From your perspective, the combining of genes from your mother and father at the moment of your conception is the start of a story. But from your genes’ perspective, it’s only the latest page in a story that began long ago. Your genes are the product of evolution, the biological process through which species develop and change across generations.

Let’s look at how evolution works. Then we’ll explore the implications of evolutionary processes for psychology.

The great British biologist Charles Darwin had a simple idea. When it came to questions of evolution, Darwin said, Earth was like a giant, long-running livestock farm (Wright, 1978). On an actual farm, farmers select animals for breeding. If, for example, they want cattle that are large, they select the largest cattle currently on the farm and allow them to mate. In the next generation, they do the same thing: Select the largest cattle, and allow them to mate. After a few generations of selection and mating, the farmer ends up with a population of cattle that are larger than the ones he originally owned. Across generations, the cattle evolve.

Evolution on a contemporary farm differs in one main way from evolutionary processes that occurred on Earth millions of years ago. Back then, there were no farmers; no individuals intentionally selected organisms for breeding. Instead, nature—that is, features of the natural environment—did the selection.

As Darwin explained, in any naturally occurring environment some of an organism’s features are more adaptive than others—that is, the features help the organism survive and reproduce in that environment. A long neck is adaptive if the environment contains food hanging on branches of tall trees. Bright feathers are adaptive if they attract mates. Across generations, features that are adaptive become more frequent; the reproducing organisms pass them down to their offspring via genetic inheritance. Just as large cattle became more common across generations on the farm environment, long-necked or brightly colored organisms (in our example) became more common across generations in certain natural environments. Through this simple process, species evolve.

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When you examine evolved species in their natural habitats, it often seems as if they had been “designed” to fit the environment. However—and this point is key to Darwin’s explanation of how species evolve—there is no “designer” (Dennett, 1995). There merely is nature itself. Natural selection is the process by which, across generations, features of a population of organisms change, depending on whether those features are adaptive in a given environment.

EVOLVED MECHANISMS. The long-term goal of any organism is survival and reproduction. To achieve that goal, organisms have to overcome many short-term obstacles: avoiding a predator, recovering from an injury, detecting a food source, and so forth. Because organisms face specific problems, evolutionary processes tend to equip them with problem-specific evolved mechanisms, that is, inherited biological subsystems that evolved due to their success in solving a specific problem faced repeatedly during the evolution of a species.

An example of a biological evolved mechanism is calluses (DeKay & Buss, 1992). Repeated friction on your hands or feet causes calluses to develop. You don’t have to learn to produce calluses; the callus-producing mechanism is “built in” as part of your evolutionary heritage. This evolved mechanism must have been advantageous over the course of evolution, enabling people, for example, to work longer hours on manual tasks without injuring their hands.

Note, however, that this evolved mechanism does not, by itself, cause you to break out in calluses. If you lead a life of leisure, with no manual labor, your hands won’t have any calluses at all. Calluses only develop in a specific circumstance, namely, one in which you repeatedly experience friction on the skin. Thus, the observed characteristic (the callus) results from a combination of nature (the evolved mechanism) and nurture (the activity that causes the friction on the skin that activates the callus-producing mechanism).

PROXIMAL AND DISTAL CAUSES. The example of calluses highlights another aspect of evolution-based explanations: the distinction between proximal and distal causes. In an evolutionary analysis, proximal causes are events and mechanisms that are present at the time an organism reacts to the environment, whereas distal causes are factors in the distant past that caused a biological mechanism to evolve.

The question “Why did you get that callus?” has two types of answers. One is proximal: Your skin cells reacted to friction on your skin. The other is distal: Across millions of years of primate evolution, organisms that could form calluses were better adapted to their environments, causing the callus-producing mechanism to evolve.

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Principles of evolution are important not only to biology, but also to psychology. Many of the problems faced in the evolutionary past were psychological in nature. To hunt animals or build shelters, people had to organize themselves into coordinated teams. To get help from others when sick or injured, they had to establish social bonds with the people who could provide the help. To reproduce, they had to attract mates. To avoid losing offspring, they needed to be effective parents. The ability to interact socially with others, therefore, was critical to survival and reproduction.

This suggests that evolutionary pressures shaped not only physical characteristics, but also mental ones. People with mental abilities that were adaptive—who could organize groups, establish friendships, attract mates, and parent effectively—were more likely to survive, reproduce, and have offspring who survived and reproduced. These mental abilities, then, were shaped by evolution.

This insight has given rise to a subfield of psychology known as evolutionary psychology, in which researchers explore the evolutionary foundations of contemporary human mental abilities, preferences, and desires (Buss, 1991; Caporeal, 2001).

All evolutionary psychologists agree that Darwinian natural selection produced the contemporary human mind that we possess today. But they do not all agree on the exact nature of that mind and the exact role of evolution in shaping it. Let’s now turn to two views of evolution and psychology. The first is the inherited mental modules perspective.

INHERITED MENTAL MODULES. One popular view is that, thanks to evolution, the human mind consists of numerous mental modules. A mental module is a subsystem of the mind and nervous system that performs a specific task in response to a specific type of environmental input (Fodor, 1983). The retina, at the back of the eye, is an example. It performs one task and one task only: responding to light that enters the eye. It won’t respond to sound. It can’t tell if it’s warm or cold. Because it is specialized to become active only in response to a specific type of input (light), it is considered modular. All psychologists agree that evolutionary processes created the modular mechanism that is the retina.

Some evolutionary psychologists propose that, over the course of evolution, the human mind acquired a large number of additional modules, each of which, like the retina, responds to a type of environmental input faced repeatedly by our evolutionary ancestors (Buss, 1991; Pinker, 1997). Consider a few examples throughout evolution:

The list could go on and on. There could be hundreds of such modules, each of which evolved hundreds of thousands of years ago.

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TRY THIS!

Earlier, in our Try This! activity, you attempted to solve two kinds of problems. (If you didn’t, go to www.pmbpsychology.com and try them now.) You probably found one of them to be difficult—the problem involving a clerical job and the categorizing of documents. The other problem, involving a crackdown against underage drinking, was relatively easy.

Why was one problem easier than the other? In their basic structure, the problems were identical: They both presented a rule to be tested (“If a person has a D rating, then his documents must be marked Code 3”; “If a person is drinking beer, then he or she must be over 20 years old”) and four cards to turn over to see whether the rule was broken.

The psychologist Leda Cosmides (1989; Cosmides & Tooby, 2013) explains that the underage drinking problem was easy because of human’s evolved biological nature. Throughout evolution, people exchanged goods: food, drink, livestock, and so forth. The goods were necessary to survival, so it was important not to be cheated out of them. People who were able to detect cheating attempts were more likely to retain their goods, survive, and reproduce. Through natural selection, therefore, a mental module to detect cheating evolved. The underage drinking problem activated this mental module because it contained a case of potential cheating (underage drinking). The module, in turn, made problem solving easy.

The clerical problem, on the other hand, did not activate an evolved mental module. Over the course of evolution, our ancient ancestors did not spend any time filing documents; thus, there is no “document-filing module” in the human brain to activate. With no specialized mental module to help you, the clerical problem was more difficult—despite its having the same overall structure as the underage drinking problem. The different types of problems thus reveal the role of nature in the structure and functioning of the human mind.

According to evolutionary psychologists, mental modules solve problems by producing preferences. Preferences that evolved in the evolutionary past shape your present likes and dislikes. For instance, why do you like pie, cake, and milkshakes when they can make you fat? During periods of evolutionary history when food was scarce, it was adaptive for humans to prefer high-calorie foods; those who didn’t like those foods were more likely to starve. Today, we inherit that ancient preference. Why do homeowners spend time and money planting flowers and arranging backyard garden landscapes, when they could spend those resources on food and clothing for themselves? Throughout evolutionary history, flowers indicated that food was near. People who were attracted to flowers encountered more food and were more likely to survive and reproduce. Today, we inherit the evolved liking of flowers (Joye, 2007).

In this view, mental modules are specified in your genes (Lickliter & Honeycutt, 2003). Just as genes contain information to build body parts such as a heart and lungs, they contain information used to build neural systems that correspond to each mental module. These genetically specified mental modules are thought to be universal, that is, possessed in basically the same way by humans everywhere (Tooby & Cosmides, 2005). Just as people around the world have the same bodily organs, they have the same “mental organs” (Pinker, 1997).

This viewpoint makes a startling claim. Thoughts and preferences that you think you learned from modern society are, in reality, inherited—“wired in” by evolutionary processes that occurred hundreds of thousands of years ago. Here are two examples.

When men evaluate the physical attractiveness of female figures, they rate “curvy” figures with a low waist-to-hip ratio (relatively thin waist and wide hips) as more attractive (Singh, 1993). Are these preferences learned through social experience? Evolutionary psychology says no; they instead are inherited due to evolution. Throughout evolutionary history, wide hips signaled that a woman possessed body fat that could be used as energy during pregnancy. Preferring wide hips thus was adaptive because it enhanced reproductive success. As a result, the evolutionary psychologist argues, contemporary men inherit a mental module that predisposes them to prefer a small waist-to-hip ratio (Singh, 1993).

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An evolutionary psychologist (Buss, 1989) found that men and women differed in their desires for a mate (i.e., a serious romantic partner). Women emphasized wealth; men’s financial resources, as reflected in job status or the ability to pay for dinner at a nice restaurant, were important to them. Men emphasized looks; they were attracted mostly to women’s physical attributes. Evolutionary psychology claims these preferences are not learned, but inherited (Buss, 1989). Throughout evolution, women have borne the physical burden of pregnancy. It was adaptive for them to seek men with status and resources, who could provide food and shelter during pregnancy and after childbirth. Men, in contrast, were free from the physical burdens of pregnancy and thus had less need for mates who could take care of them. Men’s preference for young, fit women also is said to have an evolutionary basis, because women who are young and fit are more likely to have a successful pregnancy (Buss, 1989). Men who preferred such women thus would have more offspring, and future generations of men would inherit the preference.

THE INHERITED-MODULES HYPOTHESIS: THREE LIMITATIONS. The inherited-mental-modules approach to evolutionary psychology has been promoted with extraordinary zeal in recent decades. Proponents have touted it as not only “a new paradigm for psychology science” (Buss, 1995, p. 1), but also potentially “a fundamental advance for our species” that would benefit from “a true, natural science of humanity” (Tooby & Cosmides, 2005, p. 63). The idea has had great impact in not only psychology, but also society at large. “I seemed to encounter it everywhere I turned,” one writer reports:

During almost every wait in the supermarket checkout line, I would find reference to the evolutionary psychology of human mating on the covers of … magazines. [In newspapers] I would often find articles … about the evolutionary psychology of mating, parent–child relationships, or status seeking. And it seemed to be all over television, and not just on “high-brow” channels like PBS.

—Buller (2005, p. 3)

A main proponent of the mental-modules perspective, Harvard University psychologist Steven Pinker is one of the few psychologists to appear on TIME magazine’s annual list of the most influential people in the world (Wright, 2004).

Acclaim, however, does not imply correctness. In recent years, many scientists have questioned evolutionary psychologists’ claims (e.g., Heyes, 2012). No one doubts the significance of evolution to human behavior, yet the exact ways in which evolution did, or did not, shape contemporary humans’ abilities and preferences is a point of debate. Three factors raise questions about the inherited-modules perspective:

These three limitations are significant, suggesting that an alternative perspective— a refashioned evolutionary psychology—may be needed. To meet this need, researchers have introduced “new thinking” (Heyes, 2012, p. 2091) into evolutionary psychology. Two thoughts are central to these advances. One is that evolution has produced not only mental modules that solve specific problems, but also “general-purpose” mechanisms that help solve a wide variety of problems. Just as your hand is a general-purpose physical tool—that is, a tool you use on an enormous variety of tasks—some elements of the mind are general-purpose mental tools (Heyes, 2012). Consider the mental ability to learn new skills by observing others (see Chapter 7). It is general-purpose, not problem-specific; you can learn to solve an unlimited variety of problems (fixing a household appliance; changing a baby’s diaper; fitting in at a social event) by observing others.

The second idea is one we have discussed earlier in this chapter, namely, that inheritance and experience combine to shape the brain and mind of the individual (Caporeal, 2001; Heyes, 2003, 2012). Increasingly, psychological scientists recognize that psychological abilities are not all “wired in” by evolution. Instead, they develop gradually as people interact with the environment (Karmiloff-Smith, 2009).

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CULTURAL OPPORTUNITIES

In the course of evolution, the “bottom-line” marker of success is reproduction. If members of a species fail to reproduce, the species dies out. You might expect, then, that evolution would have produced a desire to engage in sexual activity that is universal— shared by all human adults.

But that’s not what happened. Instead, people exhibit different sexual orientations, the primary form of a person’s sexual and romantic attraction. People with a heterosexual orientation are attracted to members of the opposite sex, those with a homosexual orientation are attracted to members of their same sex, and bisexual individuals are attracted to both men and women.

This three-part classification, however, does not capture the full range of variation in sexual orientation. For example, a recent study found many women who reported being “mostly” heterosexual: attracted to men romantically and sexually, but experiencing some degree of sexual attraction to women as well (Thompson & Morgan, 2008).

Surveys (e.g., Kinsey, Pomeroy, & Martin, 1948) provide estimates of how common the different sexual orientations are. It’s difficult to specify precise numbers, however, because survey responses vary depending on the exact question asked. In surveys in the United States and the United Kingdom, about 1.5% of men reported same-sex sexual activity within the past year. But a larger number (greater than 6%) said yes when survey items asked men if they had ever had same-sex activity at any time in their lives. Women’s rates of homosexuality in these surveys were lower than men’s (Dawood, Bailey, & Martin, 2009).

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EVOLUTION AND SEXUAL ORIENTATION. Although these numbers may sound small, in some ways they are big. If the 6% estimate is correct for the world at large, then nearly 300 million people have engaged in same-sex activity (there being nearly 5 billion adults overall).

This big number raises a big challenge for evolutionary theory. Homosexual orientation impairs reproduction because, of course, heterosexual pairing is the only way to reproduce. Because people who are exclusively homosexual do not leave behind offspring, you might expect that evolution would have eliminated homosexual preferences from our species. But that hasn’t happened, as nearly 300 million people could attest. How, then, can evolutionary theory explain the existence of homosexual orientation?

Scholars have suggested a range of answers to this question (Bailey & Zuk, 2009). One interesting possibility is that some genes have different effects in men and women. In men, they increase the likelihood of homosexuality. In women, the same genes increase the likelihood of childbirth; that is, they boost women’s biological capacity to have offspring. Initial evidence supporting this possibility came from a study in Italy (Camperio-Ciani, Corna, & Capiluppi, 2004). Homosexual and heterosexual men filled out surveys indicating the number of children borne by women in their family (their mother, aunts, etc.). Female relatives of homosexual men were found to have more children, on average, than relatives of heterosexual men. A follow-up study yielded the same result: larger numbers of children among women who are relatives of homosexual men (Iemmola & Camperio-Ciani, 2009).

This result can explain how homosexuality may have evolved. The explanation is that homosexuality per se did not evolve. Instead, what evolved is a particular gene that, for human reproduction, has both benefits and costs. The benefit is that it increases childbearing among many women; the cost is that it reduces reproduction among some men (those who are homosexual). In the course of evolution, the benefits outweighed the costs.

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GENETICS AND INDIVIDUAL DIFFERENCES IN SEXUAL ORIENTATION. Another source of evidence about the roots of sexual orientation is one discussed earlier in this chapter: twin studies. Researchers compare MZ to DZ twins to determine whether genetic factors contribute to individual differences in sexual orientation.

You learned earlier that genetics influences a wide range of psychological tendencies. Sexual orientation is no exception. MZ twin pairs are more similar in sexual orientation than are DZ twin pairs. In one study that focused on twins and homosexuality, researchers recruited homosexual research participants who had a twin and then determined the sexual orientation of the other member of the twin pair. Among women, 48% of MZ co-twins were homosexual or bisexual, compared with 16% for DZ twins. Likewise, among men, the MZ percentage of 52% was higher than that among DZ twins, 22% (Dawood, Bailey, & Martin, 2009).

A study of thousands of twins in Sweden similarly indicated a role for genetics (Långström et al., 2010). By comparing MZ to DZ twins, researchers found that genetics explains slightly more than one-third of the overall variability in sexual orientation. Shared environmental factors—having the same parents, the same socioeconomic status (SES), living in the same neighborhood—had no effect at all.

Genetic effects thus are significant to sexual orientation. But one-third of the variance—the finding in the Swedish study—still leaves two-thirds of the variability in sexual orientation unexplained. What other factors could be influential? Research has uncovered a surprising possibility, as you’ll see next.

PRENATAL ENVIRONMENT AND SEXUAL ORIENTATION. When people discuss nature and nurture, they usually compare genetic influences (nature) to environmental influences that people experience after they are born (nurture). There’s something else to consider, though: the prenatal environment, that is, the biological environment experienced before a person is born. Findings indicate that prenatal factors influence sexual orientation.

One clue to this influence is that men with larger numbers of older brothers are more likely to be homosexual. Each older brother increases the odds of homosexuality by about 33% (e.g., a .06 chance would rise to .08; Dawood et al., 2009). You might expect the increase to be caused by nurture, not nature; maybe spending time with older male siblings increases attraction to males. But research reveals something else: an impact of the prenatal environment. Key evidence comes from studies of adopted children (Bogaert, 2006). To see their value to research, imagine two types of male children who experience the following environments:

Findings show that children of the second, but not the first, type are more likely to be homosexual. Having older siblings increases rates of homosexuality even if siblings are not raised together (Bogaert, 2006). How can this be? One explanation is maternal immune response (Bogaert, 2006). The maternal, female immune system recognizes a male child as biologically “foreign” and produces antibodies. With more male children, there are more antibodies, which may affect fetal brain development in a way that influences sexual orientation.

Additional evidence that the prenatal environment influences sexual orientation comes from an unlikely source: finger length. The link between finger length and sexual orientation involves hormones called androgens (hormones are biochemicals that travel through the body and influence bodily organs; see Chapter 3). In the prenatal environment, androgens stimulate the development of male sexual characteristics and also affect the development of the hand (Hines, 2010). Men and women thus differ in relative finger length as follows:

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The effect of androgens on both sexual characteristics and finger length leads to an interesting hypothesis: 2D:4D ratio may predict sexual orientation. To test it, researchers conducted a meta-analysis of studies involving thousands of people in Europe and North America (Grimbos et al. 2010). They found that women with a higher (more typically female) 2D:4D ratio were more likely to be heterosexual. Among men, 2D:4D ratio and sexual orientation were not linked in this study. However, research in Japan has not only confirmed the relation between 2D:4D ratio and sexual orientation among women, it has also provided evidence of a link among men (Hiraishi et al., 2012). Overall, findings to date indicate that the “nature and nurture” of sexual orientation may include a causal factor beyond genetics and the social environment: androgens in the biological environment in which the fetus develops.

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