Unit 2 - Nurture and Human Diversity PDF

CHAPTER 4 Nature, Nurture, and Human Diversity Behavior Genetics: Predicting Individual Differences Genes: Our Codes for Life Twin and Adoption Studies Temperament and Heredity Heritability Gene–Environment Interaction Evolutionary Psychology: Explaining Human Nature and Nurture Natural Selection a...

CHAPTER 4 Nature, Nurture, and Human Diversity Behavior Genetics: Predicting Individual Differences Genes: Our Codes for Life Twin and Adoption Studies Temperament and Heredity Heritability Gene–Environment Interaction Evolutionary Psychology: Explaining Human Nature and Nurture Natural Selection and Adaptation Evolutionary Success Helps Explain Similarities An Evolutionary Explanation of Human Sexuality Cultural and Gender Diversity: Understanding Nature and Nurture How Does Experience Influence Development? Cultural Influences Gender Development  THINKING CRITICALLY ABOUT: Gender Bias in the Workplace THINKING CRITICALLY ABOUT: Sexual Aggression Reflections on Nature, Nurture, and Their Interaction What makes you you? In important ways, we are each unique. We are each a one-of-a-kind package of looks, language, personality, interests, abilities, and cultural background. Yet we are also the leaves of one tree. Our human family shares not only a common biological heritage—cut us and we bleed—but also common behavioral tendencies. Our shared brain architecture disposes us to sense the world, develop language, and feel hunger through identical mechanisms. Whether we live by the Arctic Ocean or the Atacama Desert, we prefer sweet tastes to sour. We divide the color spectrum into similar colors. And we feel drawn to behaviors that produce and protect offspring. Our kinship also appears in our social behaviors. Whether named Gonzales, Nkomo, Ahmadi, Smith, or Wong, we start fearing strangers at about 8 months, and as adults we prefer the company of those with attitudes and attributes similar to our own. Coming from different parts of the globe, we know how to read one another’s smiles and frowns. As members of one species, we affiliate, conform, return favors, punish offenses, organize hierarchies of status, and anguish over a child’s death. A visitor from outer space could drop in anywhere and find humans dancing and feasting, singing and worshiping, playing sports and games, laughing and crying, living in families and forming groups. Taken together, such universal behaviors define our human nature. What causes our striking diversity, and also our shared human nature? How much are our individual differences shaped by our differing genes? And how much by our environment—by every external influence, from maternal nutrition while in the womb to social support while nearing the tomb? To what extent are we formed by our upbringing? By our culture? By our gender? By our current circumstances? By people’s reactions to our genetic dispositions? By our own choices and efforts? This chapter tells the scientific story of how our genes (nature) and environments (nurture) together knit us.  Behavior Genetics: Predicting Individual Differences LEARNING OBJECTIVE QUESTION LOQ 4-1 What are chromosomes, DNA, genes, and the human genome? How do behavior geneticists explain our individual differences? So, if Beyoncé and JAY-Z’s eldest daughter, Blue Ivy, grows up to be a popular recording artist, should we attribute her musical talent to her “superstar genes”? To her growing up in a musically rich environment? To high expectations? Such questions intrigue behavior geneticists, who study our differences and weigh the effects and the interplay of heredity and environment. behavior genetics the study of the relative power and limits of genetic and environmental influences on behavior. heredity the genetic transfer of characteristics from parents to offspring. environment every nongenetic influence, from prenatal nutrition to the people and things around us. The nurture of nature Parents everywhere wonder: Will my baby grow up to be agreeable or aggressive? Successful or struggling at every step? What comes built in, and what is nurtured—and how? Research reveals that nature and nurture together shape our development—every step of the  way. Genes: Our Codes for Life Behind the story of our body and its brain—surely the most awesome thing on our little planet—is the heredity that interacts with our experience to create both our universal nature and our individual and social diversity. On the eve of the twentieth century, few would have guessed that every cell nucleus in your body contains the genetic master code for your entire body. It’s as if every room in Dubai Creek Tower (the world’s tallest structure) contained a book detailing the architect’s plans for the entire structure. The plans for your own book of life run to 46 chapters—23 donated by your mother’s egg and 23 by your father’s sperm. Each of these 46 chapters, called a chromosome, is composed of a coiled chain of the molecule DNA (deoxyribonucleic acid). Genes, small segments of the giant DNA molecules, form the words of those chapters (FIGURE 4.1). Altogether, you have some 20,000 genes, which are either active (expressed) or inactive. Environmental events “turn on” genes. When turned on, genes provide the code for creating protein molecules, our body’s building blocks. chromosomes threadlike structures made of DNA molecules that contain the genes. DNA (deoxyribonucleic acid) a complex molecule containing the genetic information that makes up the chromosomes. genes the biochemical units of heredity that make up the chromosomes; small segments of DNA capable of synthesizing proteins. FIGURE 4.1 The life code The nucleus of every human cell contains chromosomes, each of which is made up of two strands of DNA connected in a double helix. Genes are DNA segments that, when expressed (turned on), direct the development of proteins that influence a person’s individual development. Genetically speaking, every other human is nearly our identical twin. Human genome researchers have discovered a common sequence within human DNA. This shared genetic profile is what makes us humans, rather than tulips, bananas, or chimpanzees. genome the complete instructions for making an organism, consisting of all the genetic material in that organism’s chromosomes. “ Your DNA and mine are 99.9 percent the same…. At the DNA level, we are clearly all part of one big worldwide family.” — Francis Collins, Human Genome Project director, 2007 Yet we aren’t all that different from our chimpanzee cousins. At a genetic level, humans and chimpanzees are 96 percent identical (Mikkelsen et al., 2005). At “functionally important” DNA sites, this number reaches 99.4 percent (Wildman et al., 2003)! Yet that wee 0.6 percent difference matters. It took a human, Shakespeare, to do what a chimpanzee cannot—intricately weave 17,677 words into literary masterpieces. “ We share half our genes with the banana.” — Evolutionary biologist Robert May, president of Britain’s Royal Society, 2001 Small differences matter among other species, too. Common chimpanzees and bonobos resemble each other in many ways. They should—their genomes differ by much less than 1 percent. But they display markedly differing behaviors. Chimpanzees are aggressive and male dominated. Bonobos are peaceable and female led. The occasional variations found at particular gene sites in human DNA fascinate geneticists and psychologists. Slight person-to-person variations from the common pattern give clues to our uniqueness—why one person has a disability that another does not, why one person is tall and another short, why one is anxious and another calm. Should people use newly available gene-editing technologies to reduce their unborn children’s risk of disease? Only 26 percent of Americans approve (Scheufele et al., 2017). Most of our traits have complex genetic roots. How tall you are, for example, reflects the size of your face, vertebrae, leg bones, and so forth—each of which may be influenced by different genes interacting with your specific environment. Traits such as intelligence, happiness, and aggressiveness are each similarly influenced by a whole orchestra of genes (Holden, 2008). Indeed, one of the big take-home findings of today’s behavior genetics is that there is no single gene that predicts your smarts, sexual orientation, or personality. And gene analyses of more than 800,000 people have identified 269 genes associated with depression (Howard et al., 2019). Another study of 1.1 million people identified 1271 gene variations that together predicted about 12 percent of the differences in people’s years of schooling (Lee et al., 2018). The bottom line: Our differing traits are polygenetic—they are influenced by “many genes of small effect” (Lee et al., 2018; Matoba et al., 2019; Plomin, 2018). So, our many genes help explain both our shared human nature and our human diversity. But—another take-home finding—knowing our heredity tells only part of our story. To form us, environmental influences interact with our genetic predispositions. Nature or nurture or both? When talent runs in families, as with Wynton Marsalis, Branford Marsalis, and Delfeayo Marsalis, how do heredity and environment together do their work? RETRIEVAL PRACTICE RP-1 Put the following cell structures in order from smallest to largest: nucleus, gene, chromosome. ANSWERS IN APPENDIX E Twin and Adoption Studies  LOQ 4-2 How do twin and adoption studies help us understand the effects and interactions of nature and nurture? To scientifically tease apart the influences of heredity and environment, behavior geneticists could wish for two types of experiments. The first would control heredity while varying the home environment. The second would control the home environment while varying heredity. Such experiments with human infants would be unethical, but nature has done this work for us. See the Video: Twin Studies for a helpful tutorial animation. Identical Versus Fraternal Twins Identical (monozygotic) twins develop from a single fertilized egg that splits. Thus they are genetically identical—nature’s own human clones (FIGURE 4.2). Indeed, they are clones who share not only the same genes but the same conception and uterus, and usually the same birth date and cultural history. Two slight qualifications: Although identical twins have the same genes, they don’t always have the same number of copies of those genes repeated within their genome, and they sometimes differ in their brain’s tiny wiring structures. These variations help explain why one twin may have a greater risk for certain illnesses and disorders, including schizophrenia (Lee et al., 2019; Maiti et al., 2011). During prenatal development, most identical twins share a placenta (transfers nutrients and oxygen from mother to embryo), but one of every three sets has separate placentas. One twin’s placenta may provide slightly better nourishment, which may contribute to a few identical twin differences (Marceau et al., 2016; van Beijsterveldt et al., 2016). identical (monozygotic) twins individuals who developed from a single fertilized egg that split in two, creating two genetically identical organisms.  FIGURE 4.2 Same fertilized egg, same genes; different eggs, different genes Identical twins develop from a single fertilized egg, fraternal twins from two. Fraternal (dizygotic) twins develop from two separate fertilized eggs. Although they share a prenatal environment, they are genetically no more similar than ordinary siblings. fraternal (dizygotic) twins individuals who developed from separate fertilized eggs. They are genetically no closer than ordinary siblings, but shared a prenatal environment. Shared genes can translate into shared experiences. A person whose identical twin has autism spectrum disorder, for example, has about a 3 in 4 risk of being similarly diagnosed. If the affected twin is fraternal, the co-twin has about a 1 in 3 risk (Tick et al., 2016). To study the effects of genes and environments, several thousand medical and psychological researchers have studied nearly 15 million identical and fraternal twin pairs (Polderman et al., 2015). Are genetically identical twins also behaviorally more similar than fraternal twins? Studies in 12 countries have found that, compared with fraternal twins, identical twins are much more alike in openness, conscientiousness, agreeableness, extraversion, and neuroticism (see Chapter 14) (Vukasovic´ & Bratko, 2015). As identical twins grow older, their behaviors remain similar (McGue & Christensen, 2013). Genes also influence many specific behaviors. For example, among marijuana users, identical twins begin at a more similar age than do fraternal twins (Minică et al., 2018). Identical twins, more than fraternal twins, look alike—so much so that most have difficulty distinguishing a flashed photo of their face from their co-twin’s face (Martini et al., 2015). So, do people’s responses to their looks account for their similarities? No. In a clever approach, researcher (and fraternal twin) Nancy Segal (2013; Segal et al., 2013) compared personality similarity between identical twins and unrelated look-alike pairs. Only the identical twins reported similar personalities. Other studies have shown that identical twins whose parents treated them alike (for example, dressing them identically) were not psychologically more alike than other identical twins (Kendler et al., 1994; Loehlin & Nichols, 1976). In explaining individual differences, genes matter. Do look-alikes act alike? Genetically unrelated look-alikes, called doppelgangers, tend not to have notably similar personalities (Segal, 2013). Amazingly, these two bearded, red-haired minor league baseball  pitchers also share the same unusual name—Brady Feigl—but are unrelated and grew up unaware of each other. Separated Twins Imagine the following science fiction experiment: A mad scientist, given two pairs of identical twins, swaps one in each pair. The resulting pairs are then raised in separate environments as if they were fraternal twins. Better yet, consider a true story (Dominus, 2015; Segal & Montoya, 2018): In 2015, William Velasco was working as a butcher in Bogotá, Colombia. One day, customer Laura Vega Garzón mistook him for a colleague, Jorge, who looked the same —matching high cheek bones, same smile, same walking style. Was it Jorge, pretending to be someone else? Confused, she returned to the butcher shop to show William a picture of his look-alike, Jorge. William laughed and didn’t take it seriously, but Laura later showed Jorge a photograph of William. “That’s me!” Jorge exclaimed. Scrolling through William’s social media photos, Jorge continued to see mirror images of himself. He saw something else that surprised him—his look-alike, William, sitting next to Jorge’s fraternal twin brother (at least that’s who it looked like to Jorge). Until then, William and Jorge had lived utterly separate lives. William grew up in a rural village, whereas Jorge was city-raised. William and Jorge both—thanks to a colossal hospital mistake—believed they had fraternal twin brothers, Wilber and Carlos, respectively. In reality, Wilber and Carlos, like William and Jorge, were identical twins born in 1988. The hospital had sent William home with Wilber, and Carlos home with Jorge. Although they were raised apart, William and Jorge both were jokesters, physically strong, and supportive. Wilber and Carlos were moody and serious, always organized, and prone to crying, and they had the same speech impediment. Each of them had wondered why he felt so different from his supposed fraternal twin. Meeting their identical twin revealed the power of genetics. Genes matter, but so does environment. Urban-dwellers Jorge and Carlos had better nutrition, and were taller than rural-raised William and Wilber. Wilber didn’t have access to the speech therapy that Carlos did, which meant that only Wilber struggled with speaking as an adult. The remarkable story of the “Bogotá brothers” (see tinyurl.com/BogotaBrothers for more) resembles that of many separated twin pairs tested by psychologists Thomas Bouchard and Nancy Segal. These studies find similarities not only of tastes and physical attributes but also of personality, abilities, attitudes, interests, and fears. Separated Jims In 1979, Jim Lewis awoke next to his second wife, for whom he often left love notes around the house. As he lay in bed, he thought about others he loved, including his son, James Alan, and his dog, Toy. Jim enjoyed building furniture in his basement woodworking shop, including a white bench circling a tree in his front yard, and he also liked driving his Chevy, watching stock-car racing, and drinking Miller Lite beer. Shockingly, there existed another Jim for whom all these things were also true!1 Thirty-seven days after their birth, Jim Lewis and Jim Springer, genetically identical twins, were separated, adopted, and raised with no contact or knowledge of each other until the day Jim Lewis received a call from his genetic clone (who, having been told he had a twin, set out to find him). These 39-year-old brothers became the first of 74 separated twin pairs tested by psychologist Thomas Bouchard and his colleagues (2009; Miller, 2012c). The brothers’ voice intonations and inflections were so similar that, hearing an earlier interview, Jim Springer guessed, “That’s me.” Wrong—it was Jim Lewis. Tests measuring their personality, intelligence, heart rate, and brain waves were virtually as alike as the same person tested twice. Stories of startling twin similarities have not impressed critics, who remind us that “the plural of anecdote is not data.” They note that if any two strangers were to spend hours comparing their behaviors and life histories, they would probably discover many coincidental similarities. If researchers created a control group of biologically unrelated pairs of the same age, sex, and ethnicity, who had not grown up together but who were as similar to one another in economic and cultural background as are many of the separated twin pairs, wouldn’t these pairs also exhibit striking similarities (Joseph, 2001)? Twin researchers have replied that separated fraternal twins do not exhibit similarities comparable to those of separated identical twins. Even the impressive data from personality assessments are clouded by the reunion of many of the separated twins some years before they were tested. When adoption agencies are involved, separated twins tend to be placed in similar homes. Despite these criticisms, the striking twin-study results helped shift scientific thinking toward a greater appreciation of genetic influences. Biological Versus Adoptive Relatives For behavior geneticists, nature’s second real-life experiment—adoption—creates two groups: genetic relatives (biological parents and siblings) and environmental relatives (adoptive parents and siblings). For personality or any other given trait, we can therefore ask whether adopted children are more like their biological parents, who contributed their genes, or their adoptive parents, who contribute a home environment. And while sharing that home environment, do adopted siblings come to share traits? The stunning finding from studies of hundreds of adoptive families is that, apart from identical twins, people who grow up together—whether biologically related or not—do not much resemble one another in personality (McGue & Bouchard, 1998; Plomin, 2011; Rowe, 1990). In personality traits such as extraversion and agreeableness, for example, people who have been adopted are more similar to their biological parents than their caregiving adoptive parents. The finding is important enough to bear repeating: The normal range of environments shared by a family’s children has little discernible impact on their personalities. Two adopted children raised in the same home are no more likely to share personality traits with each other than with the child down the block. Heredity shapes other primates’ personalities, too. Macaque monkeys raised by foster mothers exhibit social behaviors that resemble their biological, rather than foster, mothers (Maestripieri, 2003). Why are children in the same family so different? Why does shared family environment have so little effect on children’s personalities? Is it because each sibling experiences unique peer influences and life events? Because sibling relationships ricochet off each other, amplifying their differences? Because siblings—despite sharing half their genes —have very different combinations of genes and may evoke very different kinds of parenting? Such questions fuel behavior geneticists’ curiosity. Siblings so different: Hermann Goering was outgoing, loved crowds, and became Hitler’s right-hand man and founder of the Nazi Gestapo. His younger brother Albert Goering was quiet and reclusive, and worked to save the Jews that brother Hermann’s regime was killing (Brennan, 2010). The genetic leash may limit the family environment’s influence on personality, but it does not mean that adoptive parenting is a fruitless venture. One study followed more than 4000 Swedish children of parents who had criminal convictions or had substance use problems. Compared with their not-adopted siblings, those raised by an adoptive family were 44 percent less likely to be convicted of a crime (Kendler et al., 2016). As an adoptive parent, I [ND] find it heartening to know that parents do influence their children’s attitudes, values, manners, politics, education, and faith (Gould et al., 2019; Kandler & Riemann, 2013). This was dramatically illustrated during World War II by separated identical twins Jack Yufe, a Jew, and Oskar Stöhr, a member of Germany’s Hitler Youth. After later reuniting, Oskar mused to Jack: “If we had been switched, I would have been the Jew, and you would have been the Nazi” (Segal, 2005, p. 70). Parenting—and the cultural environments in which parents place children—matters! Moreover, child neglect and abuse and even parental divorce are rare in adoptive homes. (Adoptive parents are carefully screened; biological parents are not.) One study looked at the parenting of siblings being raised apart—some with their biological mother, some with an adoptive mother (Natsuaki et al., 2019). Compared with the biological mothers, the adoptive mothers used gentler parenting, gave more guidance, and experienced less depression. So it is not surprising that, despite a somewhat greater risk of psychological disorder, most adopted children thrive, especially when adopted as infants (Loehlin et al., 2007; van IJzendoorn & Juffer, 2006; Wierzbicki, 1993). Seven in eight adopted children have reported feeling strongly attached to one or both adoptive parents. As children of self-giving parents, they have grown up to be more self-giving and altruistic than average (Sharma et al., 1998). Many scored higher than their biological parents and raised-apart biological siblings on intelligence tests, and most grew into happier and more stable adults (Kendler et al., 2015b; van IJzendoorn et al., 2005). In one Swedish study, children adopted as infants grew up with fewer problems than were experienced by children whose biological mothers initially registered them for adoption but then decided to raise the children themselves (Bohman & Sigvardsson, 1990). The bottom line: Most adopted children benefit from adoption. Adoption matters Country music singer Faith Hill and Olympic gold medal gymnast Simone Biles both benefited from one of the biggest gifts of love: adoption. ASK YOURSELF Do you know biological siblings who, despite having been raised together, have very different personalities? (Are you one of these siblings perhaps?) Knowing what you do of their lives and upbringing, what do you think contributed to these differences? RETRIEVAL PRACTICE  RP-2 How do researchers use twin and adoption studies to learn about psychological principles? ANSWERS IN APPENDIX E Temperament and Heredity LOQ 4-3 What have psychologists learned about temperament? As most parents with multiple children report, babies differ right out of the womb (Willoughby et al., 2019). One aspect of personality— temperament (emotional reactivity and intensity)—is quickly apparent, and it is genetically influenced (Kandler et al., 2013; Raby et al., 2012). Identical twins, more than fraternal twins, often have similar temperaments (Fraley & Tancredy, 2012; Kandler et al., 2013). Temperament differences typically persist. The most emotionally reactive newborns tend also to be the most reactive 9-month-olds (Wilson & Matheny, 1986; Worobey & Blajda, 1989). Emotionally intense preschoolers tend to become relatively intense young adults (Larsen & Diener, 1987). One study of 1037 New Zealanders found that a 45-minute assessment of 3-year-olds’ frustration tolerance, impulsivity, and intelligence could predict “with considerable accuracy” which of them would, by age 38, consume the most welfare benefits, parent then abandon the most children, and commit the most crime (Caspi et al., 2016). temperament a person’s characteristic emotional reactivity and intensity. The genetic effect appears in physiological differences. Anxious, inhibited infants have high and variable heart rates and a reactive nervous system. When facing new or strange situations, they become more physiologically aroused (Kagan & Snidman, 2004; Roque et al., 2012). Play the role of a researcher studying temperament and personality by engaging online with the activity How Would You Know If Personality Runs in Our Genes? Heritability LOQ 4-4 What is heritability, and how does it relate to individuals and groups? So, our biology helps form our personality. Yet asking whether our personality is more a product of our genes or our environment is like asking whether a basketball court’s size is more the result of its length or its width. We could, however, ask whether different court sizes are more the result of differences in their length or differences in their width. Similarly, we can ask whether person-to-person personality differences are influenced more by nature or by nurture. Using twin and adoption studies, behavior geneticists can mathematically estimate the heritability of a trait—the extent to which variation among individuals in a group can be attributed to their differing genes. For many personality traits, heritability is about 40 percent; for major depressive disorder, heritability is about 30 percent (Haworth et al., 2010; Pettersson et al., 2019). Everyday estimates for the heritability of intelligence range from 50–80 percent (Madison et al., 2016; Plomin et al., 2016; Plomin & von Stumm, 2018). This does not mean that your intelligence is 50–80 percent genetic. Rather, it means that genetic influence explains between 50–80 percent of the observed variation among people. We can never say what percentage of an individual’s personality or intelligence is inherited. It makes no sense to say that your personality is due x percent to your heredity and y percent to your environment. This point is so often misunderstood that we repeat: Heritability refers to how much differences among people are due to genes. heritability the proportion of variation among individuals in a group that we can attribute to genes. The heritability of a trait may vary, depending on the range of populations and environments studied. The heritability of traits such as intelligence varies from study to study. Consider humorist Mark Twain’s (1835–1910) fictional idea of raising boys in barrels to age 12, feeding them through a hole. If we were to follow his suggestion, the boys would all emerge with lower-than-normal intelligence scores at age 12. Yet given their equal environments, their test score differences could be explained only by their heredity. With the same environment, heritability—differences due to genes—would be near 100 percent. As environments become more similar, heredity becomes the primary source of differences. If all schools were of uniform quality, all families equally loving, and all neighborhoods equally healthy, then heritability would increase (because differences due to environment would decrease). But consider the other extreme: If all people had similar heredities but were raised in drastically different environments (some in barrels, some in luxury homes), heritability would be much lower. So, heritability is not a single fixed score; it varies with changing environments. If genetic influences help explain variations in traits among individuals in a group, can the same be said of trait differences between groups? Not necessarily. Height is 90 percent heritable, yet nutrition (an environmental factor) rather than genes explains why, as a group, today’s adults are taller (Floud et al., 2011). In 1896, the global average height for men was 5 feet 3 inches (162 centimeters) and for women was 4 feet 10 inches (151 centimeters) (Our World in Data, 2019). In the 1990s, their male and female counterparts stood 3.5 inches (9 centimeters) taller. The two groups differ, but not because human genes have changed in this eyeblink of time. South Korean men and women, with their better diets, now average 6 inches (15 centimeters) taller than genetically similar North Koreans (Johnson et al., 2009). Genes matter, but so does environment. As with height, so with personality and intelligence scores: Heritable individual differences need not imply heritable group differences. And, if some individuals are genetically disposed to be more aggressive than others, that needn’t explain why some groups are more aggressive than others. Putting people in a new social context can change their aggressiveness. Today’s peaceful Scandinavians carry many genes inherited from their Viking warrior ancestors. RETRIEVAL PRACTICE RP-3 Those studying the heritability of a trait try to determine how much of the person-to-person variation in that trait among members of a specific group is due to their differing. ANSWERS IN APPENDIX E For a 7-minute explanation of genes and environment, watch the Video: Behavior Genetics. Gene–Environment Interaction Among our similarities, the most important—the behavioral hallmark of our species—is our enormous adaptive capacity. Some human traits develop the same way in virtually every environment. But other traits are expressed only in particular environments. Go barefoot for a summer and you will develop toughened, callused feet—a biological adaptation to friction. Meanwhile, your shod neighbor will remain a tenderfoot. The difference between the two of you is an effect of environment. But it is the product of a biological mechanism—adaptation. Our shared biology enables our developed diversity (Buss, 1991). Thus, to say that genes and experience are both important is true. But more precisely, they interact. Just how our genes and our experiences interact to form us as unique individuals is one of the hottest topics in psychology today. Gene– environment interaction studies are revealing, for example, who is most at risk of permanent harm from stress or abuse and who is most likely to benefit from interventions (Byrd et al., 2019; Manuck & McCaffery, 2014). The National Institutes of Health All of Us research program is now studying 1 million people to pinpoint precisely how their genes and environment together predict physical and mental health (NIH, 2019). interaction the interplay that occurs when the effect of one factor (such as environment) depends on another factor (such as heredity). Genetic space exploration In 2015, Scott Kelly (left) spent 340 days orbiting the planet in the International Space Station. His identical twin, Mark Kelly (right), remained on Earth. Both twins underwent the same physical and psychological testing (Garrett-Bakelman et al., 2019). Compared with his Earth-bound brother, Scott Kelly’s immune system temporarily went into overdrive, possibly due to the stresses of living in space and exposure to greater-than-average levels of radiation. The study results are helping scientists understand how genes and environment—in outer space and on Earth—interact. Molecular Behavior Genetics LOQ 4-5 How is molecular genetics research changing our understanding of the effects of nature and nurture? Behavior geneticists have progressed well beyond asking “Do genes influence behavior?” The newest frontier of behavior-genetic research draws on “bottom-up” molecular genetics, which studies the molecular structure and function of genes. molecular genetics the subfield of biology that studies the molecular structure and function of genes. SEARCHING FOR SPECIFIC GENES INFLUENCING BEHAVIOR As we’ve seen, genes typically are not solo players. For example, twin and adoption studies have revealed no single “obesity gene.” Some genes influence how quickly the stomach tells the brain, “I’m full” (Adetunji, 2014). Others might dictate how much fuel the muscles need, how many calories are burned off by fidgeting, and how efficiently the body converts extra calories into fat. So, one goal of molecular behavior genetics is to find some of the many genes that together orchestrate complex traits such as body weight, sexual orientation, and impulsivity. molecular behavior genetics the study of how the structure and function of genes interact with our environment to influence behavior. EPIGENETICS: TRIGGERS THAT SWITCH GENES ON AND OFF Genes can be either active (expressed, as hot water activates a tea bag) or inactive. Epigenetics (meaning “in addition to” or “above and beyond” genetics) studies the molecular mechanisms by which environments can trigger or block genetic expression. Genes are self-regulating. Rather than acting as blueprints that lead to the same result no matter the context, genes react. An African butterfly that is green in summer turns brown in fall, thanks to a temperature-controlled genetic switch. The same genes that produce green in one situation produce brown in another. epigenetics “above” or “in addition to” (epi) genetics; the study of the molecular mechanisms by which environments can influence gene expression (without a DNA change). Our experiences also create epigenetic marks, which are often organic methyl molecules attached to part of a DNA strand (FIGURE 4.3). If a mark instructs the cell to ignore any gene present in that DNA segment, those genes will be “turned off”—they will prevent the DNA from producing the proteins normally coded by that gene. As one geneticist explained, “Things written in pen you can’t change. That’s DNA. Things written in pencil you can. That’s epigenetics” (Reed, 2012). FIGURE 4.3 Epigenetic expression Beginning in the womb, life experiences lay down epigenetic marks— often organic methyl molecules—that can influence the expression of any gene in the DNA segment they  affect. (Research from Champagne, 2010.) Environmental factors such as diet, drugs, and stress can affect the epigenetic molecules that regulate gene expression. Mother rats normally lick their infants. In experiments, infant rats deprived of this licking had more epigenetic molecules blocking access to their brain’s “on” switch for developing stress hormone receptors. When stressed, those animals had above-average levels of free-floating stress hormones and were more stressed (Champagne et al., 2003; Champagne & Mashoodh, 2009). Epigenetics provides a possible mechanism by which the effects of childhood trauma, poverty, or malnutrition may last a lifetime (Nugent et al., 2016; Peter et al., 2016; Swartz et al., 2016). Such experiences may leave their fingerprints in a person’s genome. Some epigenetic changes may even get passed down to future generations. Studies of Holocaust trauma survivors and former U.S. Civil War prisoners of war showed shared epigenetic alterations with their offspring (Costa et al., 2018; Yehuda et al., 2016). Some critics question these findings’ reliability (Horsthemke, 2018; Yasmin, 2017). Stay tuned: This scientific story is still being written. Lasting effects Canadian Senator  Murray Sinclair received a humanitarian award from the Canadian Psychological Association for an in-depth report on the devastating effects of Canada’s long running residential school program that removed Indigenous children from their families. Psychologist Susan Pinker (2015) observed that the epigenetic effects of forced family separation “can play out, not only in the survivors of residential schools but in subsequent generations.” Epigenetics research may solve some scientific mysteries, such as why only one member of an identical twin pair may develop a genetically influenced mental disorder (Spector, 2012). Epigenetics can also help explain why identical twins may look slightly different. Researchers studying mice have found that in utero exposure to certain chemicals can cause genetically identical twins to have different-colored fur (Dolinoy et al., 2007). RETRIEVAL PRACTICE RP-4 Match the following terms (i–iii) to the correct explanation (a–c). i. Epigenetics a. Study of the relative effects of our genes and our environment on our behavior. ii. Molecular b. Study of how the structure and function of specific genes behavior interact with our environment to influence behavior. genetics iii. Behavior c. Study of environmental factors that affect how our genes are genetics expressed. ANSWERS IN APPENDIX E REVIEW Behavior Genetics: Predicting  Individual Differences LEARNING OBJECTIVES Test Yourself Answer these repeated Learning Objective Questions on your own (before checking the answers in Appendix D) to improve your retention of the concepts (McDaniel et al., 2009, 2015). LOQ 4-1: What are chromosomes, DNA, genes, and the human genome? How do behavior geneticists explain our individual differences? LOQ 4-2: How do twin and adoption studies help us understand the effects and interactions of nature and nurture? LOQ 4-3: What have psychologists learned about temperament? LOQ 4-4: What is heritability, and how does it relate to individuals and groups? LOQ 4-5: How is molecular genetics research changing our understanding of the effects of nature and nurture? TERMS AND CONCEPTS TO REMEMBER Test Yourself Write down the definition in your own words, then check your answer. behavior genetics heredity environment chromosomes DNA (deoxyribonucleic acid) genes genome identical (monozygotic) twins fraternal (dizygotic) twins temperament heritability interaction molecular genetics molecular behavior genetics epigenetics MASTER THE MATERIAL Test Yourself Answer the following questions on your own first, then check your answers in Appendix E. 1. The threadlike structures made largely of DNA molecules are called. 2. A small segment of DNA that codes for particular proteins is referred to as a. 3. When the mother’s egg and the father’s sperm unite, each contributes a. one chromosome pair. b. 23 chromosomes. c. 23 chromosome pairs. d. 25,000 chromosomes. 4. Fraternal twins result when a. a single egg is fertilized by a single sperm and then splits. b. a single egg is fertilized by two sperm and then splits. c. two eggs are fertilized by two sperm. d. two eggs are fertilized by a single sperm. 5. twins share the same DNA.  6. Adoption studies seek to understand genetic influences on personality. They do this mainly by a. comparing adopted children with nonadopted children. b. evaluating whether adopted children’s personalities more closely resemble those of their adoptive parents or their biological parents. c. studying the effect of prior neglect on adopted children. d. studying the effect of children’s age at adoption. 7. From the very first weeks of life, infants differ in their characteristic emotional reactions, with some infants being intense and anxious, while others are easygoing and relaxed. These differences are usually explained as differences in. 8. is the proportion of variation among individuals in groups that we can attribute to genes. 9. Epigenetics is the study of the molecular mechanisms by which trigger or block genetic expression. Continue testing yourself with LearningCurve or Achieve Read & Practice to learn and remember most effectively.  Evolutionary Psychology: Explaining Human Nature and Nurture LOQ 4-6 How do evolutionary psychologists use natural selection to explain behavior tendencies? Behavior geneticists explore the genetic and environmental roots of human differences. Evolutionary psychologists instead focus mostly on what makes us so much alike as humans. They use Charles Darwin’s principle of natural selection —“arguably the most momentous idea ever to occur to a human mind,” said Richard Dawkins (2007)—to understand the roots of behavior and mental processes. The idea, simplified, is this: Organisms’ varied offspring compete for survival. Certain biological and behavioral variations increase organisms’ reproductive and survival chances in their particular environment. Offspring that survive are more likely to pass their genes to ensuing generations. Thus, over time, population characteristics may change. To see these principles at work, let’s consider a straightforward example in foxes. evolutionary psychology the study of the evolution of behavior and the mind, using principles of natural selection. natural selection the principle that inherited traits that better enable an organism to survive and reproduce in a particular environment will (in competition with other trait variations) most likely be passed on to succeeding generations. Natural Selection and Adaptation A fox is a wild and wary animal. If you capture a fox and try to befriend it, be careful: If the timid fox cannot flee, it may snack on your fingers. In the early 1950s, Russian scientist Dmitry Belyaev wondered how our human ancestors had domesticated dogs from their equally wild wolf forebears. Might he, within a comparatively short stretch of time, accomplish a similar feat by transforming the fearful fox into a friendly fox? How to tame a fox Over six decades, Lyudmila Trut has genetically bred silver foxes into friendly human companions. To find out, Belyaev set to work with 100 female and 30 male foxes selected from fox farms (where some domestication would have occurred through routine feeding and confinement [Gorman, 2019]). From their offspring he selected and mated the tamest 20 percent of females and 5 percent of males. (He measured tameness by the foxes’ responses to attempts to feed, handle, and stroke them.) Over 57 generations of foxes, Belyaev and his successor, Lyudmila Trut, have repeated that simple procedure (Dugatkin & Trut, 2017). After forty years and 45,000 foxes, they had a new breed of foxes that, in Trut’s (1999) words, were “docile, eager to please, and unmistakably domesticated…. Before our eyes, ‘the Beast’ has turned into ‘beauty,’ as the aggressive behavior of our herd’s wild [ancestors] entirely disappeared.” A transformation had occurred. So friendly and eager for human contact were these animals, so inclined to whimper to attract attention and to lick people like affectionate dogs, that the cash- strapped institute seized on a way to raise funds—marketing its friendly foxes as house pets. Does the same process work with naturally occurring selection? Does natural selection explain our human tendencies? Nature has indeed selected advantageous variations from the new gene combinations produced at each human conception plus the mutations (random errors in gene replication) that sometimes result. But the tight genetic leash that predisposes a dog’s retrieving, a cat’s pouncing, or a bird’s nesting is looser on humans. The genes selected during our ancestral history provide more than a long leash; they give us a great capacity to learn and therefore to adapt to life in varied environments, from the tundra to the jungle. Genes and experience together wire the brain. Our adaptive flexibility in responding to different environments contributes to our fitness—our ability to survive and reproduce. mutation a random error in gene replication that leads to a change. RETRIEVAL PRACTICE RP-1 How are Belyaev and Trut’s breeding practices similar to, and how do they differ from, the way natural selection normally occurs? ANSWERS IN APPENDIX E Evolutionary Success Helps Explain Similarities Human differences grab our attention. Guinness World Records entertain us with the tallest, oldest, longest-haired, and most-tattooed humans. But our deep similarities also demand explanation. At the Amsterdam Airport’s international arrivals area, one sees the same delighted joy on the faces of Indonesian grandmothers, Chinese children, and homecoming Dutch. “Even at a time when everything seems to be changing,” notes physician-sociologist Nicholas Christakis (2019), “fundamental ways that humans live together remain constant.” Our Genetic Legacy Our similarities arise from our shared human genome, our common genetic profile. No more than 5 percent of the genetic differences among humans arise from population group differences. Some 95 percent of genetic variation exists within populations (Rosenberg et al., 2002). The typical genetic difference between two South Africans or between two Singaporeans is much greater than the average difference between the two groups. Thus, if after a worldwide catastrophe only South Africans or Singaporeans survived, the human species would suffer only “a trivial reduction” in its genetic diversity (Lewontin, 1982). And how did we develop this shared human genome? At the dawn of human history, our ancestors faced certain questions: Who is my ally, who is my foe? With whom should I mate? What food should I eat? Some individuals answered those questions more successfully than others. For example, women who experienced nausea in the critical first three months of pregnancy were genetically predisposed to avoid certain bitter, strongly flavored, and novel foods. Avoiding such foods had survival value, since they are the very foods most often toxic to prenatal development (Profet, 1992; Schmitt & Pilcher, 2004). Early humans disposed to eat nourishing rather than poisonous foods survived to contribute their genes to later generations. Those who deemed leopards “nice to pet” often did not. Similarly successful were those whose mating helped them produce and nurture offspring. Over generations, the genes of individuals not disposed to mate or nurture tended to be lost from the human gene pool. As success-enhancing genes continued to be selected, behavioral tendencies and learning capacities emerged that prepared our Stone Age ancestors to survive, reproduce, and send their genes into the future—and into you. Consider yourself fortunate: Despite high infant mortality and rampant disease in past millennia, not one of your countless ancestors died childless. Across our cultural differences, we even share a “universal moral grammar” (Mikhail, 2007). Men and women, young and old, liberal and conservative, living in Sydney or Seoul, all respond negatively when asked, “If a lethal gas is leaking into a vent and is headed toward a room with seven people, is it okay to push someone into the vent— saving the seven but killing the one?” And they all respond more approvingly when asked if it’s okay to allow someone to fall into the vent, again sacrificing one life but saving seven. Our shared moral instincts survive from a distant past where we lived in small groups in which direct harm-doing was punished. For all such universal human tendencies, from our intense need to give parental care to our shared fears and lusts, evolutionary theory proposes a one-stop-shopping explanation (Schloss, 2009). As heirs to this prehistoric legacy, we are genetically predisposed to think and act in ways that promoted our biological ancestors’ surviving and reproducing. On campus, we easily remember survival-related information (“In case of fire, use the stairs”) and pay close attention to those we find romantically desirable (Nairne et al., 2019; Nakamura et al., 2017). But in some ways, we are biologically prepared for a world that no longer exists. We love the taste of sweets and fats, nutrients that prepared our physically active ancestors to survive food shortages. Few of us now gather and hunt for our food; instead, we too readily find sweets and fats in fast-food outlets and vending machines. Our deeply rooted natural dispositions are mismatched with today’s junk-food and often inactive lifestyle. Evolutionary Psychology Today Darwin’s theory of evolution has become one of biology’s fundamental organizing principles and lives on in the second Darwinian revolution: the application of evolutionary principles to psychology. In concluding On the Origin of Species, Darwin (1859, p. 346) anticipated this, foreseeing “open fields for far more important researches. Psychology will be based on a new foundation.” Those who are troubled by an apparent conflict between scientific and religious accounts of human origins may find it helpful to consider that different perspectives on life can be complementary. For example, the scientific account attempts to tell us when and how; religious creation stories usually aim to tell about an ultimate who and why. As Galileo explained to the Grand Duchess Christina, “The Bible teaches how to go to heaven, not how the heavens go.” Elsewhere in this text, we address questions that intrigue evolutionary psychologists: Why do infants start to fear strangers about the time they become mobile? Why do more people develop a specific phobia in response to spiders, snakes, and heights than to modern threats such as guns? And why do we fear air travel more than driving (which is riskier)? To see how evolutionary psychologists think and reason, let’s pause to explore their answers to two questions: How are males and females alike? How and why does their sexuality differ? An Evolutionary Explanation of Human Sexuality LOQ 4-7 How might an evolutionary psychologist explain male-female differences in sexuality and mating preferences? Having faced many similar challenges throughout history, all genders have adapted in similar ways: We eat the same foods, avoid the same dangers, and perceive, learn, and remember similarly. When looking for a mate, we all prize many of the same traits— kindness, honesty, and intelligence—and we all avoid our close genetic relatives (Dandine-Roulland et al., 2019). It is only in those domains where we have faced differing adaptive challenges—most obviously in behaviors related to reproduction— that we differ, say evolutionary psychologists. Male-Female Differences in Sexuality And differ we do. Consider sex drives. Men and women are sexually motivated, some women more so than many men. Yet on average, who thinks more about sex? Hooks up more often? Masturbates more often? Views more pornography? The answers worldwide: men, men, men, and men (Baumeister et al., 2001; Hall et al., 2017; Lippa, 2009; Petersen & Hyde, 2010). Even among 65- to 80-year-old Americans, 12 percent of women and 50 percent of men reported being “very” or “extremely” interested in sex (Malani et al., 2018). Unattached heterosexual men are alert for women’s interest, and often misperceive a woman’s friendliness as a sexual come-on (Abbey, 1987). This sexual overperception bias was evident in speed-dating men, who believed their dating partners expressed more sexual interest than the partners reported actually expressing (Perilloux et al., 2012). (This may be partially explained by unattached heterosexual women underreporting their own sexual interests [Engeler & Raghubir, 2018]). Long-term couples do better at gauging their partners’ sexual interest (Dobson et al., 2018). Many gender similarities and differences transcend sexual orientation. Compared with gay women, gay men (like straight men) report more responsiveness to visual sexual stimuli, and more concern with their partner’s physical attractiveness (Bailey et al., 1994; Doyle, 2005; Schmitt, 2007; Sprecher et al., 2013). Gay male couples also report having sex more often than do gay female couples (Peplau & Fingerhut, 2007). And (also like straight men) gay men report more interest in uncommitted sex (Schmitt, 2003). To listen to experts discuss evolutionary psychology and sex differences, see the 4-minute Video: Evolutionary Psychology and Sex Differences. Natural Selection and Mating Preferences Natural selection is nature selecting traits and appetites that contribute to survival and reproduction. Over time, traits that give an individual or a species a reproductive advantage are selected and will prevail. Evolutionary psychologists use the principle of sex selection to explain how females and males tend to differ in mating-related domains, whether they’re chimpanzees or elephants, rural peasants or corporate presidents (Buss & Schmitt, 2019; Geary, 2010). Our natural yearnings, they say, are our genes’ way of reproducing themselves. Why do women tend to be choosier than men when selecting sexual partners? Women have more at stake. To send her genes into the future, a woman must—at a minimum— conceive and protect a fetus growing inside her body for up to 9 months, and may nurse for months following birth. No surprise then, that heterosexual women prefer partners who will offer their joint offspring support and protection—stick-around dads over likely cads (Meeussen et al., 2019). Straight women are attracted to tall men with slim waists and broad shoulders—all signs of reproductive success (Mautz et al., 2013). And straight American, Chinese, and European women overwhelmingly prefer men who seem affluent (Wang et al., 2018). One study of hundreds of Welsh pedestrians asked people to rate a driver pictured at the wheel of a humble Ford Fiesta or a swanky Bentley. Men said a female driver was equally attractive in both cars. Women, however, found a male driver more attractive if he was in the luxury car (Dunn & Searle, 2010). Similarly, when people viewed pictures of others in luxury or standard apartments, women found men more attractive when they appeared in the luxury apartment, but men’s perceptions were not affected by the backdrop (Dunn & Hill, 2014). The data are in, say evolutionists: Men pair widely; women pair wisely. And what traits do straight men find desirable? Some, such as smooth skin and a youthful shape, cross place and time, and they convey health and fertility (Buss & Von Hippel, 2018). Mating with such women might increase a man’s chances of sending his genes into the future. And sure enough, men feel most attracted to women whose waist is roughly a third narrower than their hips—a likely sign she has not yet had children (Lassek & Gaulin, 2018, 2019; Lewis et al., 2015). Even blind men show this preference for women with a low waist-to-hip ratio (Karremans et al., 2010). Men are most attracted to women whose ages in the ancestral past (when ovulation began later than today) would be associated with peak fertility (Kenrick et al., 2009). Thus, teen boys are most excited by women several years older than themselves, mid-twenties men prefer women around their own age, and older men prefer younger women. This pattern consistently appears across European singles ads, Indian marital ads, and marriage records from North and South America, Africa, and the Philippines (Singh, 1993; Singh & Randall, 2007). Nature selects behaviors that increase genetic success. As mobile gene machines, say evolutionary psychologists, we are designed to prefer whatever worked for our ancestors in their environments. Had they not been genetically predisposed to act in ways that would produce children, we wouldn’t be here. As carriers of their genetic legacy, we are similarly predisposed. Critiquing the Evolutionary Perspective LOQ 4-8 What are the key criticisms of evolutionary explanations of human sexuality, and how do evolutionary psychologists respond? Most psychologists agree that natural selection prepares us for survival and reproduction. But critics say there is a weakness in evolutionary psychology’s explanation of our mating preferences. Let’s consider how an evolutionary psychologist might explain the findings in a startling, classic study of heterosexual women and men (Clark & Hatfield, 1989), and how a critic might object. In this experiment, a woman or man posing as a stranger approached men or women and remarked, “I have been noticing you around campus. I find you to be very attractive.” The “stranger” then asked a question, which was sometimes “Would you go to bed with me tonight?” What percentage of women and men do you think agreed? An evolutionary explanation of sexuality would predict that women would be choosier than men in selecting their sexual partners. Indeed, not a single woman agreed—but 70 percent of the men did. A French replication of this study produced a similar result (Guéguen, 2011). Did the research support evolutionary psychology? Critics note that evolutionary psychologists start with an effect—in this case, the survey result showing that men were more likely to accept casual sex offers—and work backward to explain what happened. What if research showed the opposite effect? If men refused an offer for casual sex, might we not reason that men who partner with one woman for life make better fathers, whose children more often survive? Other critics ask why we should try to explain today’s behavior based on decisions our distant ancestors made thousands of years ago. Don’t cultural expectations also bend the genders? Alice Eagly and Wendy Wood (1999, 2013) point to the smaller behavioral differences between women and men in cultures with greater gender equality. Such critics believe that social learning theory offers a better, more immediate explanation. We all learn social scripts—our culture’s guide to how people should act in certain situations. By watching and imitating others in their culture, women may learn that sexual encounters with strange men can be dangerous, that casual sex may not offer much sexual pleasure, and that women (more than men) who engage in casual sex face reputational harm (Conley, 2011; Muggleton et al., 2019). This alternative explanation suggests that women are reacting to sexual encounters in socially scripted ways. And men’s reactions may reflect their learned social scripts: “Real men” take advantage of every opportunity to have sex. social script a culturally modeled guide for how to act in various situations. A third criticism focuses on evolutionary psychology’s possible social effects. Are straight men truly hardwired to have sex with any woman who approaches them? If so, does it mean that men have no moral responsibility to remain faithful to their partners? Does this explanation excuse inappropriate sexual behavior as men’s evolutionary legacy? Might evolutionary psychology weaken social movements aimed at reducing sexual aggression, such as #MeToo? Evolutionary psychologists agree that much of who we are is not hardwired. “Evolution forcefully rejects a genetic determinism,” insisted one research team (Confer et al., 2010). Genes are not destiny. And evolutionary psychology research confirms that men and women, having faced similar adaptive problems, are far more alike than different. Natural selection has prepared us to be flexible. We humans have a great capacity for learning and social progress. We adjust and respond to varied environments. We adapt and survive, whether we live in the arctic or the desert. “It is dangerous to show a man too clearly how much he resembles the beast, without at the same time showing him his greatness. It is also dangerous to allow him too clear a vision of his greatness without his baseness. It is even more dangerous to leave him in ignorance of both.” — Blaise Pascal, Pensées, 1659 Evolutionary psychologists also agree with their critics that some traits and behaviors, such as suicide, are hard to explain in terms of natural selection (Barash, 2012; Confer et al., 2010). But they ask us to remember evolutionary psychology’s scientific goal: to explain behaviors and mental traits by offering testable predictions using principles of natural selection (Lewis et al., 2017). We may, for example, predict that people are more likely to perform favors for those who share their genes or can later return those favors. Is this true? (The answer is Yes.) And evolutionary psychologists remind us that studying how we came to be need not dictate how we ought to be. Understanding our tendencies can help us overcome them. ASK YOURSELF Based on what you’ve learned so far, how would you say that genes and environment work together to influence sexual behavior? RETRIEVAL PRACTICE RP-2 How do evolutionary psychologists explain male-female differences in sexuality? RP-3 What are the three main criticisms of the evolutionary explanation of human sexuality? ANSWERS IN APPENDIX E To observe an experiment showing men’s and women’s attitudes toward casual sex, see the Video: Openness to Casual Sex—A Study of Men Versus Women. And for an interactive demonstration of evolutionary psychology and mating preferences, visit Topic Tutorial: PsychSim6, Dating and Mating. REVIEW Evolutionary Psychology: Explaining Human Nature and Nurture LEARNING OBJECTIVES Test Yourself Answer these repeated Learning Objective Questions on your own (before checking the answers in Appendix D) to improve your retention of the concepts (McDaniel et al., 2009, 2015). LOQ 4-6: How do evolutionary psychologists use natural selection to explain behavior tendencies? LOQ 4-7: How might an evolutionary psychologist explain male-female differences in sexuality and mating preferences? LOQ 4-8: What are the key criticisms of evolutionary explanations of human sexuality, and how do evolutionary psychologists respond? TERMS AND CONCEPTS TO REMEMBER Test Yourself Write down the definition in your own words, then check your answer. evolutionary psychology natural selection mutation social script MASTER THE MATERIAL Test Yourself Answer the following questions on your own first, then check your answers in Appendix E. 1. Behavior geneticists are most interested in exploring (commonalities/differences) in our behaviors. Evolutionary psychologists are most interested in exploring (commonalities/differences). 2. Evolutionary psychologists are most likely to focus on a. how individuals differ from one another. b. the social consequences of learned behaviors. c. the natural selection of traits that helped our ancestors survive and  reproduce. d. social scripts. 3. How do evolutionary psychologists use the principle of natural selection to explain differences in mating preferences in women and men? Continue testing yourself with LearningCurve or Achieve Read & Practice to learn and remember most effectively.  Cultural and Gender Diversity: Understanding Nature and Nurture From conception onward, we are the product of a cascade of interactions between our genetic predispositions and our surrounding environments (McGue, 2010). Our genes affect how people react to and influence us. And they affect our family environment (Barlow, 2019; Kong et al., 2018). Forget nature versus nurture; think nature via nurture. Imagine two babies, one genetically predisposed to be attractive, sociable, and easygoing, the other less so. Assume further that the first baby attracts more affectionate and stimulating care and so develops into a warmer and more outgoing person. As the two children grow older, the more naturally outgoing child may seek more activities and friends that encourage further social confidence. What has caused their resulting personality differences? Neither heredity nor experience acts alone. Environments trigger gene activity. And our genetically influenced traits evoke significant responses in others. Thus, a child’s impulsivity and aggression may evoke an angry response from a parent or teacher, who reacts warmly to well-behaved children in the family or classroom. In such cases, the child’s nature and the parents’ nurture interact. Gene and scene dance together. Identical twins not only share the same genetic predispositions, they also seek and create similar experiences that express their shared genes (Kandler et al., 2012). This helps explain why identical twins raised in different families have recalled their parents’ warmth as remarkably similar—almost as similar as if they had been raised by the same parents (Plomin et al., 1988, 1991, 1994). Fraternal twins have more differing recollections of their early family life—even if raised in the same family. “Children experience us as different parents, depending on their own qualities,” noted Sandra Scarr (1990). How Does Experience Influence Development? Our genes, when expressed in specific environments, influence our developmental differences. We are like coloring books, with certain lines predisposed and experience filling in the full picture. We are formed by nature and nurture, but what are the most influential components of our nurture? How do our early experiences, our family and peer relationships, and all our other experiences guide our development and contribute to our diversity? The formative nurture that conspires with nature begins at conception, with the prenatal environment in the womb, where embryos receive differing nutrition and varying levels of exposure to toxic agents. Nurture then continues outside the womb, where our early experiences foster brain development. Experience and Brain Development LOQ 4-9 How do early experiences modify the brain? Developing neural connections prepare our brain for thought, language, and other later experiences. So how do early experiences leave their fingerprints in the brain? Mark Rosenzweig, David Krech, and their colleagues (1962) opened a window on that process when they raised some young rats in solitary confinement and others in a communal playground. When they later analyzed the rats’ brains, those who died with the most toys had won. The rats living in the enriched environment, which simulated a natural environment, usually developed a heavier and thicker brain cortex (FIGURE 4.4).  FIGURE 4.4 Experience affects brain development Researchers raised rats either alone in an environment without playthings, or with other rats in an environment enriched with playthings that changed daily (Rosenzweig et al., 1962). In 14 of 16 repetitions of this basic experiment, rats in the enriched environment developed significantly more cerebral cortex (relative to the rest of the brain’s tissue) than did those in the impoverished environment. Rosenzweig was so surprised that he repeated the experiment several times before publishing his findings (Renner & Rosenzweig, 1987; Rosenzweig, 1984). So great are the effects that, shown brief video clips of the rats, you could tell from their activity and curiosity whether their environment had been impoverished or enriched (Renner & Renner, 1993). After 60 days in the enriched environment, the rats’ brain weights increased 7 to 10 percent and the number of brain synapse connections mushroomed by about 20 percent (Kolb & Whishaw, 1998). The enriched environment literally increased brain power. In humans, too, lack of stimulation can slow brain and cognitive development (Farah, 2017). Such results have motivated improvements in environments for laboratory, farm, and zoo animals—and for children in institutions. Stimulation by touch or massage also benefits infant rats and premature babies (Field et al., 2007; Sarro et al., 2014). “Handled” infants of both species develop faster neurologically and gain weight more rapidly. Preemies who have had skin-to-skin contact with their parents sleep better, experience less stress, and show better cognitive development 10 years later (Britto et al., 2017; Feldman et al., 2014). Nature and nurture interact to sculpt our synapses. Brain maturation provides us with an abundance of neural connections. Experience—sights and smells, touches and tastes, music and movement—activates and strengthens some neural pathways while others weaken from disuse. Similar to paths through a forest, less-traveled neural pathways gradually disappear and popular ones are broadened (Dahl et al., 2018; Gopnik et al., 2015). By puberty, this pruning process results in a massive loss of unemployed connections. Stringing the circuits young String musicians who started playing before age 12 have larger and more complex neural circuits controlling the note- making left-hand fingers than do string musicians whose training started later (Elbert et al., 1995). Here at the juncture of nurture and nature is the biological reality of early childhood learning. During early childhood—while excess connections are still on call— youngsters can most easily master such skills as the grammar and accent of another language. Lacking any exposure to language before adolescence, a person will never master any language. Likewise, lacking visual experience during the early years, a person whose vision is later restored by cataract removal will never achieve normal perceptions (Gregory, 1978; Wiesel, 1982). Without that early visual stimulation, the brain cells normally assigned to vision will die or be diverted to other uses. The maturing brain’s rule: Use it or lose it. Although normal stimulation during the early years is critical, brain development does not end with childhood. Thanks to the brain’s amazing neuroplasticity, our neural tissue is ever changing and reorganizing in response to new experiences. New neurons are also born. If a monkey pushes a lever with the same finger many times a day, brain tissue controlling that finger will change to reflect the experience (Karni et al., 1998). Human brains work similarly. Whether learning to keyboard, skateboard, or navigate London’s streets, we perform with increasing skill as our brain incorporates the learning (Ambrose, 2010; Maguire et al., 2000). Genes and experiences are just two ways of doing the same thing—wiring synapses.” — Joseph LeDoux, The Synaptic Self, 2002 ASK YOURSELF What skills did you practice the most as a child—sports, music, cooking, video gaming? How do you think this affected your brain development? How will you continue to develop your brain with new learning and new skills? How Much Credit or Blame Do Parents Deserve? LOQ 4-10 In what ways do parents and peers shape children’s development? Biological parents shuffle their gene decks and deal a life-forming hand to their child- to-be, who is then subjected to countless influences beyond their control. But all parents—whether biological or adoptive—feel enormous satisfaction in their children’s successes or guilt and shame over their failures. They beam over the child who wins trophies and titles. They wonder where they went wrong with the child who is repeatedly in trouble. Freudian psychiatry and psychology encouraged such ideas, blaming problems from asthma to schizophrenia on “bad mothering.” Believing that parents shape their offspring as a potter molds clay, many people praise parents for their children’s virtues and blame them for their children’s vices, and for the psychological harm that toxic parents presumably inflict on their fragile children. No wonder having and raising children can seem so risky. But do parents really produce wounded future adults by being (take your pick from the toxic-parenting lists) overbearing—or uninvolved? Pushy—or indecisive? Overprotective—or distant? Should we then blame our parents for our failings, and ourselves for our children’s failings? Or does talk of wounding fragile children through normal parental mistakes trivialize the brutality of real abuse? To paraphrase developmental psychologist Alison Gopnik (2016), are parents less like potters and more like gardeners who provide the soil for their children’s natural growth? Parents do matter. But parenting wields its largest effects at the extremes: the abused children who become abusive, the deeply loved but firmly handled who become self- confident and socially competent. The power of the family environment also appears in the remarkable academic and vocational successes of many children of people who leave their home countries, such as those of refugees who fled war-torn Vietnam and Cambodia—successes attributed to close-knit, supportive, even demanding families (Caplan et al., 1992). Asian Americans and European Americans often differ in their parenting expectations. An Asian-American mother may push her children to do well, but usually not in a way that strains their relationship (Fu & Markus, 2014). Having a supportive “Tiger Mother”—one who pushes her children and works alongside them— tends to motivate such children (whose culture prepares them to expect such pushing) to work harder. European Americans, however, might see this as going overboard and undermining children’s motivation (Deal, 2011).  Mom in charge Amy Chua, law professor and author of Battle Hymn of the Tiger Mother (2011), sparked controversy by comparing strict “Chinese” and more permissive “Western” parenting styles. In raising her two daughters, Chua came to appreciate the benefits and the costs of the more externally controlled traditional Chinese parenting. Yet in personality measures, shared environmental influences from the womb onward typically account for less than 10 percent of children’s differences. Referring to our traits rather than our values, behavior geneticist Robert Plomin (2018b) noted, “We would essentially be the same person if we had been adopted at birth and raised in a different family.” This research has stunning implications. To developmental psychologist Sandra Scarr (1993), it implies that “parents should be given less credit for kids who turn out great and blamed less for kids who don’t.” So, knowing that children’s personalities are not easily sculpted by parental nurture, perhaps parents can relax and love their children for who they are. Peer Influence As children mature, what other experiences do the work of nurturing? At all ages, but especially during childhood and adolescence, we seek to fit in with our groups (Blakemore, 2018; Harris, 1998, 2000): Preschoolers who disdain a certain food often will eat that food if put at a table with a group of children who like it. Children who hear English spoken with one accent at home and another in the neighborhood and at school will invariably adopt the accent of their peers, not their parents. Accents (and slang) reflect culture, “and children get their culture from their peers,” as Judith Rich Harris (2007) has noted. Teens who start smoking typically have friends who model smoking, suggest its pleasures, and offer cigarettes (Liu et al., 2017). Part of this peer similarity may result from a selection effect, as adolescents seek out peers with similar attitudes, interests, and traits (Domingue et al., 2018). Those who smoke (or don’t) may select as friends those who also smoke (or don’t). Peer power As we develop, we play, mate, and partner with peers. No wonder children and youths are so sensitive and responsive to peer influences. The power to select a child’s neighborhood and schools gives some parents the ability to influence the culture that shapes the child’s peer group. And because neighborhood influences matter, parents may want to become involved in intervention programs that aim at a whole school or neighborhood. If the vapors of a toxic climate are seeping into a child’s life, that climate—not just the child—needs reforming. Even so, peers are but one medium of cultural influence. As an African proverb declares, “It takes a village to raise a child.” RETRIEVAL PRACTICE RP-1 What is the selection effect, and how might it affect a teen’s decision to join sports teams at school? ANSWERS IN APPENDIX E Cultural Influences LOQ 4-11 How does culture affect our behavior? Compared with the narrow path taken by flies, fish, and foxes, the road along which environment drives humans is wider. The mark of our species—nature’s great gift to us —is our ability to learn and adapt. We come equipped with a huge cerebral mobile device, ready to receive cultural apps. Culture is the behaviors, ideas, attitudes, values, and traditions shared by a group of people and transmitted from one generation to the next (Brislin, 1988; Cohen, 2009). Human nature, noted Roy Baumeister (2005), seems designed for culture. We are social animals, but more. Wolves are social animals; they live and hunt in packs. Ants are incessantly social, never alone. But “culture is a better way of being social,” observed Baumeister. Wolves function pretty much as they did 10,000 years ago. We enjoy electricity, indoor plumbing, antibiotics, and the internet—things unknown to most of our ancestors. Culture works. culture the enduring behaviors, ideas, attitudes, values, and traditions shared by a group of people and transmitted from one generation to the next. Other animals exhibit smaller kernels of culture. Chimpanzees sometimes invent customs—using leaves to clean their bodies, slapping branches to get attention, and doing a “rain dance” by slowly displaying themselves at the start of rain—and pass them on to their peers and offspring (Whiten et al., 1999). Culture supports survival and reproduction by transmitting learned behaviors that give a group an edge. But human culture does more. Thanks to our culture’s mastery of language, we humans enjoy the preservation of innovation. Within the span of this day, we have used Google, our phones, digital hearing technology [DM], and a digital guitar tuner [ND]. On a grander scale, we have culture’s accumulated knowledge to thank for the last century’s 30-year extension of the average human life expectancy (in most countries where this book is being read). Moreover, culture enables an efficient division of labor. Although two lucky people get their name on this book (which transmits accumulated cultural wisdom), it actually results from the coordination and commitment of a team of gifted people, no one of whom could produce it alone. Across cultures, we differ in our language, money, sports, religion, and customs. But beneath differences is our great similarity—our capacity for culture. Culture transmits the customs and beliefs that enable us to communicate, to exchange money for things, to play, to eat, and to drive with agreed-upon rules and without crashing into one another. Variation Across Cultures We see our adaptability in cultural variations among our beliefs and our values, in how we nurture our children and bury our dead, and in what we wear (or whether we wear anything at all). We are ever mindful that the worldwide readers of this book are culturally diverse. You and your ancestors reach from Australia to Algeria and from Singapore to Sweden.  Gender equity: Among the Aka people of Central Africa, men and women interchangeably perform roles such as hunting and child care. As a result, fathers form an especially close bond with their infants, even suckling the babies with their own nipples when hunger makes the child impatient for Mother’s return. Fathers in this culture are holding or within reach of their babies 47 percent of the time (Hewlett, 1991). Riding along with a unified culture is like biking with the wind: As it carries us along, we hardly notice it is there. When we try biking against the wind, we feel its force. Face-to-face with a different culture, we become aware of the cultural winds. Visiting Europe, most North Americans notice the smaller cars, the left-handed use of the fork, the uninhibited swimwear. Stationed in Iraq, Afghanistan, and Kuwait, American and European soldiers alike realized how comparatively liberal their home cultures were. Arriving in North America, visitors from Japan and India struggle to understand why so many people wear their dirty street shoes in the house. But humans in varied cultures nevertheless share some basic moral ideas. Even before they can walk, babies prefer helpful people over naughty ones (Hamlin et al., 2011). Worldwide, people prize honesty, fairness, and kindness (McGrath, 2015). Yet each cultural group also evolves its own norms—rules for accepted and expected behavior. The British have a norm for orderly waiting in line. Many in South Asia, Africa, and the Middle East use only the right hand for eating. Sometimes social expectations seem oppressive: “Why should it matter how I dress?” Yet, norms—how to greet, how to eat— grease the social machinery. norm an understood rule for accepted and expected behavior. Norms prescribe “proper” behavior. When cultures collide, their differing norms often befuddle. Should we greet people by shaking hands, bowing, or kissing each cheek? Knowing what sorts of gestures and compliments are culturally appropriate, we can relax and enjoy one another without fear of embarrassment or insult. When we don’t understand what’s expected or accepted, we may experience culture shock. People from Mediterranean cultures have perceived northern Europeans as efficient but cold, and preoccupied with punctuality (Triandis, 1981). People from time- conscious Japan—where pedestrians walk briskly and postal clerks fill requests speedily—have found themselves growing impatient when visiting Indonesia, where the pace of life is more leisurely (Levine & Norenzayan, 1999). Someone from the European community, which requires 20 paid vacation days each year, may also experience culture shock when working in the United States, which does not guarantee workers any paid vacation (Ray et al., 2013). Variation Over Time Like biological creatures, cultures vary and compete for resources, and thus evolve over time (Mesoudi, 2009). Consider how rapidly cultures may change. English poet Geoffrey Chaucer (1342–1400) is separated from a modern Briton by only 25 generations, but the two would have great difficulty communicating. At the beginning of the last century, your ancestors lived in a world without cars, radio broadcasting, or electric lighting. And in the thin slice of history since 1960, most Western cultures have changed with astonishing speed. Middle-class people enjoy the convenience of air- conditioned housing, online shopping, anywhere-anytime electronic communication, and—enriched by doubled per-person real income—they eat out more than twice as often as did their grandparents. People now enjoy expanded human rights. And with greater economic independence, today’s women more often marry for love and less often endure abusive relationships. But some changes seem not so wonderfully positive. Had you fallen asleep in the United States in 1960 and awakened today, you would open your eyes to a culture with more depression and more economic inequality. You would also find North Americans —like their counterparts in Britain, Australia, and New Zealand—spending more hours at work, fewer hours with friends and family, and fewer hours asleep (BLS, 2011; Twenge, 2017). Whether we love or loathe these changes, we cannot fail to be impressed by their breathtaking speed. And we cannot explain them by changes in the human gene pool, which evolves far too slowly to account for high-speed cultural transformations. Cultures vary. Cultures change. And cultures shape our lives. Culture and the Self LOQ 4-12 How do individualist and collectivist cultures differ in their values and goals? Imagine that someone ripped away your social connections, making you a solitary refugee in a foreign land. How much of your identity would remain intact? If you are an individualist, a great deal. You would have an independent sense of “me,” and an awareness of your unique personal convictions and values. Individualists prioritize personal goals. They define their identity mostly in terms of personal traits. They strive for personal control and individual achievement. individualism giving priority to one’s own goals over group goals and defining one’s identity in terms of personal attributes rather than group identifications. The human need to belong is universal, so individualists do seek out and join groups. But they are less focused on group harmony and doing their duty to the group (Brewer & Chen, 2007). As children, they value a sense of free will (Chernyak et al., 2019). Being more self-contained, individualists move in and out of social groups more easily. They feel relatively free to switch places of worship, change jobs, or even leave their extended families and migrate to a new place. Marriage is often for as long as they both shall love. If set adrift in a foreign land as a collectivist, you might experience a greater loss of identity. Cut off from family, groups, and loyal friends, you would lose the connections that have defined who you are. Group identifications provide a sense of belonging, a set of values, and an assurance of security. Collectivists have deep attachments to their groups—their family, clan, company, or country. Elders receive respect. For example, Chinese law states that parents aged 60 or above can sue their sons and daughters if they fail to provide “for the elderly, taking care of them and comforting them, and cater[ing] to their special needs.” collectivism giving priority to the goals of one’s group (often one’s extended family or work group) and defining one’s identity accordingly. “One needs to cultivate the spirit of sacrificing the little me to achieve the benefits of the big me.” — Chinese saying Collectivists are like athletes who take more pleasure in their team’s victory than in their own performance. They find satisfaction in advancing their groups’ interests, even at the expense of personal needs. They preserve group spirit by avoiding direct confrontation, blunt honesty, and uncomfortable topics. They value humility, not self- importance (Bond et al., 2012). They view forgiveness as a way to strengthen group harmony (Joo et al., 2019). Instead of dominating conversations, collectivists hold back and display shyness when meeting strangers (Cheek & Melchior, 1990). Given the priority on “we,” not “me,” that satisfying, super-customized North American latte might seem selfishly demanding in Seoul (Kim & Markus, 1999). A question: What do you think of people who willingly change their behavior to suit different people and situations? People in individualist countries (for example, the United States and Brazil) typically describe such people as “dishonest,” “untrustworthy,” and “insincere” (Levine, 2016). In traditionally collectivist countries (China, India, and Nepal, for example), people more often describe them as “mature,” “honest,” “trustworthy,” and “sincere.” To be sure, there is also diversity within cultures. Many countries also have distinct subcultures related to religion, economic status, and region (Cohen, 2009). Southern Chinese farmers typically grow rice, a crop that requires group cooperation and builds collectivist values. Northern Chinese farmers often grow wheat, a crop that involves individual effort and leads to more individualist values. These differences lead people from Southern and Northern China to adopt different ways of thinking, feeling, and acting (Dong et al., 2019; Obschonka et al., 2018; Talhelm et al., 2014). In one clever study, researchers moved chairs together in Starbucks coffeehouses across China to block the aisle. They observed who acted like a typical individualist—controlling the environment by moving one of the chairs out of the way—and who acted like a typical collectivist—adapting to the environment by squeezing through the chairs (Talhelm et al., 2018). Compared with the more collectivist Southern Chinese, the Northern Chinese were more likely to move the chair. In collectivist Japan, a spirit of individualism marks the “northern frontier” island of Hokkaido (Kitayama et al., 2006). And even in the most individualist countries, people have some collectivist values. But in general, people (especially men) in competitive, individualist cultures have more personal freedom, are less geographically bound to their families, enjoy more privacy, and take more pride in personal achievements (TABLE 4.1). TABLE 4.1 Value Contrasts Between Individualism and Collectivism Concept Individualism Collectivism Self Independent (identity from individual traits) Interdependent (identity from belonging to groups) Life task Discover and express one’s uniqueness Maintain connections, fit in, perform role What Me—personal achievement and fulfillment; Us—group goals and solidarity; social responsibilities matters rights and liberties; self-esteem and relationships; family duty Coping Change reality Accommodate to reality method Morality Defined by the individual (self-based) Defined by social networks (duty-based) Relationships Many, often temporary or casual; confrontation Few, close, and enduring; harmony is valued acceptable Attributing Behavior reflects the individual’s personality Behavior reflects social norms and roles behavior and attitudes Information from Thomas Schoeneman (1994) and Harry Triandis (1994). Individualists even prefer unusual names, as psychologist Jean Twenge noticed while seeking a name for her first child. When she and her colleagues (2010a, 2016a) analyzed the first names of 358 million American babies born between 1880 and 2015, they discovered that the most common baby names had become less common. As FIGURE 4.5 illustrates, the percentage of boys and girls given one of the 10 most common names for their birth year has plunged. Collectivist Japan provides a contrast: Half of Japanese baby names are among the country’s 10 most common names (Ogihara et al., 2015). FIGURE 4.5 A child like no other Americans’ individualist tendencies are reflected in their choice of names for their  babies. In recent years, the percentage of American babies receiving one of that year’s 10 most common names has plunged. (Data from Twenge et al., 2010a, 2016a.) The individualist-collectivist divide appears in reactions to Olympic medals received. U.S. gold medal winners and the U.S. media covering them have attributed the achievements mostly to the athletes themselves (Markus et al., 2006). “I think I just stayed focused,” explained swimming gold medalist Misty Hyman. “It was time to show the world what I could do.” Japan’s gold medalist in the women’s marathon, Naoko Takahashi, had a different explanation: “Here is the best coach in the world, the best manager in the world, and all of the people who support me—all of these things … became a gold medal.” Individualists frequently demand romance and personal fulfillment in marriage (Dion & Dion, 1993). In contrast, collectivist love songs have often expressed enduring commitment and friendship, as in this one from China: “We will be together from now on…. I will never change from now to forever” (Rothbaum & Tsang, 1998). What predicts change in one culture over time, or differences between cultures? Social history matters. Individualism and independence have been fostered by voluntary emigration, a capitalist economy, and a sparsely populated, challenging environment (Kitayama et al., 2009, 2010; Varnum et al., 2010). In Western cultures over the last century and now in all but the poorest countries, individualism has increased, following closely on the heels of increasing affluence (Grossmann & Varnum, 2015; Santos et al., 2017). Might biology also play a role? One study comparing collectivists’ and individualists’ brain activity suggested that collectivists experienced greater emotional pain when they viewed others in distress (Cheon et al., 2013). As we have seen, we are biopsychosocial creatures.  Collectivist culture Although the United States is largely individualist, many cultural subgroups remain collectivist. This is true for many Alaska Natives, who demonstrate respect for tribal elders, and whose identity springs largely from their group affiliations. ASK YOURSELF Do you consider yourself to be more of a collectivist or an individualist? How do you think this sense of self has influenced your behavior, emotions, and thoughts? To explore your individualist versus collectivist self, engage online with Assess Your Strengths—What Is Your Self- Concept? Culture and Child Raising Variations in child-raising values reflect individual and cultural diversity, as well as changes over time. Do you prefer children who are independent, or children who comply with what others think? Compared with families in Asian cultures, families in Western cultures more often prefer independence: “You are responsible for yourself.” “Follow your conscience. Be true to yourself. Discover your gifts.” Some Western parents have recently gone further, telling their children, “You are more special than other children” (Brummelman et al., 2015). Many with Western cultural values no longer prioritize obedience, respect, and sensitivity to others (Alwin, 1990; Remley, 1988). In the 1950s, Western parents were more likely to teach their children, “Be true to your traditions.” “Be loyal to your heritage and country.” “Show respect toward your parents and other superiors.” Cultures vary. And cultures change. Children across place and time have thrived under various child-raising systems. Upper-class British parents traditionally handed off routine caregiving to nannies, then sent their 10-year-olds away to boarding school. Those from Asian and African cultures more often value emotional closeness. Infants and toddlers may spend their days close to a family member (Morelli et al., 1992; Whiting & Edwards, 1988). Many of these cultures encourage a strong sense of family self—which may mean that what shames the child shames the family, and what brings honor to the family brings honor to the self. In traditional Gusii society, in western Kenya, babies nursed freely but spent most of the day on their mother’s or a sibling’s back—with lots of body contact but little face-to- face and language interaction. What might appear as a lack of interaction to many Westerners might to these Gusii parents seem far preferable to the lesser body contact experienced by babies pushed in strollers, carried in car seats, and left in playpens (Small, 1997). In some rural villages in Senegal, cultural traditions discourage caregivers’ talking with young children. Programs encouraging verbal interaction improved the children’s language development one year later (Weber et al., 2017). But were the programs worth the cultural disruption? Diversity in child raising cautions us against presuming that any one culture’s way is the only way to raise children successfully.  Cultures vary Parents everywhere care about their children, but raise and protect them differently depending on the surrounding culture. In big cities, parents keep children close. In smaller, close-knit communities, such as Scotland’s Orkney Islands’ town of Stromness, social trust has enabled parents to park their toddlers outside shops. Developmental Similarities Across Groups Mindful of how others differ from us, we often fail to notice the similarities predisposed by our shared biology. One 49-country study revealed smaller than expected nation-to-nation differences in personality traits, such as conscientiousness and extraversion (Terracciano et al., 2006). National stereotypes exaggerate differences that, although real, are modest: Australians see themselves as outgoing. German- speaking Swiss see themselves as conscientious. And Canadians see themselves as agreeable. Actually, compared with the per