Evolutionary Theory & Human Behavior: Natural Selection

Darwin and the Development of Evolutionary Theory (Super Simple Version) Darwin was curious, patient, and hardworking. Even though he thought his skills were just okay, he ended up changing science forever. His big idea? Life changes over time through evolution—organisms adapt and develop based on...

Darwin and the Development of Evolutionary Theory (Super Simple Version) Darwin was curious, patient, and hardworking. Even though he thought his skills were just okay, he ended up changing science forever. His big idea? Life changes over time through evolution—organisms adapt and develop based on biological changes passed down through generations. Humans and other animals have changed a lot over millions of years—brains, bodies, behaviours, even how we talk. Evolution explains why animals have the traits they do (like wings or long necks). These changes help them survive in their environments. Darwin’s ideas didn’t just impact biology—they also shaped psychology. He even predicted that psychology would someday be based on evolution. Today, psychologists study both ultimate causes (long-term evolutionary reasons for behaviour) and proximate causes (immediate environment-related causes). Evolutionary psychology is a field that looks at how evolution shaped the way we think and act—especially for survival and reproduction. To understand our behaviour now, we need to understand how we evolved. Darwin’s Big Adventure (Super Simple Version) After studying theology, Darwin joined a five-year trip on a ship called the HMS Beagle. He wasn’t paid but went as a naturalist to explore places like South America, Australia, and various islands. Along the way, he collected tons of animals, plants, and fossils. Even though he saw lots of interesting and diverse life forms, Darwin still believed in creationism at the time—that God made all living things as they are. He didn’t come up with his theory of evolution until later. How Darwin Came Up with Natural Selection (Super Simple Version) After returning to England, Darwin kept studying all the plants and animals he’d collected. He was especially interested in how humans breed animals with good traits—like choosing the fattest cows to make beefier ones. This got him thinking: could something like that happen in nature too? After reading a book about how populations grow and compete (by Malthus), Darwin had a lightbulb moment: in nature, only the animals with the best traits survive and have more babies. Over time, these traits spread, and new species can form. That’s natural selection. Even though he knew his idea was huge, Darwin didn’t rush to publish it. But when another scientist, Alfred Russel Wallace, came up with the same idea, they presented it together. Darwin published The Origin of Species a year later, and it was a hit. Darwin’s Big Ideas (Simple Version) Darwin’s theory of evolution is based on two main ideas: ​ Adaptation = animals and plants change over generations to fit their environment better.​ ​ Natural selection = only the helpful traits get passed on; the rest fade out.​ His 4 Key Points: 1.​ Species change over time – some survive, others go extinct.​ 2.​ Evolution is slow and steady, unless there’s a big environmental change.​ 3.​ All life comes from a common ancestor.​ 4.​ Natural selection helps species change and stay stable if their environment doesn’t change much.​ How Natural Selection Works Darwin got inspired by Malthus, who said that food grows slower than populations, so animals compete. Only the best-suited ones survive and reproduce. This is called reproductive success – it’s not just about being strong, but about how many babies you have. Key idea: Natural selection isn’t planned. Giraffes didn’t decide to grow long necks. It just happened that longer-necked giraffes survived better and passed on their genes. Variation and Competition – The Two Big Drivers ​ Variation = differences in looks or behavior between individuals (like beak size in finches). These come from genetics (genotype) and environment (like diet).​ ​ Phenotype = what you see (tallness, beak shape, strength) = genes + environment.​ Finch Study:​ In a drought, only birds with thick beaks survived. Later, when food changed again, birds with thin beaks did better. This shows how natural selection works depending on the environment. ​ Competition = animals fight (directly or indirectly) for food, mates, territory. Winners survive and reproduce, spreading their genes.​ Do People Accept Evolution? ​ In the US, acceptance of evolution has been mixed and often low, especially compared to Europe and Japan.​ ​ Many Americans either reject evolution or are unsure.​ ​ In contrast, countries like France, Sweden, and Japan have much higher acceptance rates.​ ​ A lot of people don’t understand basic biology – for example, many don’t know what DNA is or how similar we are to other animals. Human Evolution Overview ​ Reconstructing human evolution is like piecing together a broken jigsaw puzzle with missing and damaged parts.​ ​ We use methods like carbon dating and DNA analysis to estimate when ancestors lived, but the results are still guesses.​ ​ Humans share a lot of DNA with chimps (99% of the protein-related DNA), but the differences come from how our genes are regulated.​ Key Ancestors: 1.​ Homo habilis: Small, early human with bigger brains and tools.​ 2.​ Homo erectus: More advanced, used fire, made tools, and spread across continents.​ 3.​ Homo sapiens: Modern humans who evolved in Africa and spread worldwide.​  4.​ Neanderthals: Early humans who lived in Europe and had cultural practices but went extinct around 25,000 years ago.​ Natural Selection and Human Evolution Overview: Human evolution has been heavily shaped by natural selection, favoring two key traits: 1.​ Bipedalism: Walking on two feet, which developed over 4 million years ago. It helped early humans reach food in trees and freed their hands for other tasks like using tools and carrying objects.​ 2.​ Encephalisation: Increased brain size, which started around 2 million years ago and contributed to more complex thinking, problem-solving, and planning.​ As early hominids evolved, their brain volume increased significantly, from 650 cm³ in early ancestors to 1,500 cm³ in modern humans. This growth allowed for: ​ Improved survival through better hunting, tool use, and defense.​ ​ Planning: The ability to anticipate future events, like organizing hunts or social gatherings.​ ​ Better communication: Language developed, helping humans share vital information and strengthen social bonds.​ This combination of bipedalism, brain growth, and language helped humans thrive, live longer, and pass on knowledge, setting the foundation for the development of cultures. Heredity and Genetics Overview: Darwin’s theory of natural selection opened new areas for exploration, particularly genetics, the study of genes, how they function, and how they are passed down from one generation to the next. Genetics helps explain how genes shape physical and behavioral traits. However, Darwin didn’t have an explanation for how traits were inherited, a key element of his theory. In the 1860s, Gregor Mendel, an Austrian monk, discovered the basic principles of heredity through experiments with pea plants. He showed how traits like height and flower color could be passed down from parent to offspring, forming the foundation of genetics. Genes, the units of heredity, are made of DNA (deoxyribonucleic acid). DNA is a double helix, resembling a twisted ladder, where pairs of nucleotides (adenine, thymine, guanine, and cytosine) form the "rungs" of the ladder. The sequence of these nucleotides directs the production of proteins, which regulate the development of the body and its functions. Our genetic code is composed of about 3 billion pairs of these nucleotide molecules, guiding our biological development. Protein Synthesis and Heredity Overview: Protein Synthesis: Genes influence development and behavior through protein synthesis, where sequences of DNA nucleotides (adenine, thymine, guanine, and cytosine) determine the production of proteins. These proteins, composed of amino acids, are essential for creating physical structures and regulating physiological processes in the body. While there are no specific "genes for behavior," genes govern the physical structures that enable behaviors, such as those involved in mental health disorders like schizophrenia. The Human Genome Project mapped the DNA sequence and identified about 25,000 genes in each human cell, contributing to the complexity of how genes manifest in our traits and behavior. Chromosomes and Meiosis: Genes are located on chromosomes, structures made of DNA found in the cell's nucleus. Humans inherit 46 chromosomes (23 pairs), including sex chromosomes (XX for females, XY for males) that determine sex characteristics. Meiosis is the special type of cell division that produces sperms and ova (egg cells), each containing 23 chromosomes. The process of meiosis ensures genetic diversity because it randomly shuffles chromosome pairs, leading to numerous possible combinations. Thus, the genetic makeup of siblings varies, even though they share the same parents. Sex Determination: The sex of offspring is determined by the 23rd chromosome pair. If a sperm carrying a Y chromosome fertilizes the egg, a male child is produced (XY). If an X-bearing sperm fertilizes the egg, a female child is produced (XX). Dominant and Recessive Alleles: ​ Alleles are different forms of the same gene that can be inherited from parents. These can be homozygous (same alleles from both parents) or heterozygous (different alleles from each parent).​ ​ Dominant alleles have a stronger influence on the expression of traits, such as brown eyes, which will appear if at least one dominant allele is present.​ ​ Recessive alleles have a weaker influence and require two copies of the same allele (one from each parent) to express traits, such as blue eyes.​ ​ Other traits like eye color can be influenced by additional genes, creating variations (e.g., hazel or black eyes due to different pigment amounts).​ Genetic Complexity: ​ Traits like behavior are polygenic, meaning they are influenced by multiple genes and environmental factors, such as running speed, which depends on genetics for muscles and coordination, plus environmental influences like nutrition and exercise.​ Genetic Diversity: ​ Sexual reproduction promotes genetic diversity, which helps species adapt to environmental changes. Genetic variation increases the likelihood that some individuals will have traits that enable survival under new conditions.​ Influence of Sex on Heredity: ​ Haemophilia, a blood disorder, is caused by a recessive gene on the X chromosome. Females have two X chromosomes, so they can carry the allele but still clot blood normally if the other X has the correct gene. Males, with only one X chromosome, will express the disorder if they inherit the faulty gene.​ ​ Sex-influenced genes, like pattern baldness, are controlled by genes that affect one sex more than the other. In males, baldness can appear with just one allele, while females need both alleles to show the trait. Mutations and Chromosomal Aberrations: ​ Mutations are accidental changes in the DNA code within a single gene and are the source of genetic diversity. While most mutations are harmful, some can be beneficial, particularly in specific environments. Mutations can occur spontaneously or be caused by external factors like high-energy radiation.​ ​ Haemophilia is a notable example of a mutation, which, through a spontaneous mutation in Queen Victoria’s family, spread among royal families of Europe due to their practice of marrying within nobility.​ ​ Chromosomal Aberrations involve changes in chromosome parts or the total number of chromosomes. An example is cri-du-chat syndrome, where a portion of chromosome 5 is deleted, causing severe developmental issues, including gastrointestinal and cardiac problems, and intellectual disabilities. However, early special education can help individuals with this syndrome develop self-care and communication skills.​ Epigenetics: ​ Epigenetics refers to changes in gene expression caused by environmental factors, without altering the DNA itself. External factors like trauma, drug abuse, or affection can affect the DNA’s function by adding molecules that either block or promote the gene’s activity. This alteration affects protein synthesis, which is crucial for bodily and brain functions.​ ​ Gene silencing occurs when molecules prevent access to specific genes, altering their expression. An example is an experiment with rats: those whose mothers licked or groomed them showed reduced anxiety and stress, as the corticosterone receptor gene was more active in these rats, helping them manage stress better.​ ​ Brain-derived neurotrophic factor (BDNF) is a protein important for cell function. Low BDNF levels are associated with depression, and distressing events may alter the DNA that encodes this protein. In an experiment, bullied mice had lower BDNF levels, and this protein was regulated by a molecule that inhibited its expression. Treatment with antidepressants increased BDNF levels.​ ​ Knockout Genes are used in experiments where a gene is “knocked out” by exposing animals to radiation, preventing it from expressing itself. An example is rats with a knocked-out gene for spatial learning, which impaired their ability to perform tasks like finding a submerged platform, demonstrating the gene's role in cognitive abilities. Heredity and Behaviour Genetics: ​ Heredity and Environment: Each person is shaped by both their genetic instructions and the environment they are born into. Differences in traits such as size, personality, and abilities (e.g., athletic or artistic) are influenced by both genetics and the environment. Heredity refers to the genetic transmission of traits, whereas heritable traits reflect the degree to which these differences are influenced by genetics.​ ​ Heritability: This statistical term measures the extent to which a trait's variability within a population is due to genetic differences. It is often misunderstood as inheritance, but while inheritance refers to the transmission of traits from parents to offspring, heritability measures genetic variation within a population. Behaviour genetics is the field focused on understanding how heredity influences behaviour and psychological traits.​ ​ Behaviour Genetics: Researchers study how genetics contribute to psychological traits such as memory, autism, personality, aging, emotional recognition, and expression. For example, studies on episodic memory (the ability to recall past experiences) suggest heritability of 30-60%. Variants of genes like HTR2A, BDNF, and COMT have been linked to memory, but the relationship between specific genes  and phenotypic expression is still being explored.​ ​ Cognitive Decline and Alzheimer's: The APOE E4 allele is linked to cognitive decline and the development of Alzheimer's disease, though research on this is still inconclusive.​ Studying Genetic Influences: ​ Historical Background: The study of heredity and behaviour traces back to Mendel’s experiments, which helped establish the idea that traits are heritable. Galton's studies further explored family correlations in intelligence, marking the beginning of genetic studies on human traits.​ Artificial Selection in Animals: ​ Artificial Selection: This method involves selecting animals with desired traits for breeding. Tryon's (1940) study on maze learning in rats demonstrated that heritable traits could be selected. He selected rats that learned the maze fastest ("bright" rats) and those that learned slowly ("dull" rats). After successive generations, the traits for learning ability were passed down, showing how genetics could influence behaviour.​ ​ Environmental Influences: Cooper and Zubek (1958) demonstrated that the environment plays a significant role in the expression of genetic traits. They showed that when rats from both "bright" and "dull" strains were raised in enriched or impoverished environments, the differences in maze performance diminished. This suggests that genetic potential for learning is influenced by the environment in which the organism develops.​ ​ Implications: Tryon’s research shows how a trait’s frequency in a population can be influenced by artificial selection, but the exact genes responsible for traits like learning are not always clear. The study also highlighted that motivation or other unmeasured traits could have influenced the results.​ Genetic Manipulation in Humans: ​ Gene Manipulation: While studies like Tryon’s show how genetics can shape behaviour, the possibility of gene manipulation in humans raises ethical and scientific questions. Techniques like gene mapping are helping researchers understand the genetic basis of behaviour, memory, and mental disorders. Advances in molecular genetics are crucial for understanding psychological disorders and may eventually lead to genetic therapies.​ In sum, genetics and the environment both play significant roles in shaping behaviour. Behaviour genetics seeks to uncover the complex interaction between inherited genetic factors and external environmental influences in producing individual difference Twin Studies in Behaviour Genetics Twin studies are an essential tool for understanding the role of genetics in shaping behaviour. These studies help psychologists and geneticists overcome the ethical and environmental challenges of directly manipulating human genetics or environments. By comparing identical (monozygotic, MZ) and fraternal (dizygotic, DZ) twins, researchers can assess the influence of heredity on various traits, including personality, intelligence, and attitudes. Key Concepts in Twin Studies: ​ MZ vs. DZ Twins: MZ twins are genetically identical, arising from the splitting of a single fertilized ovum, while DZ twins come from two separate fertilized eggs and are genetically similar to non-twin siblings. Therefore, MZ twins share 100% of their genes, while DZ twins share, on average, 50% of their genes, just like other siblings.​ ​ Concordance and Discordance: Concordance refers to the degree to which both twins share a particular trait, such as intelligence or shyness. If both twins express a trait, they are concordant for that trait; if only one twin expresses it, they are discordant. High concordance for MZ twins compared to DZ twins suggests a genetic influence on that trait.​ ​ Heredity and Traits: If a trait shows a higher concordance rate for MZ twins than DZ twins, it likely has a genetic component. For instance, traits like blood type have 100% heritability, as they are determined entirely by genetics. On the other hand, psychological traits, such as personality or intelligence, also exhibit varying degrees of genetic influence, as shown by twin studies.​ Examples of Twin Study Findings: ​ Attitudes and Personality: In a study by Olson et al. (2001), MZ twins were more likely to share similar attitudes (e.g., on topics such as religion or leadership) than DZ twins. This suggests that genetic factors may influence certain attitudes, but the exact mechanisms are still debated. The researchers proposed that certain general traits, like sociability, might underlie these attitudes. Sociability, being a heritable personality trait, could influence a range of attitudes and behaviours. For example, more sociable people might be more likely to have positive views on leadership.​ ​ Brain Structure and Function: Jahanshad et al. (2010) found that MZ twins tend to have similar brain structures, with more similar volumes in the fibres connecting different parts of the brain compared to DZ twins. This highlights a genetic influence  on brain anatomy, which in turn could affect cognitive abilities and behaviour.​ The Challenge of Interpretation: While twin studies provide valuable insights into the genetic basis of traits, interpreting the results can be complex. Even when concordance rates are higher for MZ twins, it does not necessarily mean that a specific gene directly causes a trait. Often, environmental factors and gene-environment interactions complicate these relationships. For example, while sociability may be a heritable trait that influences attitudes towards leadership, environmental factors like societal expectations or personal experiences may also play a significant role in shaping these attitudes. Moreover, the similarity between MZ twins may not always point to a specific genetic trait for a given attitude or behaviour. In some cases, shared family environments or cultural factors may explain the similarities observed between twins. Implications of Twin Studies: Twin studies have helped further our understanding of the nature vs. nurture debate, shedding light on the extent to which genetics and the environment contribute to traits such as personality, intelligence, and psychological disorders. While they provide evidence for genetic influence, they also highlight the complexity of human behaviour, which is shaped by a combination of genetic and environmental factors. What is Sociobiology? Sociobiology is the study of how our social behaviors (like how we act with others) are connected to biology (how our bodies and brains work). It looks at how behaviors, like forming relationships or making decisions, might have evolved over time. Sociobiologists look at animals to understand these behaviors and then apply that to humans. But not everyone agrees with sociobiologists. Some people think sociobiology is too simple and doesn't explain everything about human behavior. Mating and Parenting: One big thing sociobiologists study is how animals (including humans) mate and take care of their babies. They look at how different ways of mating help species survive. For example, most relationships in Western cultures are monogamous, meaning one man and one woman get together and raise children. But this isn’t the only way animals can reproduce. There are also other ways, like: ​ Polygyny: One male mates with multiple females.​ ​ Polyandry: One female mates with multiple males.​  ​ Polygynandry: Several males and females mate with each other.​ Sociobiologists believe these different ways of mating evolved because males and females invest differently in raising babies. Parental investment is all the effort and energy that parents put into raising their children. For example, in most animals, females spend more time and energy taking care of the babies than males do. Why Do Women Choose Mates Carefully? Since females (like humans) invest more in their babies (pregnancy, nurturing, etc.), they are more careful about who they mate with. Women usually pick men who seem to have qualities like strength, power, or good resources. The idea is that these traits help make sure the babies will survive and thrive. For example, in Trivers' theory (which sociobiologists like), women are very selective in choosing a mate because they spend a lot of time carrying and taking care of babies. So, they look for men who have resources (like money or status) to help them raise their children. If a man doesn’t help or diverts his resources to other women, a woman might decide to leave him. Competition Among Males for Mates: In some species, males compete with each other to attract females. The males that are bigger, stronger, or more aggressive usually win the competition for mates. They then get to mate with the females and have offspring. This competition happens because males can father many more offspring than females can give birth to. For example, a man can father many children with different women, while a woman can only have a limited number of children in her lifetime. How Do Men Attract Mates? Men don’t just compete physically; they also use things like humor, music, or creativity to attract women. Women seem to like these things, especially if they are thinking about a long-term relationship. For example, men with deep voices or who tell funny jokes might attract women more than men with higher voices. Reproductive Strategies in Human Societies: Now, let’s talk about the different ways humans mate. Most human societies practice polygyny, where one man has multiple wives. About 84% of human societies allow men who are wealthy or powerful to practice polygyny. The next most common strategy is monogamy, where one man and one woman are together. Only about 15% of human societies follow this. Polyandry (one woman with multiple men) and polygynandry (multiple women and multiple men) are very rare, making up less than 1% of all societies. Why Do Females Choose Mates Carefully in Polygyny? In species that practice polygyny (where one male mates with many females), the females are very careful about who they choose as a mate. Sociobiologists say this happens because females invest a lot of energy into raising babies, and they want to make sure they mate with males who have qualities like strength and aggressiveness. These qualities are thought to help the males protect the females and their offspring. So, the key takeaway is: sociobiologists believe that the way animals and humans reproduce (who mates with whom and how babies are raised) is deeply connected to biological differences between males and females, like how much energy and time each one spends on reproduction. Physical Attractiveness ​ Body Shape and Size: Some studies suggest that body mass index (BMI), or how much fat and muscle a person has, is important when judging attractiveness. For example, when people see a person’s body from the front, they might care more about BMI. When they see the body from the side (profile), they might care more about the shape of the body.​ ​ What’s More Attractive? One study by Tovee and Cornelissen (2001) found that BMI (how much fat or muscle a person has) is a better predictor of attractiveness than waist-to-hip ratio (WHR), which is the difference in size between a person’s waist and hips. Both men and women agreed on what looked attractive. They liked bodies that had a curvy shape (low WHR) and a good BMI.​ ​ Why Do These Ratios Matter? BMI might show how fit and healthy someone is, while WHR might show something more specific like fertility (the ability to have children). WHR can be tricky, though, because people with similar WHR might be in different health states. For example, anorexic women can have the same WHR as healthy women, but the anorexic ones aren’t fertile.​ ​ Men’s Body Preferences: For men, researchers found that the waist-to-chest ratio (WCR), which is the difference in size between the waist and chest, matters. Men with broad shoulders and narrow waists are seen as more attractive because they seem physically strong.​ ​ Thinner Men vs. Overweight Men: Thinner men tend to be seen as more socially desirable than overweight men, and they are rated more highly for qualities like trustworthiness, friendliness, intelligence, and potential as a mate (Wade et al., 2007).​ Faces and Attractiveness ​ Faces Are Key: Faces are usually visible, unlike bodies that might be covered. So, we often judge attractiveness by someone’s face. People with attractive faces are seen as healthier, sexier, more fertile, and more attractive overall, no matter their WHR (Furnham et al., 2001). This is against the idea that we focus on WHR first when choosing a mate. Instead, faces seem to matter more.​ ​ Facial Symmetry: Symmetry in a face (when the left and right sides are nearly the same) is seen as a sign of health and attractiveness. Symmetrical people also seem to have more confidence, and they perform better at activities like dancing (Brown et  al., 2005).​ Non-Physical Traits Matter Too ​ Personality Counts: Physical appearance isn’t the only thing that matters. A study by Swami et al. (2007) showed that women who were described as extraverted (sociable) were judged as more attractive and friendly than those who were introverted (shy).​ ​ What Women Want in a Mate: Women care about traits like intelligence, stability, and compatibility in terms of politics and religion. Men, however, are more likely to prioritize physical appearance over personality when choosing a partner.​ The Perils of Being Attractive ​ Attractiveness Can Be a Drawback: There is evidence that being too attractive can actually be a disadvantage. For example, in a study by Chu et al. (2007), women preferred attractive men who were of medium socio-economic status (SES), rather than highly attractive, high-status men. Why? Because they thought high-status, attractive men were more likely to just want sex rather than settle down and have a family.​ ​ Financial Risk-Taking: Attractive men might cause other men to take more financial risks. This happens because men in competition with more attractive men might gamble or spend more to increase their own desirability (Chan, 2015).​ ​ Speed-Dating Results: In a speed-dating study (Eastwick and Finkel, 2008), women said that physical attractiveness and earning potential were important, but this didn’t actually predict who they chose during the dating event. Similarly, it didn’t predict who they chose as a partner later on.​ The Matching Phenomenon ​ We Choose Similar Mates: One theory is that people tend to choose a partner who is similar in attractiveness to themselves. This is called the matching phenomenon (Walster et al., 1966). But the reasons for this are unclear – some think it’s because people feel more secure with someone who looks like them or shares their level of attractiveness.​ ​ Self-Perception Affects Judgement: Some studies suggest that people see others through the lens of how they see themselves. For example, if someone feels they are average-looking, they might rate someone else as more attractive than they rate themselves. In one study, people rated their partners as more attractive than they thought of themselves (Swami et al., 2009).​ Monogamy and Parental Investment ​ Why Monogamy Exists: Monogamy (when a couple sticks together and raises children) evolved in species where two parents’ help increased the survival of offspring. In these species, both parents are needed to raise the children successfully.​ ​ Who Contributes More? In monogamous species (including humans), females tend to contribute more to raising children. This is because pregnancy and nursing take a lot of energy. However, while both parents share duties, females usually invest more time and energy into parenting than males.​ Monogamy, Males, and Hormones ​ Hormones and Monogamy: There is a theory that hormones play a role in monogamy. Scientists studied two types of voles (a small mammal) – one species, the prairie vole, is mostly monogamous, while the other, the montane vole, is more promiscuous. Researchers think that hormones may help explain these different behaviors (Young et al., 1998). ​ Hormones and Monogamy: The hormones oxytocin (OT) and vasopressin (AVP) play significant roles in monogamous behavior, especially in prairie voles, where OT promotes mating behaviors in females and AVP influences male preferences for exclusivity and paternal care. However, studies on humans suggest that oxytocin increases behaviors like caring, trust, and generosity, though some of these findings have been debated for their reliability and validity.​ ​ Infidelity and Evolutionary Psychology: Men and women have different reactions to infidelity based on evolutionary theories. Men tend to react more strongly to sexual infidelity, while women tend to be more affected by emotional infidelity. However, when reflecting on real instances of infidelity, both men and women were more distressed by emotional betrayal than sexual betrayal. Personality traits, such as agreeableness and emotional stability, also play a significant role in marital satisfaction and infidelity.​ ​ Mate Poaching: Research on mate poaching suggests that individuals with higher levels of attractiveness and resources are more successful at attracting partners away from existing relationships. Men generally rely on their resources, such as wealth or physical attractiveness, to poach, while women use tactics related to enhancing their own attractiveness. The concept also touches on the psychological and neural mechanisms involved, suggesting that regions of the brain associated with reward (like the ventral striatum) and decision-making (like the orbitofrontal cortex) are activated when men consider pursuing someone already in a relationship.​ These findings highlight the complexity of human attraction and relationships, emphasizing biological, psychological, and social factors that influence mate selection, attraction, and behaviors within romantic contexts.  1.​ Definition of Jealousy: Jealousy is described as a response to the perception of a threat to one's relationship, often caused by an actual or imagined rival (Massar & Buunk, 2010). This emotional response is typically seen as more common in men than women, although the triggers for jealousy tend to differ by sex.​ 2.​ Sex Differences in Jealousy: Men generally exhibit stronger reactions to sexual infidelity, while women tend to be more upset by emotional infidelity, such as their partner forming a deep emotional bond with someone else (Sagarin et al., 2012). These patterns have been found globally, indicating that these sex differences in jealousy are consistent across various cultures and countries (Buss, 2013).​ 3.​ Neural Basis of Jealousy: Research has suggested that these differences in jealousy might be reflected in brain activity. Takahashi et al. (2006) conducted a study where men and women were exposed to jealousy-provoking scenarios. While both sexes experienced similar feelings of jealousy, their brain activity differed. Men showed more activation in the amygdala (involved in emotions like fear and aggression) during sexual jealousy and in the hypothalamus (related to sexual behavior) during emotional jealousy. Women, on the other hand, showed increased activation in the posterior superior temporal sulcus (STS), an area linked to detecting others' intentions and social norm violations.​ 4.​ Priming Jealousy: Studies also indicate that jealousy can be unconsciously triggered. For instance, Massar and Buunk (2010) exposed young women to photos of attractive or unattractive women and then asked them to imagine a jealousy-inducing scenario. Those who were exposed to attractive women reported significantly more jealousy, suggesting that jealousy can be primed or elicited subconsciously.​ Overall, this passage emphasizes the complex emotional response of jealousy, highlighting both the psychological and neural mechanisms involved, as well as the differences in how men and women experience jealousy in response to different types of infidelity. 1.​ Promiscuity and Personality:​ Promiscuity, defined as engaging in sexual activity with multiple partners, has been linked to certain personality traits. Sensation-seekers, individuals with lower levels of conscientiousness, lower agreeableness, higher extraversion, and more antagonistic tendencies tend to report having more sexual partners. Additionally, dominant individuals (both men and women) and those who are personally warm also tend to have more sexual partners than their counterparts (Markey & Markey, 2007).​ 2.​ Perception of Attractiveness in Unrestricted vs. Restricted Men:​ Men who engage in unrestricted sexual activity (i.e., transient sexual relationships) tend to rate women with lower BMI and low waist-to-hip ratio (WHR) as more attractive compared to restricted men. Both unrestricted and restricted men, however, based their attractiveness judgments primarily on BMI, not WHR (Swami et al.,  2008).​ 3.​ Polyandry: High Male and Low Female Parental Investment:​ Polyandry, a rare reproductive strategy where a woman has multiple husbands, is observed in some human populations, particularly in remote Himalayan villages. Here, brothers often share the same wife in order to prevent the dissolution of family farms. This strategy ensures that the family's primary resource, the farm, is preserved and that the woman and her multiple husbands can adequately support the family.​ 4.​ Polygynandry: Group Parental Investment:​ In species like chimpanzees, polygynandry involves promiscuous mating in which both males and females have multiple mates. The primary benefit of this strategy is cooperative care of offspring. Since males are uncertain of which offspring are theirs, they invest in the care and protection of all offspring, contributing to the well-being of the colony. This unity minimizes aggression among males and ensures that the offspring are well-cared for. In some cases, males engage in consortships, forming temporary monogamous unions with females to increase the certainty that the offspring are theirs, but this can come with the risk of injury and reduced paternal investment.​ These reproductive strategies highlight the diverse ways in which humans and other animals adapt to their environments and ensure the survival and success of their offspring, with varying degrees of male and female parental investment. 1.​ Altruism and Evolution:​ Altruism involves self-sacrifice for the benefit of others, often at the expense of the altruist’s own survival or reproductive success. While altruism may appear to contradict the principles of natural selection, sociobiologists explain it as a behavior that has evolved because it benefits the survival of shared genes. This is particularly relevant in situations where individuals help close relatives, which is known as kin selection (Hamilton, 1964). The closer the genetic relationship, the more likely the individual will engage in altruistic behaviors.​ 2.​ Kin Selection and Inclusive Fitness:​ William D. Hamilton (1964) proposed that natural selection favors inclusive fitness, which involves the reproductive success of an individual’s relatives who share many of the same genes. In this framework, altruistic behavior is not about individual survival but about ensuring the survival of shared genetic material across generations. Parenting, for example, promotes inclusive fitness by ensuring the survival and future reproductive success of offspring.​ 3.​ Emotional Closeness as a Mediator of Altruism:​ While genetic relatedness is important, emotional closeness can also play a significant role in altruistic behavior. A study by Korchmaros and Kenny (2001)  showed that individuals were more likely to behave altruistically toward family members with whom they had an emotional bond, even if the genetic relationship was more distant.​ 4.​ Stepfamilies and Altruism:​ There is evidence supporting the idea that altruism is less likely to extend to non-genetic relatives. Studies have shown that children in stepfamilies are more likely to suffer harm, such as child abuse (Daly & Wilson, 1988, 1996), indicating that non-genetic parents or caregivers are less likely to invest in the welfare of unrelated children. This finding aligns with the idea that individuals are more altruistic toward those who share their genes.​ 5.​ Cohabitation and Spousal Violence:​ Research has shown that women in cohabiting relationships are at a higher risk of being murdered by their partner compared to married women (Shackelford, 2001). One explanation for this is that men in cohabiting relationships may feel more insecure and possessive due to the higher likelihood of relationship dissolution. This insecurity can lead to extreme actions to prevent abandonment, such as homicide.​ 6.​ Reciprocal Altruism:​ Reciprocal altruism refers to altruistic behavior directed at non-relatives, which can be explained by the idea that individuals cooperate because it enhances their survival chances. According to Trivers (1971), this form of altruism occurs when individuals help others with the expectation that the favor will be returned in the future. Cooperation and mutual support are common in human and primate societies, as seen in examples such as male baboons cooperating to gain access to mates.​ In summary, altruism, while seemingly paradoxical from an evolutionary standpoint, can be understood through the lens of kin selection and inclusive fitness. Emotional closeness and reciprocal altruism further contribute to the complexities of altruistic behavior, emphasizing both genetic and social factors in human cooperation and survival. 1.​ Sociobiology and its Criticism:​ Sociobiology, introduced by E.O. Wilson in 1975, attempts to explain social behavior through the lens of natural selection and genetic inheritance. It posits that behaviors that maximize inclusive fitness (the reproductive success of an individual's genes) are favored by natural selection. However, critics, particularly evolutionary psychologists, argue that sociobiology oversimplifies human behavior. They contend that it erroneously treats humans as "fitness maximizers," ignoring the complex, context-dependent nature of human adaptation (Buss, 1991, 1995).​ 2.​ Evolutionary Psychology's Response:​ Evolutionary psychologists disagree with the "fitness maximization" view of sociobiology. They argue that humans are not designed to maximize fitness but to solve specific adaptive problems using evolved mental mechanisms. These  mechanisms are domain-specific—for example, solving problems related to mate selection differs from those related to food acquisition or child-rearing. Evolutionary psychologists focus on how these adaptive mechanisms were activated in response to the challenges humans faced throughout evolutionary history (Tooby & Cosmides, 1990; Buss, 1995).​ 3.​ The Criticism of Sociobiology:​ The most intense criticism of sociobiology comes from a political standpoint rather than a scientific one. Opponents argue that sociobiology can justify unethical behaviors, such as the domination of one group over another, by framing them as "natural." Sociobiologists, however, argue that critics confuse the idea of what is "natural" with what is "good" or "superior." They deny that sociobiology inherently promotes such ideologies, asserting that the theory is often misinterpreted (e.g., Nazi racial superiority).​ 4.​ Limitations of Evolutionary Psychology:​ Evolutionary psychology is not without its limitations. Some behaviors, like homosexuality (which does not directly promote reproductive success) and suicide, are difficult to explain through evolutionary theories of natural selection (Confer et al., 2010). Furthermore, there is a lack of concrete evidence about our evolutionary past, making it challenging to provide a full account of human nature. Evolutionary psychologists can only hypothesize about the adaptive challenges humans faced and the corresponding psychological mechanisms that may have evolved in response.​ 5.​ Cultural and Individual Differences:​ While evolutionary psychology is adept at explaining species-typical behaviors and sex differences, it struggles with explaining individual and cultural variations. For instance, while it may explain why women are better at certain tasks, like spatial location memory, it cannot account for the wide variability within these abilities among individuals of the same sex.​ In summary, while sociobiology and evolutionary psychology both provide valuable insights into human behavior, they differ in their perspectives on how evolutionary forces shape behavior. Sociobiology focuses on the genetic transmission of traits, while evolutionary psychology emphasizes the activation of specialized mental mechanisms to solve adaptive problems. Both approaches have strengths and limitations, and the field continues to evolve as new evidence and theories emerge.