The Impact of Evolution on Human Behavior

Gordon G Gallup Jr., Jeremy Atkinson, Daniel D Moriarty. 21st Century Psychology: A Reference Handbook. Editor: Stephen F Davis & William Buskist. Volume 1. Thousand Oaks, CA: Sage Publications, 2008.

Evolutionary psychology (EP) uses the theory of evolution to predict and explain human behavior. EP posits that the brain, just like any other organ (e.g., lungs, heart, etc.), has been shaped by natural selection and thus can be best understood from an evolutionary framework. This chapter outlines the theory of evolution and provides a number of concrete examples of EP using the latest research, complete with clinical and societal implications. Additionally, there is a separate section that addresses many common criticisms of EP and evolution.


Forget everything you thought you knew about evolution. Evolution does not create ideal creatures or abilities, nor is it based on the “survival of the fittest.” Nobody is perfect and everybody dies; death and failure are an inevitable consequence of life. When it comes to evolution, it is not a question of whether you live, die, succeed, or fail; it is a question of whether you reproduce. Evolution is not about competition for food or other scarce resources but rather competition for genetic representation in subsequent generations. It is ultimately based on the perpetuation of genes, not individuals.

In its most rudimentary sense, evolution implies change. The simplest way to define evolution and represent such change is in terms of gradual changes in the composition of a gene pool over time. A gene pool is a hypothetical conglomerate of all of the genes being carried by all of the members of a particular species. In any particular gene pool, if individuals carrying certain configurations of genes leave relatively more (or relatively fewer) descendants than those carrying another configuration of genes, these gene frequencies will become progressively more (or less) prevalent in subsequent generations. In other words, the composition of the gene pool at any particular point in time is a direct reflection of the reproductive success of individuals in the immediately preceding generation. You could be the strongest, most disease resistant, most intelligent person alive, but if you don’t reproduce your contribution to evolution is zero. This means that the decision to remain childless is equivalent to committing genetic suicide; the unique configuration of genes that make you who you are will not be included in the human gene pool in the future.

In order for anything to evolve, the probability of passing on its genes must be positively reinforced. An adaptive trait, therefore, is one that confers a reproductive advantage by increasing the probability of passing on one’s genes, while a maladaptive trait is one that diminishes the likelihood of producing descendants. From this vantage point, selection can be defined as a correlation between the genetic basis for a trait and the probability of reproductive success. The size of the correlation determines the rate of selection, whereas the sign of the correlation determines the direction of selection. If the correlation is positive, there will be selection for the trait and it will become more prevalent; if the correlation is negative, there will be selection against the trait and it will eventually disappear. A few examples of adaptive traits that have been important to the evolution of modern humans are increased cranial capacity, bipedalism, and self-awareness.

Common Misconceptions of EP

Evolution works to improve the species. Evolution does not occur by design. Evolution occurs by selection, and the raw material for such selection consists of nothing more than random genetic accidents (mutations). Evolution does not work to improve the species, only the probability of reproductive success. The fact that over 99 percent of all the species that ever lived have gone extinct suggests that evolution works to the demise of most life forms.

Evolution has a purpose. Evolution lacks a conscious, intentional, or deliberate component. Each species is a byproduct of a complex array of random genetic accidents. Differential reproduction is the key to evolution and what promotes and maintains various adaptations.

Evolution sustains human contentment. Evolution does not work to promote human happiness. The capacity for happiness, sadness, jealousy, envy, and all of the other human emotions are a by-product of evolution. For example, the ability to experience pain is an adaptive capacity. Organisms that experience pain learn to refrain from engaging in behaviors that are not in their biological best interests, such as putting one’s hand on a hot stove. Likewise, happiness has been shaped by selection to occur when it is in our best interest to engage in a specific behavior.

EP tells “just-so stories.” A common criticism of EP is that it generates an adaptive story that conveniently explains all the data amassed to date, and then claims that it has generated a perfect theory because all the data are explained! EP is based on science, not artful storytelling. Any hypotheses generated by EP must be parsimonious and testable, and the research results must be replicable by other scientists. If the results of research do not fit the theory, then either the theory must be modified to fit the evidence or the theory must be discarded. Furthermore, a theory must generate novel predictions. If a theory fails to fulfill any of these tenets, then the problem is not with science, but rather the theory.

Evolution is only a theory. The common usage of the word “theory” implies that there is no strong evidence in favor of the idea or concept, or that plausible alternatives exist. Evolution is based on a huge body of accumulating evidence from many scientific disciplines, including biology, geology, mathematics, genetics, and of course paleontology. There is no other scientific theory that can even begin to explain this body of evidence.

Evolutionists don’t all agree on what evolution is (or, there are scientists who disagree with evolution). In any science, progress is based on controversy and disagreement. New ideas are tested, old ones rejected. EP is the same in this regard. However, one idea that is never disregarded or dismissed is evolutionary theory. Scientists disagree all the time about the nuances of evolutionary theory, but they never question evolution itself because there is so much diverse evidence in support of it.

Intelligent design disproves evolution. Intelligent design, try as it might, is simply not science. At some level it ultimately relies upon an unknown, unseen, and unproven agent that supposedly directs cosmic affairs. In the last analysis, intelligent design is based on faith, not fact. Evolution, on the other hand, is based entirely on scientific evidence.

EP is immoral. EP and evolution use science to describe the world around them, including human behavior. Just because EP finds that humans have violent impulses or that murder is a way to increase your fitness does not mean that this is the way society or humanity ought to be. EP can provide valuable insight into why people break the law, but it can’t be used to condemn or condone breaking the law. Do not fall victim to the naturalistic fallacy: Simply because something evolved does not mean that it is right or justifiable. Nor does it mean that it cannot be changed.

EP is antifeminist. That males and females are biologically different is a fact. There is also a lot of evidence to show that males and females are psychologically different. EP does not attempt to justify such differences; rather, it is an attempt to study and better understand these differences. Males can be great parents and females great generals. Denying EP simply on ideological grounds is unscientific.

EP is antireligious. Simply because EP is based on science does not mean that there is an inherent disconnect between it and religion. Indeed, the mere existence of religion may be a by-product of evolution (Maser & Gallup, 1990). EP explains the adapted mind, and religion attempts to find the place of humanity in the cosmos. Perhaps the best way to resolve the issue is to ask whom one would rather see for a serious medical illness—a doctor or a priest? Many people would answer both, yet while both perform important roles, only one can legally practice medicine. Additionally, most major religions now concede that the tenets of evolution are not contrary to their religious teachings, including such a statement by Pope John Paul II.

Evolution and EP

In laymen’s terms, EP looks for the adaptive benefits of specific human behaviors. Adaptiveness can be measured by determining the genetic fitness of the individual, or how many copies of an organism’s genes are passed on to subsequent generations as a result of the behavior in question. In lieu of actual progeny, EP frequently employs substitute measurements such as the number of sexual partners a person has had, his or her attractiveness or status (as rated by members of either or both sexes), or his or her success in a particular endeavor or field (e.g., salary, GPA). However, as we will outline, there are myriad ways to measure fitness. EP also looks for and explains why some behaviors in a modern setting appear to be maladaptive.

Maladaptive Behaviors

Many people are afraid of spiders and snakes. Most people are far less afraid of speeding cars or driving above the posted speed limit. However, the chances of being killed in a traffic accident are much higher than of being killed or injured by a snake or a spider. Why do people show such “maladaptive” fearlessness of traffic? The answer is quite simple: Snakes and spiders posed significant problems to people during human evolutionary history; automobiles did not. Evolution is not a forward-looking process. Current adaptations were shaped by what has happened in the past. Novel situations, like weightlessness, crack-cocaine, or automobiles, would take thousands of years to shape specific evolutionary adaptations.

Costs and Benefits of Adaptations

Evolution never creates a perfectly adapted organism, even in a traditional environment. This is because each adaptation entails costs as well as benefits. For example, the benefits that derive from increased cranial capacity in humans had to be balanced against the increased risk of mortality during childbirth; bigger heads have a greater chance of getting stuck in the birth canal, which can result in the deaths of both mother and child. Even something purely behavioral, like a susceptibility to arachnophobia, incurs costs to the organism; it has to set aside space in the brain to maintain the behavior and spend energy to develop it while maturing. Add to this the chance that the behavior may never be used or that the behavior may develop incorrectly (many people often lose consciousness when suddenly confronted with a snake or spider) and the benefits do not always outweigh the costs. However, if, on average, the benefits exceed the costs, then the behavior in question will contribute toward genetic fitness and the trait will be maintained in the gene pool. The cost benefit ratio can be illustrated by examining your behaviors and their reproductive consequences. People who go to college have fewer children, on average, than people who do not, which would suggest that they have reduced genetic fitness. On the other hand, people who go to college tend to make more money, which increases the likelihood of the survival of each child, thereby increasing his or her chances of marrying and having children as well. Therefore, the reproductive costs of attending college can be offset by resulting economic advantages, which can increase your genetic fitness by increasing the chances that your progeny and their progeny will live to reproduce. During evolution, organisms that consistently behaved in ways in which the reproductive costs exceeded the benefits would have been selected against, and only those that behaved in ways in which the benefits more than compensated for the costs were able to prevail.

Inclusive Fitness and Cost/Benefit Ratios

Competition with other individuals for genetic representation in the next generation involves more than your reproductive success. Through a process known as kin selection, you also have a vested interest in the reproductive success of your relatives. This is because you share approximately 50 percent of your genes in common with members of your immediate family. Therefore, if you behave in ways that enable your brother or sister to produce two children that they would not otherwise have had, it would be equal to producing a child of your own. Likewise, because you share 25 percent of your genes in common with each of your nieces and nephews, any action that costs you one child but produces four children for one of your nieces would be neutral from a cost/benefit point of view. Taken together, this means that every human has a selfish, vested interest in aiding his relatives as long as the benefits outweigh the costs.

Sex Differences

Contrary to popular opinion, there is no such thing as equality when it comes to sex and reproduction. Because men and women have very different reproductive best interests, the ways in which they maximize their fitness can be very different; each sex faces different constraints in reproduction and thus has different costs and benefits associated with the same action or choice. In order to understand these cost/benefit differences, the three underlying biological dimensions that distinguish males from females will be examined.

Genetic Assurance

Females have a significant advantage when it comes to the question of sharing genes in common with their offspring. Maternity is always certain. Mothers have an ironclad guarantee of sharing 50 percent of their genes in common with each of their children. Paternity, on the other hand, is rarely certain. Because of the possibility of rape or female infidelity, males have to contend with the prospect of being cuckolded (i.e., being duped into caring for children sired by other males). The incidence of nonpaternity can be substantial. Among males with low paternity confidence (i.e., those that believe they have reason to question the paternity of their ostensible children), the worldwide incidence of nonpaternity is as high as 30 percent (Anderson, 2006).

Parental Investment

Another important underlying biological dimension that distinguishes males from females is the issue of investment in children. Whereas the benefits that accrue to your fitness as a consequence of producing a child are the same whether you are a male or a female, the costs are not equally distributed. Females pay the lion’s share of the parental investment tab. There is no such thing as a “Dutch treat” when it comes to reproduction. Females are the ones who get pregnant, experience childbirth, and serve as the primary caretaker for the child. This latter point is particularly true of mammals, for which the mother’s milk is the primary source of nutrition for the offspring. Among many sexually reproducing species, the male’s role in reproduction is to simply serve as a source of complementary gametes. Whereas the male’s role in reproduction often focuses on insemination, for females insemination is the mere beginning of the reproductive process, a process that can take years of investment coupled with extended periods of caring and commitment by the female until the child reaches adolescence and can begin to fend for itself.

Reproductive Potential

Sperm are plentiful, but eggs are scarce. A single human ejaculate can contain as many as 250 million or more sperm. Every male produces millions of gametes on a daily basis. In stark contrast, females are born with all of the eggs they will ever have, and ovulation usually involves the release of a single ovum. Women typically ovulate only once during a normal menstrual cycle.

When a woman gets pregnant, it puts her capacity for further reproduction on hold. Pregnancy produces hormonal changes that serve to inhibit further ovulation, and as a consequence pregnancy prevents reimpregnation. Moreover, breast-feeding also leads to hormonal changes that serve to inhibit ovulation (a result called lactational anovulation), and during human evolutionary history mothers probably breast-fed infants for two to three years or longer.

Not only do males have an almost (theoretically) unlimited capacity to reproduce, but they also remain reproductively viable much longer than females. Assuming the typical male begins to produce viable semen at 15 years of age, many males continue to be capable of having children well into their 60s and even 70s. Thus a male who lives to be 75 years old may be reproductively viable for up to 80 percent of his life. If given access to enough reproductively viable women, a man could produce hundreds of offspring in a lifetime. Females, however, have to contend with a much more limited reproductive capacity and a truncated reproductive life span. As a consequence of menopause, most females lose the capacity to reproduce long before they die. Assuming that females begin to ovulate at about age 15 and menopause can occur in the late 30s and early 40s, females are capable of having children for only about 35 percent of their lives.

Reproduction is very costly for females. Unlike males, females can ill afford to make many reproductive mistakes. Thus females have a strong interest in the other 50 percent of their child’s genes that derive from the father. Evolutionary theory would predict that women should be careful comparison shoppers when it comes to mate selection. Whether a woman’s children can compete effectively for a limited number of reproductive opportunities in subsequent generations and perpetuate her genes would be influenced not only by the genes that she gives them but also by the characteristics they get from the father. Clearly, females who made judicious mate choices would have left more descendants than those who were indiscriminant about whom they had sex with.

On the other hand, because paternity is uncertain and males have an enormous capacity to reproduce, men have been selected to be opportunistic maters. This point is made in a compelling way by the results of the following classic study (Clark & Hatfield, 1989). Both male and female college students were approached individually by an attractive member of the opposite sex (a confederate of the experimenter), who then made one of three graded hypothetical invitations. Students were invited to (a) date the person, (b) go back to the person’s apartment, or (c) have sex with the person. Whereas 50 percent of both the male and female students agreed to the date, only 6 percent of the females agreed to return to the person’s apartment, and none accepted the invitation to have sex with someone they did not know. In stark contrast, more than 67 percent of the males agreed to go back to the person’s apartment, and 75 percent agreed to have sex.

Mental Modules

The next four sections examine a specific adaptive behavior. The section on incest avoidance introduces the concept of a mental module—an adaptive behavior that has evolved to solve a specific problem, task, or environmental feature encountered by most members of a species or sex. A mental module can evolve if the same situation is encountered over many generations; the preceding example of our species’ susceptibility to arachnophobia is a good example of a mental module.

Incest Avoidance

A good example of the ability of evolutionary psychology to explain existing data, discount alternative theories, as well as generate and test novel hypotheses can be found by examining incest avoidance. There is ample evidence to suggest that the incest taboo is the result of an innate, evolved mental module, as opposed to a result of cultural values or society (e.g., the environment).

To begin with, there are large fitness costs associated with close-relative mating (i.e., between full siblings or offspring and parents). Much of this cost is due to inbreeding depression. All individuals carry a number of deleterious mutations that are masked because they also have a functional copy of the gene. Most of these mutations are inherited from parents and grandparents. Thus, if you mate with a close relative, you dramatically increase the chances of receiving two defective copies of the same gene. Researchers have documented inbreeding depression in many species. In humans, hemophilia, infertility, and microcephaly have all been traced to inbreeding. Additionally, a number of genes such as those that regulate the immune system maximize an organism’s fitness when they are all different (i.e., the organism is heterozygous for the genes in question). Acquiring a novel set of these genes requires mating with a nonrelative (Potts, Manning, & Wakeland, 1991). As with inbreeding depression, mating with a close relative would increase your chances of receiving two copies of the same gene, and lead to a depressed immune system. In humans, matings between close relatives result in at least twice the rate of mortality and morbidity compared to control populations (Adams & Neel 1967; Seemanova, 1971).

Second, incest is a universal taboo (Brown, 1991) and is rarely practiced in human cultures. In almost every society, both past and present, there are very stringent rules prohibiting either sexual relationships or marriages between closely related individuals (Bixler, 1981). Furthermore, in the few societies that do condone a form of incest, the incestuous relationships have been almost entirely limited to the royalty or nobility (Bixler, 1982a, 1982b). Likewise, in the multitude of animal species examined to date, close-relative incestuous matings generally comprise less than 2 percent of all matings (Harvey & Ralls, 1996)

Third, there is also a large body of evidence that incest avoidance is triggered by mechanisms activated by cohabitation at a young age; for example, if you lived together with someone as a child, you will not be sexually interested in them as an adult. In other words, the fact that familiarity breeds contempt may be the result of selection against inbreeding. Some of the best evidence in support of this comes from a detailed study of the marriage patterns of former members of a kibbutz (a communal housing complex for children in Israel). Sherper (1971a, 1971b, 1983) found that out of 2,769 subjects, not a single one married a peer with whom they had been continuously raised for the first six years of life. Furthermore, detailed interviews with 65 individuals found only one incidence of postpubertal heterosexual activity among them, in which the person in question joined the kibbutz group at age 10.

Another line of evidence comes from Sim Pua marriages in China. These marriages, known as “minor form” marriages, involve the transfer of the bride to the groom’s home at a young age, often as young as three years old. Wolf (1966, 1968, 1970, 1995) compared 132 Sim Pua marriages to 172 traditional marriages and found higher rates of divorce, separation, and adultery in Sim Pua marriages. Most interestingly, Sim Pua marriages produced 30 percent fewer children than regular marriages. These results are consistent with the suggestion that there is an innate incest taboo mechanism operating in humans mediated by early childhood rearing experiences.

There are a number of implications that arise from this analysis. Because the formation of the innate incest taboo requires cohabitation at young ages, families that don’t expose siblings to one another as young children risk the possibility of incestuous relationships between the siblings later in life. Likewise, there is a risk of “quasi”-incestuous relationships between stepsiblings if they were not raised together as young children.

What we know about inbreeding depression demonstrates that the biblical account of human origins based on the story of Adam and Eve is untenable. This account implies that human origins were inherently incestuous. After Adam and Eve reproduced and had children, the only way to get to the next generation would have been for their children to have sex with one another, or for the children to have sex with their parents. Because there simply are no other alternative means by which their genes could be propagated, the resulting inbreeding depression would have had disastrous consequences for the human species.

Signaling and Measuring Genetic Fitness

The next three sections examine how humans detect genetic fitness in others and signal their own genetic fitness. There are many mental modules that function to determine the genetic fitness of other people. We selected these modules to focus on honest signals of fitness, signals that are truly indicative of the ability of an individual to pass on his or her genes. Each remaining section title is an example of an honest signal of genetic fitness. However, humans have also been selected to subvert honest signal detection modules. This is frequently accomplished by employing dishonest signals of fitness, signals that mimic honest signals in order to fool other people into thinking you (or your kin) are more genetically fit than you really are.

Paternal Resemblance

A common refrain among mothers is that all of their children look like their father, not them. In order to appreciate why this is the case, recall that women have at least one substantial reproductive advantage over men: Females can be virtually certain of sharing 50 percent of their genes in common with each of their children. However, as a consequence of rape or female infidelity, males have to contend with paternal uncertainty and being cuckolded (investing in offspring sired by other males).

Paternal resemblance, or the degree to which children look like their father, is one hypothesized phenotypic feature that fathers could use to gauge relatedness to their purported offspring. By presenting people with pictures of infants and asking them to identify their parents out of an array of adult photos, Christenfeld and Hill (1995) found that participants were better at matching children’s pictures to their father than to their mother, although researchers have not always replicated this finding (Brédart & French, 1999). However, another study that had third parties directly compare the degree of resemblance of children to both parents found that after the age of three, boys resemble their fathers more than their mothers (Alvergne, Faurie, & Raymond, in press). If children show a paternal resemblance bias it may be a consequence of the fact that during human evolutionary history males invested preferentially in children who looked like them. Because of the enormous costs of cuckoldry, males who were indifferent to the question of paternity would have been at a significant disadvantage compared to males who cared about the paternity of their mate’s children.

To test this hypothesis, Platek, Burch, Panyavin, Wasserman, and Gallup (2002) combined individual pictures of college students with one of several infant pictures using a computerized morphing program. Students were then presented with an array of five baby pictures that had been morphed with different adult faces, including the one that had been combined with their own face. When asked to make hypothetical investment decisions about the babies in these pictures (e.g., which one of these children would you adopt, or spend $50 on?), men, in direct contrast to women, showed a clear preference for investing in children who shared some of their own facial features. Another study that interviewed British men found paternal investment increased the more a father believed his children resembled him, and this effect was more pronounced for divorced fathers (Apicella & Marlowe, 2003). These results suggest that men do in fact invest preferentially in children as a function of their shared resemblance.

The existence of a paternal resemblance investment bias has been replicated in several subsequent studies (e.g., Anderson, Kaplan, & Lancaster, 2007; Platek, Critton, et al., 2003), and has been extended to show that male (but not female) brains are uniquely activated by pictures of children they resemble (Platek, Rainesa, et al., 2003).

There is even evidence that males modulate the importance of specific physical features based on how relevant these cues are to their specific phenotype in determining paternal certainty. Laeng, Mathisen, and Johnsen (2007) found that blueeyed men (but not brown-eyed men or women of either eye color) found pictures of women with blue eyes more attractive than the same pictures featuring the same women with brown eyes. These findings remained statistically significant even after controlling for the color of either parent’s eyes. Researchers hypothesized that blueeyed men showed this difference because a blueeyed man who mated with a blue-eyed woman would have only blue-eyed offspring. Thus, a child who exhibited a different eye color would obviously not be his. It is interesting to note that brown-eyed men, who would not gain any additional information by preferring blue-eyed women, exhibited no preference for eye color. The same study also found evidence for assortative mating: Blueeyed men preferentially choose light-eyed partners.

Given the importance of paternal certainty, it is no wonder that there is a substantial body of evidence that mothers and the matriline comment more frequently on an infant’s purported paternal resemblance than on its maternal resemblance (Daly & Wilson, 1982; Regalski & Gaulin, 1993). For example, McLain, Settersa, Moultona, and Pratt (1999) found that mothers of one- to three-day-old newborns made more comments about paternal than maternal resemblance, particularly when the father was present.

Preferential investment based on paternal resemblance has some interesting practical implications: In the case of adoption, by matching features of the adopted children (e.g., hair color, eye color) with features of the adoptive father there is reason to believe you might be able to promote better adoption outcomes. Indeed, these findings would even suggest that the risk of child abuse might be inversely proportional to the degree of paternal resemblance. There is some evidence to support this hypothesis: Burch and Gallup (2000) found that for men enrolled in a domestic violence treatment program, paternal resemblance was positively correlated with the self-reported quality of the men’s relationships with their children and inversely proportional to the severity of injuries suffered by their spouses. In light of this study and the findings of Laeng, Mathisen, and Johnsen (in press), social service agencies might do well to target instances of obvious failures to show paternal resemblance for special intervention.

Both fathers and mothers act in ways that suggest paternal resemblance is very important. As with all evolved behaviors, the mental processes involved in assessing paternal resemblance need not be explicitly known to the individual. However, as both fathers and mothers actively and easily assess paternal resemblance, and modulate their behavior accordingly, it is quite probable that paternal resemblance is an example of an evolutionary adaptive behavior that is, at some level, consciously processed.

Fluctuating Asymmetry

Not everyone has features on one side of their body that perfectly mirror the features on the other side. For example, the length of the ring finger, the diameter of the wrist, or the length of the ear lobe may be slightly different from one side to the other. Researchers believe that random morphological deviations from perfect bilateral symmetry, known as fluctuating asymmetry (FA), are a consequence of stress-induced developmental instabilities that occur primarily during prenatal growth. Although the body is programmed to develop symmetrically, it is thought that environmental stressors (e.g., diseases, parasites, malnutrition) disrupt the normal developmental program. Individual differences in the magnitude of FA are taken as evidence for underlying genetic differences in the ability to buffer against such stressors. An individual with high FA thus would have either an increased exposure or a compromised ability to mitigate the effects of environmental stressors. Whatever the cause, it would be an indicator of decreased fitness. Indeed, there is a wealth of evidence that low FA individuals have increased fitness.

Researchers have correlated elevated FA with increased vulnerability to disease as well as higher levels of morbidity (Livshits & Kobyliansky, 1991), whereas low FA has been correlated with increased genetic, physical, and mental health (Thornhill & Møller, 1997) as well as increased fertility (Thornhill & Gangestad, 1994) and athleticism. Additionally, intelligence appears to be related to fluctuations in FA, with low FA males scoring significantly higher on standardized intelligence tests (Furlow, Gangestad, & Thornhill, 1997).

In light of this evidence, it should not be surprising that people preferentially respond to low FA (high fitness) individuals. For example, men and women with low FA are consistently judged as being more attractive than those with high FA (Thornhill & Gangestad, 1994). Low FA men have more sexual partners, and they are more likely to engage in instances of sexual infidelity (Gangestad & Thornill, 1997). Women who are in committed sexual relationships with low FA men experience more frequent orgasms than those with high FA partners (Thornhill, Gangestad, & Comer, 1995). Even dancing ability appears to be related to FA—low FA dancers were rated as better dancers than their high FA counterparts, particularly by low FA individuals (Brown et al., 2005).

Low FA is even related to body odor. Thornhill and Gangestad (1999a) had a sample of male volunteers agree to wear freshly laundered T-shirts to bed and return the shirts to the laboratory the next day. The shirts were placed in coded plastic bags and each of the donors was measured for deviations from bilateral symmetry across a variety of physical traits. Women, who did not know the source of the shirts or the identity of the men who had worn them, smelled the shirts and rated the extent to which they found the odor of the different T-shirts to be attractive. Women who were not taking birth control pills and were in the ovulatory phase of their menstrual cycle rated the shirts worn by low FA men as smelling more attractive than those worn by high FA men. In other words, fertile women found the body odor of low FA men more attractive.

People also find that low FA individuals sound more attractive. Hughes, Harrison, and Gallup (2002) measured FA in male and female college students and had them individually count from 1 to 10 while their voices were being recorded. These recordings were then rated for attractiveness by a sample of men and women who did not know and had not seen the people whose voices had been recorded. Hughes et al. discovered that the voices of people with low FA were rated as being more attractive than those of people with high FA.

Mate choice is an important dimension of fitness. When it comes to competing for genetic representation in subsequent generations it is important to pick a mate who has high-quality genes. Offspring with high-quality parents have a better chance of showing traits that will bode well for their own reproductive success, which in turn will serve to propagate their parents’ genes. The above evidence strongly suggests that humans have been selected to prefer a variety of subtle features in mating partners that are correlated with low FA in order to maximize their genetic fitness.

Facial Attractiveness

Faces are important. We recognize people based on their facial features, and the allure of those features plays an important role in interpersonal attraction. Many people believe that facial attractiveness is a social construction, driven by learned, arbitrary sociocultural influences. However, data have begun to emerge that challenge this position (for an earlier review of this evidence see Thornhill & Gangestad, 1999b). Not only is there relatively good consensus among people as to which faces are attractive, but attractive faces also appear to be a cross-cultural universal. That is, faces that people in Asia find attractive are also rated as attractive by people in North America, and vice versa. Ratings of facial attractiveness are consistent not only across cultures and ethnic groupsbut across sexes, sexual orientations, and ages as well. Coupled with the fact that even neonates spend more time looking at faces that we judge attractive, these data suggest that facial attractiveness may have a biological component.

There is growing evidence that individual differences in facial attractiveness signal important underlying biological properties. People with attractive faces are healthier (Shackelford & Larsen, 1999), and they tend to have lower FA (Grammer & Thornhill, 1994). Contrary to the social constructionist position, we may have been selected during human evolutionary history to find certain faces attractive because they were associated with honest signals of fitness. In other words, there may have been selective benefits to mating with people based on their facial features.

Several recent studies provide strong support for this position. In one recent study, Soler et al. (2003) took facial photographs and collected semen samples from 66 male college students. Semen quality for each man was determined by measuring sperm count, sperm motility, and sperm morphology. Then, a large sample of women were shown the photos of these men and asked to rate them for attractiveness. As with low FA males, ratings of facial attractiveness were significantly correlated with semen assay results; men with attractive faces tended to have higher-quality semen.

In another study, Henderson and Anglin (2003) selected 50 facial photographs, equally representing men and women, from old high school yearbooks. Using a public database that included the dates of birth and death for the people in these yearbooks, longevity data were calculated to the nearest month. Male and female college students were instructed to rate each of the yearbook photos for attractiveness and health. Ratings of facial attractiveness were highly correlated with ratings of health. Moreover, ratings of facial attractiveness of the high school photos were significantly correlated with longevity. Both men and women with attractive faces tended to live longer.


EP has the ability to explain and predict many facets of human behavior. Based on the principles of evolution, it follows that behavior is also subject to natural selection, just like physical features. EP can be used to suggest new clinical and social treatment programs as well as to examine existing regimes for their efficacy. In recent years, EP has been gaining acceptance in all fields of mainstream psychology. Some people believe it has the potential to unite these disparate fields under a single and overarching theoretical framework.