the reproduction of its genes by helping brothers, sisters, nieces, or nephews to survive and reproduce. All these relatives have some probability of carrying copies of the organism’s genes. Hamilton’s genius was in the recognition that the definition of classical fitness was too narrow and should be broadened to be inclusive fitness. Technically, inclusive fitness is not a property of an individual or an organism but rather a property of its actions or effects. Thus, inclusive fitness can be viewed as the sum of an individual’s own reproductive success (classical fitness) plus the effects the individual’s actions have on the reproductive success of his or her genetic relatives. For this second component, the effects on relatives must be weighted by the appropriate degree of genetic relatedness to the target organism—for example, 0.50 for brothers and sisters (because they are genetically related by 50 percent with the target organism), 0.25 for grandparents and grandchildren (25 percent genetic relatedness), and 0.125 for first cousins (12.5 percent genetic relatedness) (see Figure 1.1). One implication of inclusive fitness theory is that acts of altruism will be directed more toward closely related individuals than more distantly related individuals. William D. Hamilton revolutionized evolutionary biology with his theory of inclusive fitness, published in 1964. He continued to make profound theoretical contributions on topics as diverse as the evolution of spite and the origins of sexual reproduction. Figure 1.1 Genetic Relatedness Among Different Types of Relatives The inclusive fitness revolution marshaled a new era that may be called “gene’s eye thinking.” If you were a gene, what would facilitate your replication? First, you might try to ensure the well-being of the “vehicle” or body in which you reside (survival). Second, you might try to induce the vehicle to reproduce. Third, you might want to help the survival and reproduction of vehicles that contain copies of you. Genes, of course, do not have thoughts, and none of this occurs with consciousness or intentionality. The key point is that the gene is the fundamental unit of inheritance, the unit that is passed on intact in the process of reproduction. Genes producing effects that increase their own replicative success will replace other genes, producing evolution over time. Adaptations are selected and evolve because they promote inclusive  fitness. Thinking about selection from the perspective of the gene offered a wealth of insights unknown in Darwin’s day (Buss, 2009a). The theory of inclusive fitness has profound consequences for how we think about the psychology of the family, altruism, helping, the formation of groups, and even aggression— topics we explore in later chapters. As for W. D. Hamilton himself, after a stint at the University of Michigan, Oxford University made him an offer he couldn’t refuse, and he became an esteemed professor there. Unfortunately, Hamilton met an untimely death in 2000 from a disease acquired in the Congo jungle, where he had traveled to gather evidence for a novel theory on the origins of the virus that causes AIDS. But his influence on modern evolutionary theory continues to this day. Clarifying Adaptation and Natural Selection The rapid inclusive fitness revolution in evolutionary biology owes part of its debt to George C. Williams, who in 1966 published a now-classic work titled Adaptation and Natural Selection. This seminal book contributed to at least three key shifts in thinking in the field of evolutionary theory. First, Williams (1966) challenged the prevailing endorsement of group selection, the notion that adaptations evolved for the benefit of the group through the differential survival and reproduction of groups (Wynne- Edwards, 1962), as opposed to benefit of the gene and arising through the differential reproduction of genes. According to the theory of group selection, for example, an animal might limit its personal reproduction to keep the population low, thus avoiding the destruction of the food base on which the population relied. According to group selection theory, only species that possessed characteristics beneficial to their group survived. Those that acted more selfishly perished because of the overexploitation of the critical food resources on which the species relied. Williams argued persuasively that group selection, although theoretically possible, was likely to be a weak force in evolution, for the following reason. Imagine a bird species with two types of individuals—one that sacrifices itself by committing suicide so as not to deplete its food resources and another that selfishly continues to eat the food, even when supplies are low. In the next generation, which type is likely to have descendants? The answer is that the suicidal birds will have died out and failed to reproduce, whereas those who refused to sacrifice themselves for the group will have survived and left descendants. Selection operating on individual differences within a group, in other words, undermines the power of selection operating between groups. Within 5 years of the book’s publication, most biologists had relinquished their endorsement of group selection, although recently there has been a resurgence of interest in the potential potency of group selection (Sober & Wilson, 1998; Wilson, van Vugt, & O’Gorman, 2008; Wilson & Sober, 1994; for critiques of group selection, see Pinker, 2012; Price, 2012). Williams’s second contribution was in translating Hamilton’s mathematical theory of inclusive fitness into clear prose that could be comprehended by everyone. Once biologists understood inclusive fitness, they began vigorously researching its implications. To mention one prominent example, inclusive fitness theory partially solved the “problem of altruism”: How could altruism evolve—incurring reproductive costs to oneself to benefit the reproduction of others—if evolution favors genes that have the effect of self-replication? Inclusive fitness theory solved this problem (in part) because altruism could evolve if the recipients of help were one’s genetic relatives. Parents, for example, might sacrifice their own lives to save the lives of their children, who carry copies of the parents’ genes within them. The same logic applies to making sacrifices for other genetic relatives, such as sisters or cousins. The benefit to one’s relatives in fitness currencies must be greater than the costs to the self. If this condition is satisfied, then kin altruism can evolve. In later chapters, we review evidence showing that genetic relatedness is indeed a powerful predictor of helping among humans. The third contribution of Adaptation and Natural Selection was Williams’s careful analysis of adaptation, which he referred to as “an onerous concept.” Adaptations may be defined as evolved solutions to specific problems that contribute either directly or indirectly to successful reproduction. Sweat glands, for example, may be adaptations that help solve the survival problem of thermal regulation. Taste preferences may be adaptations that guide the successful consumption of nutritious food. Mate preferences may be adaptations that guide the successful selection of fertile mates. The problem is how to determine which attributes of organisms are adaptations. Williams established several standards for invoking adaptation and believed that it should be invoked only when necessary to explain the phenomenon at hand. When a flying fish leaps out of a wave and falls back into the water, for example, we do not have to invoke an adaptation for “getting back to water.” This behavior is explained more simply by the physical law of gravity. Williams provided criteria for determining when we should invoke the concept of adaptation: reliability, efficiency, and economy. Does the mechanism regularly develop in most or all members of the species across all “normal” environments and perform dependably in the contexts in which it is designed to function (reliability)? Does the mechanism solve a particular adaptive problem well and effectively (efficiency)? Does the mechanism solve the adaptive problem without extorting huge costs from the organism (economy)? In other words, adaptation is invoked not merely to explain the usefulness of a biological mechanism but to explain improbable usefulness (i.e., too precisely functional to have arisen by chance alone) (Pinker, 1997). Hypotheses about adaptations are, in essence, probability statements about why a reliable, efficient, and economic set of design features could not have arisen by chance alone (Tooby & Cosmides, 1992, 2005; Williams, 1966). In Chapter 2, we explore the key concept of adaptation in greater depth. For now, it is sufficient to note that Williams’s book brought the scientific community one step closer to the Darwinian revolution by creating the downfall of group selection as a preferred and dominant explanation, by illuminating Hamilton’s theory of inclusive fitness, and by putting the concept of adaptation on a more rigorous and scientific footing. Williams was extremely influential in showing that understanding adaptations requires being “gene centered.” As put eloquently by Helena Cronin in a book dedicated to George Williams, “The purpose of adaptations is to further the replication of genes…. Genes have been designed by natural selection to exploit properties of the world that promote their self-replication; genes are ultimately machines for turning out more genes” (Cronin, 2005, pp. 19–20). Trivers’s Seminal Theories In the late 1960s and early 1970s, a graduate student at Harvard University, Robert Trivers, studied Williams’s 1966 book on adaptation. He was struck by the revolutionary consequences that gene-level thinking had for conceptualizing entire domains. A brief paragraph in Williams’s book or Hamilton’s articles might contain the seed of an idea that could blossom into a full theory if nurtured properly. Trivers contributed three seminal papers, all published in the early 1970s. The first was the theory of reciprocal altruism among non-kin—the conditions under which mutually beneficial exchange relationships or transactions could evolve (Trivers, 1971). The second was parental investment theory, which provided a powerful statement of the conditions under which sexual selection would occur for each sex (1972). The third was the theory of parent–offspring conflict—the notion that even parents and their progeny will get into predictable sorts of conflicts because they share only 50 percent of their genes (1974). Parents may try to wean children before the children want to be weaned, for example, in order to free up resources to invest in other children. More generally, what might be optimal for a child (e.g., securing a larger share of parental resources) might not be optimal for the parents (e.g., distributing resources more equally across children). We explore these theories in greater depth in Chapter 4 (theory of parental investment), Chapter 7 (theory of parent–offspring conflict), and Chapter 9 (theory of reciprocal altruism) because they have influenced thousands of empirical research projects, including many on humans. The Sociobiology Controversy Eleven years a er Hamilton’s pivotal paper on inclusive fitness was published, a Harvard biologist named Edward O. Wilson caused a  scientific and public uproar that rivaled the outrage caused by Charles Darwin in 1859. Wilson’s 1975 book Sociobiology: The New Synthesis was monumental in both size and scope, at nearly 700 double-column pages. It offered a synthesis of cellular biology, integrative neurophysiology, ethology, comparative psychology, population biology, and behavioral ecology. Further, it examined species from ants to humans, proclaiming that the same fundamental explanatory principles could be applied to all. Sociobiology is not generally regarded as containing fundamentally new theoretical contributions to evolutionary theory. The bulk of its theoretical tools—such as inclusive fitness theory, parental investment theory, parent– offspring conflict theory, and reciprocal altruism theory—had already been developed by others (Hamilton, 1964; Trivers, 1972, 1974). What it did do is synthesize under one umbrella a tremendous diversity of scientific endeavors and give the emerging field a visible name. The chapter on humans, the last in Wilson’s book and running a mere 29 pages, created the most controversy. At public talks, audience members shouted him down, and once a pitcher of water was dumped on his head. His work sparked attacks from Marxists, radicals, creationists, other scientists, and even members of his own department at Harvard. Part of the controversy stemmed from the nature of Wilson’s claims. He asserted that sociobiology would “cannibalize psychology,” which of course was not greeted warmly by most psychologists. Further, he speculated that many cherished human phenomena, such as culture, religion, ethics, and even aesthetics, would ultimately be explained by the new synthesis. These assertions strongly contradicted the dominant theories in the social sciences. Culture, learning, socialization, rationality, and consciousness, not evolutionary biology, were presumed by most social scientists to explain human behavior. Despite Wilson’s grand claims for a new synthesis that would explain human nature, he had little empirical evidence on humans to support his views. The bulk of the scientific evidence came from non-human animals, many far removed phylogenetically from humans. Most social scientists could not see what ants and fruit flies had to do with people. Although scientific revolutions always meet resistance, often from within the ranks of established scientists (Sulloway, 1996), Wilson’s lack of relevant scientific data on humans did not help. Furthermore, the tremendous resistance to Wilson’s inclusion of humans within the purview of evolutionary theory was based on several common misunderstandings about evolutionary theory and its application to humans. It is worth highlighting a few of these before turning to movements within psychology that laid the groundwork for evolutionary psychology. Common Misunderstandings About Evolutionary Theory The theory of evolution by selection, although elegant in its simplicity, generates a number of common misunderstandings (Confer et al., 2010). Perhaps its very simplicity leads people to think that they can understand it completely a er only brief exposure to it—a er reading an article or two in the popular press, for example. Even professors and researchers in the field sometimes get mired in these misunderstandings. Misunderstanding 1: Human Behavior Is Genetically Determined Genetic determinism is the doctrine that argues that behavior is controlled exclusively by genes, with little or no role for environmental influence. Much of the resistance to applying evolutionary theory to the understanding of human behavior stems from the misconception that evolutionary theory implies genetic determinism. Contrary to this misunderstanding, evolutionary theory represents a truly interactionist framework. Human behavior cannot occur without two ingredients: (1) evolved adaptations and (2) environmental input that triggers the development and activation of these adaptations. Consider calluses as an illustration. Calluses cannot occur without an evolved callus-producing adaptation, combined with the environmental influence of repeated friction to the skin. Therefore, to invoke evolutionary theory as an explanation for calluses, we would never say “calluses are genetically determined and occur regardless of input from the environment.” Instead, calluses are the result of a specific form of interaction between an environmental input (repeated friction to the skin) and an adaptation  that is sensitive to repeated friction and contains instructions to grow extra new skin cells when the skin experiences repeated friction. Indeed, the reason that adaptations evolve is that they afford organisms tools to grapple with the problems posed by the environment. So notions of genetic determinism—behaviors caused by genes without input or influence from the environment—are simply false. They are in no way implied by the evolutionary theory or by evolutionary psychology. Misunderstanding 2: If It’s Evolutionary, We Cannot Change It A second misunderstanding is that evolutionary theory implies that human behavior is impervious to change. Consider the simple example of calluses again. Humans can and do create physical environments that are relatively free of friction. These friction-free environments mean that we have designed change—a change that prevents the activation of the underlying callus-producing mechanisms. Knowledge of these mechanisms and the environmental input that triggers their activation gives us the power to decrease callus production. In a similar manner, knowledge of our evolved social psychological adaptations along with the social inputs that activate them gives us power to alter social behavior, if that is the desired goal. Consider the following example. There is evidence that men have lower thresholds than women for inferring sexual intent. When a woman smiles at a man, male observers are more likely than female observers to infer that the woman is sexually interested (Abbey, 1982; Perilloux, Easton, & Buss, 2012). This sexual overperception bias is most likely part of an evolved psychological adaptation in men that motivates them to seek casual sexual opportunities (Buss, 2016 b). Knowledge of this mechanism, however, allows for the possibility of change. Men, for example, can be educated with the information that they have lower thresholds for inferring sexual intent when a woman smiles at them. This knowledge can then be used by men, in principle, to reduce the number of times they act on their faulty inferences of sexual interest and decrease the number of unwanted sexual advances they make toward women. Knowledge about our evolved psychological adaptations along with the social inputs that they were designed to be responsive to, far from dooming us to an unchangeable fate, can have the liberating effect of paving the way for changing behavior in areas in which change is desired. This does not mean that changing behavior is simple or easy. More knowledge about our evolved psychology, however, gives us more power to change. Misunderstanding 3: Current Mechanisms Are Optimally Designed The concept of adaptation, the notion that mechanisms have evolved functions, has led to many outstanding discoveries over the past century (Dawkins, 1982). This does not mean, however, that the current collection of adaptive mechanisms that make up humans is in any way “optimally designed.” An engineer might cringe at some of the ways that our mechanisms are structured, which sometimes appear to be assembled with a piece here and a bit there. In fact, many factors cause the existing design of our adaptations to be far from optimal. Let’s consider two of them (see Dawkins, 1982, Chapter 3; Al-Shawaf & Zreik, 2018). One constraint on optimal design is evolutionary time lags. Recall that evolution refers to change over time. Each change in the environment brings new selection pressures. Because evolutionary change occurs slowly, requiring dozens or thousands of generations of recurrent selection pressure, existing humans are necessarily designed for the previous environments of which they are a product. Stated differently, we carry around a Stone Age brain in a modern environment. In other words, “we are walking archives of ancestral wisdom” (Cronin, 1991). A strong taste preference for fat and sugar, adaptive in a past environment of scarce food resources, now leads to clogged arteries, Type 2 diabetes,  and heart attacks. The lag in time between the environment that fashioned our mechanisms (the hunter-gatherer past that formed much of our selective environment) and today’s environment means that some of our existing evolved mechanisms may not be optimally designed for the current environment. A second constraint on optimal design pertains to the costs of adaptations. Consider as an analogy the risk of being killed while driving a car. In principle, we could reduce this risk to near zero if we imposed a 5-mile-per- hour speed limit and forced everyone to drive in armored trucks with 10 feet of padding on the inside (Symons, 1993). But we consider the costs of this solution to be ridiculously high. Similarly, we might consider a hypothetical example in which natural selection built into humans such a severe terror of snakes and spiders that people never ventured outdoors. Such a fear would surely reduce the incidence of snake and spider bites, but it would carry a prohibitively high cost. It would prevent people from solving other adaptive problems, such as gathering fruits, plants, and other food resources necessary for survival. In short, the existing fears of snakes and spiders that characterize humans are not optimally designed—after all, thousands of people do get bitten by snakes every year, and some die as a result. But it works reasonably well, on average. All adaptations carry costs. Selection favors a mechanism when its benefits outweigh the costs relative to other designs existent at the time. Humans have evolved mechanisms that are reasonably good at solving adaptive problems efficiently, but they are not designed as optimally as they might be if costs were not a constraint. Evolutionary time lags and the costs of adaptations are just two of the many reasons why adaptations are not optimally designed (Williams, 1992). In summary, part of the resistance to the application of evolutionary theory to humans is based on several common misconceptions. Contrary to these misconceptions, evolutionary theory does not imply genetic determinism. It does not imply that we are powerless to change things. It does not mean that our existing adaptations are optimally designed. Importantly, students who take a course in evolutionary psychology show a dramatic decrease in these and other evolutionary misconceptions—even more knowledge improvement than students who take courses in biology (Short & Hawley, 2015). With these common misunderstandings about evolutionary theory clarified, let’s turn now to the origins of modern humans, the development of the field of psychology, and an examination of the landmarks that led to the emergence of evolutionary psychology. Milestones in the Origins of Modern Humans One of the most fascinating endeavors for those struggling to understand the modern human mind is to explore what is known about the critical historical developments that eventually contributed to who we are today. Table 1.1 shows some of these milestones. The first interesting item to note is the enormity of the timescale. It took roughly 3.7 billion years to get from the origins of the first life on earth to modern humans in the 21st century. Table 1.1 Milestones in Human Evolutionary History Time Event 15 billion years ago The Big Bang—origin of universe (bya) 4.7 bya Earth forms 3.7 bya First life emerges 1.2 bya Sexual reproduction evolves 500–450 million First vertebrates years ago (mya) 365 mya Fish evolve lungs and walk on land 248–208 First small mammals and dinosaurs evolve mya 208–65 mya Large dinosaurs flourish 114 mya Placental mammals evolve 85 mya First primates evolve  65 mya Dinosaurs go extinct; mammals then increase in size and diversity 35 mya First apes evolve 6–8 mya Common ancestor of humans and African apes evolves 4.4 mya First primate with bipedal locomotion (Ardipithecus ramidus) 3.0 mya The australopithecines evolve in savannas of Africa Earliest stone tools develop—Oldowan (found in Ethiopia and 2.5 mya Kenya, Africa); used to butcher carcasses for meat and to extract marrow from bones; linked with Homo habilis Hominids (Homo erectus) spread beyond Africa to Asia—first 1.8 mya major migration 1.6 mya Fire evidence; likely hearths; linked with African Homo erectus Invention of Acheulean hand axe; linked with Homo ergaster— 1.5 mya tall stature, long limbs 1.2 mya Brain expansion in Homo line begins 1.0 mya Hominids spread to Europe 800 thousand Crude stone tool kit used—found in Spain, linked with Homo years ago antecessor (kya) 600–400 Long, crafted wooden spears made and used; linked with Homo kya heidelbergensis found in Germany 500–100 Period of most rapid brain expansion in Homo line kya 200–30 kya Neanderthals flourish in Europe and western Asia 150–120 Common ancestor for all modern humans (Africa) evolves kya 100–50 kya Exodus from Africa—second major migration [“Out of Africa”] Explosion of diverse stone tools, bone tools, blade tools, well– 50–35 kya designed fireplaces, elaborate art; found mainly among Homo sapiens, rarely among Neanderthals 40–35 kya Homo sapiens (Cro–Magnons) arrive in Europe  30 kya Neanderthals go extinct 27 kya– Homo sapiens colonize entire planet; all other hominid species present are now extinct Note: These dates are based in part on information from a variety of sources, including Johanson and Edgar (1996), Klein (2000), Lewin (1993), Tattersall (2000), Wrangham, Jones, Laden, Pilbeam, and Conklin–Brittain (1999), and the references contained therein. Humans are mammals; the first mammals originated more than 200 million years ago. Mammals are warm-blooded, having evolved mechanisms that regulate internal body temperature to maintain a constant warm level despite environmental perturbations. Warm- bloodedness gave mammals the advantage of being able to run metabolic processes at a constant temperature. Except for some marine mammals such as whales, mammals are usually covered with fur, an adaptation that helps to keep body temperature constant. Mammals are also distinguished by a unique method of feeding their young: through secretions via mammary glands. Indeed, the term mammal comes from “mamma,” the Latin word for breast. Mammary glands exist in both males and females but become functional for feeding only in females. Human breasts are merely one modern form of an adaptation whose origins can be traced back more than 200 million years. Another major development was the evolution of placental mammals around 114 million years ago, as contrasted with egg-laying nonplacentals. In placental mammals, the fetus attaches to the mother inside her uterus through a placenta, which allows the direct delivery of nutrients. The fetus remains attached to the mother’s placenta until it is born alive, unlike its egg-laying predecessors, whose prebirth development was limited by the amount of nutrients that could be stored in an egg. These initially small, warm-blooded, furry mammals began a line that eventually led to modern humans. Roughly 85 million years ago, a new line of mammals evolved: primates. Early primates were small, perhaps the size of squirrels. They developed hands and feet that contained nails instead of claws and opposable digits on hands (and sometimes feet) that enabled increased grasping and manipulative abilities. Primates have well-developed stereoscopic vision with eyes facing forward, which gave them an advantage in jumping from branch to branch. Their brains are large in relation to their bodies (compared to nonprimate mammals), and their mammary glands have been reduced in number to two (rather than several pairs). One of the most critical developments of the primate line that led to modern humans occurred roughly 4.4 million years ago: bipedal locomotion, the ability to walk, stride, and run on two feet rather than on four. Although no one knows the precise evolutionary impetus for bipedalism, it undoubtedly offered a bounty of benefits on the African savanna where it evolved. It afforded the ability to rapidly cover long distances in an energetically efficient manner, enabled a greater visual angle for the detection of predators and prey, decreased the surface area of the body that was exposed to harmful sun rays, and freed up the hands. The liberation of hands from the work of walking not only enabled this early ancestor to carry food from place to place but also opened up a niche for the subsequent evolution of toolmaking and tool use. It is in these bipedal primates that we first recognize the glimmerings of early humans. Many scientists believe that the evolution of bipedalism paved the way for many subsequent developments in human evolution, such as toolmaking, large-game hunting, and the enlargement of the brain. It took roughly 2 million years of additional evolution, however, before the first crude tools appear in the paleontological record about 2.5 million years ago. These were Oldowan stone tools, fashioned by stone flaking to create a sharp edge. These tools were used to separate meat from bone on carcasses and to extract the nutritious marrow from the larger bones. Although Oldowan stone tools are simple and crude when viewed from today’s modern perspective, making them required a level of skill and technological mastery that even a well-trained chimpanzee cannot duplicate (Klein, 2000). Oldowan stone tools apparently were so successful as a technology that they remained essentially unchanged for more than a million years. And they were linked with the first group in the genus Homo, called Homo habilis, or “handy man,” which existed between 2.5 and 1.5 million years ago. Roughly 1.8 million years ago, bipedal toolmaking primates evolved into a successful branch known as Homo erectus and started to migrate out of Africa and into Asia. Fossils dated at 1.8 million years old have been found in both Java and China (Tattersall, 2000). The term “migration” is a bit misleading, in that it implies setting out on a quest to colonize a distant land. More likely, the “migration” occurred through gradual population expansion into lands with abundant resources. It is not clear whether this expanding Homo erectus group knew how to use fire. Although the earliest traces of controlled fire are found in Africa 1.6 million years ago, clear evidence of fire in Europe does not appear until a million years later. The descendants of this first major migration out of Africa ended up colonizing many parts of Asia and eventually Europe and later evolved into the Neanderthals. The next major technological advancement was the Acheulean hand axe 1.5 million years ago. These axes varied considerably in size and shape, and little is known about their precise uses. Their common quality is the flaking on two opposing surfaces, resulting in a sharp edge around the periphery of the implement. These axes took considerably more skill to produce than the crude Oldowan stone tools. They show symmetry of design and standardization of production that are not seen with the earlier stone tools. Around 1.2 million years ago, brains in the Homo line began to expand rapidly, more than doubling in size to the modern human level of approximately 1,350 cubic centimeters. The period of most rapid brain expansion occurred between 500,000 and 100,000 years ago. There are many speculations about the causes of this rapid brain size increase, such as the rise of toolmaking, tool use, complex communication, cooperative large-game hunting, climate, and social competition. It is possible that all these factors played some role in the expansion of the human brain (Bailey & Geary, 2009). Around 200,000 years ago, Neanderthals dominated many parts of Europe and western Asia. Neanderthals had weak chins and receding foreheads, but their thick skulls encased a large brain of 1,450 cubic centimeters. They were built for tough living and cold climates. Short limbed and stocky, their solid bodies housed a thick skeletal structure, which was needed for muscles far more powerful than those of modern humans. Their tools were advanced, their hunting skills formidable. Their teeth bore the marks of heavy wear and tear, suggesting frequent chewing of tough foods or the use of teeth to soften leather for clothing. There is evidence that Neanderthals buried their dead. They lived through ice and cold, thriving all over Europe and the Middle East. Then something dramatic happened 30,000 years ago. Neanderthals suddenly went extinct, after having flourished through ice ages and sudden changes in resources for more than 170,000 years. Their disappearance strangely coincided with another key event: the sudden arrival of anatomically modern Homo sapiens, called Homo sapiens sapiens. Why? (See Box 1.1.) Landmarks in the Field of Psychology Whereas changes have been taking place in evolutionary biology since Darwin’s 1859 book, psychology proceeded along a different path. Sigmund Freud, whose contributions came a few decades a er Darwin’s, was significantly influenced by Darwin’s theory of evolution by natural selection. So was William James. In the 1920s, however, psychology took a sharp turn away from evolutionary theory and embraced a radical behaviorism that reigned for half a century. Then important empirical discoveries made radical behaviorism untenable, encouraging a turn back to evolutionary theory. In this section, we briefly trace the historical influence—and lack of influence—of evolutionary theory on the field of psychology. 1.1 Out of Africa Versus Multiregional Origins: The Origins of Modern Humans A hundred thousand years ago, three distinct groups of hominids roamed the world: Homo neanderthalensis in Europe, Homo erectus in Asia, and Homo sapiens in Africa (Johanson, 2001). By 30,000 years ago, this diversity had been drastically reduced. All human fossils from 30,000 years ago to today share the same modern anatomical form: a distinct skull shape, a large brain (1,350 cubic centimeters), a chin, and a lightly built skeleton. Precisely what caused this radical transformation to a singular human form has been the subject of contentious debate among scientists. There are two competing theories: the multiregional continuity theory (MRC) and the Out of Africa theory (OOA). According to the MRC, a er the first migration from Africa 1.8 million years ago, the different groups of humans in different parts of the world slowly evolved in parallel with each other, all gradually becoming modern humans (Wolpoff & Caspari, 1996; Wolpoff, Hawks, Frayer, & Huntley, 2001). According to this theory, the emergence of modern humans did not occur in a single area but rather occurred in different regions of the world wherever humans lived (hence the term multiregional). The multiregional evolution of the different groups into the anatomically modern human form occurred, according to MRC, as a consequence of gene flow between the different groups, which interbred enough to prevent divergence into separate species. In sharp contrast, the OOA proposes that modern humans evolved quite recently in one location—Africa—and then migrated into Europe and Asia, replacing all previous populations, including the Neanderthals (Stringer & McKie, 1996). According to OOA, the different existing groups, such as the Neanderthals and Homo sapiens, had evolved into essentially different species, so interbreeding was unlikely or rare. In short, OOA posits a single location of modern human origins that occurred only recently, during the past 100,000 years, as contrasted with multiple regions of human origins posited by MRC. Scientists have examined three sources of evidence to test which of these theories is correct: anatomical evidence, archeological evidence, and genetic evidence. The anatomical evidence suggests that Neanderthals and Homo sapiens differed dramatically. The Neanderthals had a large cranial vault; pronounced brow ridges; a massive facial skeleton; large, heavily worn incisors; a protruding mid-face; short stature; and a thick-boned, stocky body build. The early Homo sapiens, in contrast, looked like modern humans: a cranial vault with a vertical (rather than sloping) forehead; a reduced facial skeleton without the protruding mid-face; a lower jaw with a clearly pronounced chin; and more slightly built bones. These large anatomic differences suggest that Neanderthals and early modern humans were isolated from each other rather than mating with each other and possibly evolved into two somewhat distinct species—findings that support the OOA. The archeological evidence—the tools and other artifacts le behind— shows that 100,000 years ago, Neanderthals and Homo sapiens were quite similar. Both