By Alan S. Miller Ph.D., Satoshi Kanazawa Ph.D., published on July 1, 2007 - last reviewed on December 10, 2012
Human nature is one of those things that everybody talks about but no one can define precisely. Every time we fall in love, fight with our spouse, get upset about the influx of immigrants into our country, or go to church, we are, in part, behaving as a human animal with our own unique evolved nature—human nature.
This means two things. First, our thoughts, feelings, and behavior are produced not only by our individual experiences and environment in our own lifetime but also by what happened to our ancestors millions of years ago. Second, our thoughts, feelings, and behavior are shared, to a large extent, by all men or women, despite seemingly large cultural differences.
Human behavior is a product both of our innate human nature and of our individual experience and environment. In this article, however, we emphasize biological influences on human behavior, because most social scientists explain human behavior as if evolution stops at the neck and as if our behavior is a product almost entirely of environment and socialization. In contrast, evolutionary psychologists see human nature as a collection of psychological adaptations that often operate beneath conscious thinking to solve problems of survival and reproduction by predisposing us to think or feel in certain ways. Our preference for sweets and fats is an evolved psychological mechanism. We do not consciously choose to like sweets and fats; they just taste good to us.
The implications of some of the ideas in this article may seem immoral, contrary to our ideals, or offensive. We state them because they are true, supported by documented scientific evidence. Like it or not, human nature is simply not politically correct.
Adapted from Why Beautiful People Have More Daughters, by Alan S. Miller and Satoshi Kanazawa, to be published by Perigee in September 2007.
Long before TV—in 15th- and 16th- century Italy, and possibly two millennia ago—women were dying their hair blond. A recent study shows that in Iran, where exposure to Western media and culture is limited, women are actually more concerned with their body image, and want to lose more weight, than their American counterparts. It is difficult to ascribe the preferences and desires of women in 15th-century Italy and 21st-century Iran to socialization by media.
Women's desire to look like Barbie—young with small waist, large breasts, long blond hair, and blue eyes—is a direct, realistic, and sensible response to the desire of men to mate with women who look like her. There is evolutionary logic behind each of these features.
Men prefer young women in part because they tend to be healthier than older women. One accurate indicator of health is physical attractiveness; another is hair. Healthy women have lustrous, shiny hair, whereas the hair of sickly people loses its luster. Because hair grows slowly, shoulder-length hair reveals several years of a woman's health status.
Men also have a universal preference for women with a low waist-to-hip ratio. They are healthier and more fertile than other women; they have an easier time conceiving a child and do so at earlier ages because they have larger amounts of essential reproductive hormones. Thus men are unconsciously seeking healthier and more fertile women when they seek women with small waists.
Until very recently, it was a mystery to evolutionary psychology why men prefer women with large breasts, since the size of a woman's breasts has no relationship to her ability to lactate. But Harvard anthropologist Frank Marlowe contends that larger, and hence heavier, breasts sag more conspicuously with age than do smaller breasts. Thus they make it easier for men to judge a woman's age (and her reproductive value) by sight—suggesting why men find women with large breasts more attractive.
Alternatively, men may prefer women with large breasts for the same reason they prefer women with small waists. A new study of Polish women shows that women with large breasts and tight waists have the greatest fecundity, indicated by their levels of two reproductive hormones (estradiol and progesterone).
Blond hair is unique in that it changes dramatically with age. Typically, young girls with light blond hair become women with brown hair. Thus, men who prefer to mate with blond women are unconsciously attempting to mate with younger (and hence, on average, healthier and more fecund) women. It is no coincidence that blond hair evolved in Scandinavia and northern Europe, probably as an alternative means for women to advertise their youth, as their bodies were concealed under heavy clothing.
Women with blue eyes should not be any different from those with green or brown eyes. Yet preference for blue eyes seems both universal and undeniable—in males as well as females. One explanation is that the human pupil dilates when an individual is exposed to something that she likes. For instance, the pupils of women and infants (but not men) spontaneously dilate when they see babies. Pupil dilation is an honest indicator of interest and attraction. And the size of the pupil is easiest to determine in blue eyes. Blue-eyed people are considered attractive as potential mates because it is easiest to determine whether they are interested in us or not.
The irony is that none of the above is true any longer. Through face-lifts, wigs, liposuction, surgical breast augmentation, hair dye, and color contact lenses, any woman, regardless of age, can have many of the key features that define ideal female beauty. And men fall for them. Men can cognitively understand that many blond women with firm, large breasts are not actually 15 years old, but they still find them attractive because their evolved psychological mechanisms are fooled by modern inventions that did not exist in the ancestral environment.
The history of western civilization aside, humans are naturally polygamous. Polyandry (a marriage of one woman to many men) is very rare, but polygyny (the marriage of one man to many women) is widely practiced in human societies, even though Judeo-Christian traditions hold that monogamy is the only natural form of marriage. We know that humans have been polygynous throughout most of history because men are taller than women.
Among primate and nonprimate species, the degree of polygyny highly correlates with the degree to which males of a species are larger than females. The more polygynous the species, the greater the size disparity between the sexes. Typically, human males are 10 percent taller and 20 percent heavier than females. This suggests that, throughout history, humans have been mildly polygynous.
Relative to monogamy, polygyny creates greater fitness variance (the distance between the "winners" and the "losers" in the reproductive game) among males than among females because it allows a few males to monopolize all the females in the group. The greater fitness variance among males creates greater pressure for men to compete with each other for mates. Only big and tall males can win mating opportunities. Among pair-bonding species like humans, in which males and females stay together to raise their children, females also prefer to mate with big and tall males because they can provide better physical protection against predators and other males.
In societies where rich men are much richer than poor men, women (and their children) are better off sharing the few wealthy men; one-half, one-quarter, or even one-tenth of a wealthy man is still better than an entire poor man. As George Bernard Shaw puts it, "The maternal instinct leads a woman to prefer a tenth share in a first-rate man to the exclusive possession of a third-rate one." Despite the fact that humans are naturally polygynous, most industrial societies are monogamous because men tend to be more or less equal in their resources compared with their ancestors in medieval times. (Inequality tends to increase as society advances in complexity from hunter-gatherer to advanced agrarian societies. Industrialization tends to decrease the level of inequality.)
When there is resource inequality among men—the case in every human society—most women benefit from polygyny: women can share a wealthy man. Under monogamy, they are stuck with marrying a poorer man.
The only exceptions are extremely desirable women. Under monogamy, they can monopolize the wealthiest men; under polygyny, they must share the men with other, less desirable women. However, the situation is exactly opposite for men. Monogamy guarantees that every man can find a wife. True, less desirable men can marry only less desirable women, but that's much better than not marrying anyone at all.
Men in monogamous societies imagine they would be better off under polygyny. What they don't realize is that, for most men who are not extremely desirable, polygyny means no wife at all, or, if they are lucky, a wife who is much less desirable than one they could get under monogamy.
According to the Oxford University sociologist Diego Gambetta, editor of Making Sense of Suicide Missions, a comprehensive history of this troubling yet topical phenomenon, while suicide missions are not always religiously motivated, when religion is involved, it is always Muslim. Why is this? Why is Islam the only religion that motivates its followers to commit suicide missions?
The surprising answer from the evolutionary psychological perspective is that Muslim suicide bombing may have nothing to do with Islam or the Koran (except for two lines in it). It may have nothing to do with the religion, politics, the culture, the race, the ethnicity, the language, or the region. As with everything else from this perspective, it may have a lot to do with sex, or, in this case, the absence of sex.
What distinguishes Islam from other major religions is that it tolerates polygyny. By allowing some men to monopolize all women and altogether excluding many men from reproductive opportunities, polygyny creates shortages of available women. If 50 percent of men have two wives each, then the other 50 percent don't get any wives at all.
So polygyny increases competitive pressure on men, especially young men of low status. It therefore increases the likelihood that young men resort to violent means to gain access to mates. By doing so, they have little to lose and much to gain compared with men who already have wives. Across all societies, polygyny makes men violent, increasing crimes such as murder and rape, even after controlling for such obvious factors as economic development, economic inequality, population density, the level of democracy, and political factors in the region.
However, polygyny itself is not a sufficient cause of suicide bombing. Societies in sub-Saharan Africa and the Caribbean are much more polygynous than the Muslim nations in the Middle East and North Africa. And they do have very high levels of violence. Sub-Saharan Africa suffers from a long history of continuous civil wars—but not suicide bombings.
The other key ingredient is the promise of 72 virgins waiting in heaven for any martyr in Islam. The prospect of exclusive access to virgins may not be so appealing to anyone who has even one mate on earth, which strict monogamy virtually guarantees. However, the prospect is quite appealing to anyone who faces the bleak reality on earth of being a complete reproductive loser.
It is the combination of polygyny and the promise of a large harem of virgins in heaven that motivates many young Muslim men to commit suicide bombings. Consistent with this explanation, all studies of suicide bombers indicate that they are significantly younger than not only the Muslim population in general but other (nonsuicidal) members of their own extreme political organizations like Hamas and Hezbollah. And nearly all suicide bombers are single.
Sociologists and demographers have discovered that couples who have at least one son face significantly less risk of divorce than couples who have only daughters. Why is this?
Since a man's mate value is largely determined by his wealth, status, and power—whereas a woman's is largely determined by her youth and physical attractiveness—the father has to make sure that his son will inherit his wealth, status, and power, regardless of how much or how little of these resources he has. In contrast, there is relatively little that a father (or mother) can do to keep a daughter youthful or make her more physically attractive.
The continued presence of (and investment by) the father is therefore important for the son, but not as crucial for the daughter. The presence of sons thus deters divorce and departure of the father from the family more than the presence of daughters, and this effect tends to be stronger among wealthy families.
It is commonly believed that whether parents conceive a boy or a girl is up to random chance. Close, but not quite; it is largely up to chance. The normal sex ratio at birth is 105 boys for every 100 girls. But the sex ratio varies slightly in different circumstances and for different families. There are factors that subtly influence the sex of an offspring.
One of the most celebrated principles in evolutionary biology, the Trivers-Willard hypothesis, states that wealthy parents of high status have more sons, while poor parents of low status have more daughters. This is because children generally inherit the wealth and social status of their parents. Throughout history, sons from wealthy families who would themselves become wealthy could expect to have a large number of wives, mistresses and concubines, and produce dozens or hundreds of children, whereas their equally wealthy sisters can have only so many children. So natural selection designs parents to have biased sex ratio at birth depending upon their economic circumstances—more boys if they are wealthy, more girls if they are poor. (The biological mechanism by which this occurs is not yet understood.)
This hypothesis has been documented around the globe. American presidents, vice presidents, and cabinet secretaries have more sons than daughters. Poor Mukogodo herders in East Africa have more daughters than sons. Church parish records from the 17th and 18th centuries show that wealthy landowners in Leezen, Germany, had more sons than daughters, while farm laborers and tradesmen without property had more daughters than sons. In a survey of respondents from 46 nations, wealthy individuals are more likely to indicate a preference for sons if they could only have one child, whereas less wealthy individuals are more likely to indicate a preference for daughters.
The generalized Trivers-Willard hypothesis goes beyond a family's wealth and status: If parents have any traits that they can pass on to their children and that are better for sons than for daughters, then they will have more boys. Conversely, if parents have any traits that they can pass on to their children and that are better for daughters, they will have more girls.
Physical attractiveness, while a universally positive quality, contributes even more to women's reproductive success than to men's. The generalized hypothesis would therefore predict that physically attractive parents should have more daughters than sons. Once again, this is the case. Americans who are rated "very attractive" have a 56 percent chance of having a daughter for their first child, compared with 48 percent for everyone else.
For nearly a quarter of a century, criminologists have known about the "age-crime curve." In every society at all historical times, the tendency to commit crimes and other risk-taking behavior rapidly increases in early adolescence, peaks in late adolescence and early adulthood, rapidly decreases throughout the 20s and 30s, and levels off in middle age.
This curve is not limited to crime. The same age profile characterizes every quantifiable human behavior that is public (i.e., perceived by many potential mates) and costly (i.e., not affordable by all sexual competitors). The relationship between age and productivity among male jazz musicians, male painters, male writers, and male scientists—which might be called the "age-genius curve"—is essentially the same as the age-crime curve. Their productivity—the expressions of their genius—quickly peaks in early adulthood, and then equally quickly declines throughout adulthood. The age-genius curve among their female counterparts is much less pronounced; it does not peak or vary as much as a function of age.
Paul McCartney has not written a hit song in years, and now spends much of his time painting. Bill Gates is now a respectable businessman and philanthropist, and is no longer a computer whiz kid. J.D. Salinger now lives as a total recluse and has not published anything in more than three decades. Orson Welles was a mere 26 when he wrote, produced, directed, and starred in Citizen Kane.
A single theory can explain the productivity of both creative geniuses and criminals over the life course: Both crime and genius are expressions of young men's competitive desires, whose ultimate function in the ancestral environment would have been to increase reproductive success.
In the physical competition for mates, those who are competitive may act violently toward their male rivals. Men who are less inclined toward crime and violence may express their competitiveness through their creative activities.
The cost of competition, however, rises dramatically when a man has children, when his energies and resources are put to better use protecting and investing in them. The birth of the first child usually occurs several years after puberty because men need some time to accumulate sufficient resources and attain sufficient status to attract their first mate. There is therefore a gap of several years between the rapid rise in the benefits of competition and similarly rapid rise in its costs. Productivity rapidly declines in late adulthood as the costs of competition rise and cancel its benefits.
These calculations have been performed by natural and sexual selection, so to speak, which then equips male brains with a psychological mechanism to incline them to be increasingly competitive immediately after puberty and make them less competitive right after the birth of their first child. Men simply do not feel like acting violently, stealing, or conducting additional scientific experiments, or they just want to settle down after the birth of their child but they do not know exactly why.
The similarity between Bill Gates, Paul McCartney, and criminals—in fact, among all men throughout evolutionary history—points to an important concept in evolutionary biology: female choice.
Women often say no to men. Men have had to conquer foreign lands, win battles and wars, compose symphonies, author books, write sonnets, paint cathedral ceilings, make scientific discoveries, play in rock bands, and write new computer software in order to impress women so that they will agree to have sex with them. Men have built (and destroyed) civilization in order to impress women, so that they might say yes.
Many believe that men go through a midlife crisis when they are in middle age. Not quite. Many middle-aged men do go through midlife crises, but it's not because they are middle-aged. It's because their wives are. From the evolutionary psychological perspective, a man's midlife crisis is precipitated by his wife's imminent menopause and end of her reproductive career, and thus his renewed need to attract younger women. Accordingly, a 50-year-old man married to a 25-year-old woman would not go through a midlife crisis, while a 25-year-old man married to a 50-year-old woman would, just like a more typical 50-year-old man married to a 50-year-old woman. It's not his midlife that matters; it's hers. When he buys a shiny-red sports car, he's not trying to regain his youth; he's trying to attract young women to replace his menopausal wife by trumpeting his flash and cash.
On the morning of January 21, 1998, as Americans woke up to the stunning allegation that President Bill Clinton had had an affair with a 24-year-old White House intern, Darwinian historian Laura L. Betzig thought, "I told you so." Betzig points out that while powerful men throughout Western history have married monogamously (only one legal wife at a time), they have always mated polygynously (they had lovers, concubines, and female slaves). With their wives, they produced legitimate heirs; with the others, they produced bastards. Genes make no distinction between the two categories of children.
As a result, powerful men of high status throughout human history attained very high reproductive success, leaving a large number of offspring (legitimate and otherwise), while countless poor men died mateless and childless. Moulay Ismail the Bloodthirsty, the last Sharifian emperor of Morocco, stands out quantitatively, having left more offspring—1,042—than anyone else on record, but he was by no means qualitatively different from other powerful men, like Bill Clinton.
The question many asked in 1998—"Why on earth would the most powerful man in the world jeopardize his job for an affair with a young woman?"—is, from a Darwinian perspective, a silly one. Betzig's answer would be: "Why not?" Men strive to attain political power, consciously or unconsciously, in order to have reproductive access to a larger number of women. Reproductive access to women is the goal, political office but one means. To ask why the President of the United States would have a sexual encounter with a young woman is like asking why someone who worked very hard to earn a large sum of money would then spend it.
What distinguishes Bill Clinton is not that he had extramarital affairs while in office—others have, more will; it would be a Darwinian puzzle if they did not—what distinguishes him is the fact that he got caught.
An unfortunate consequence of the ever-growing number of women joining the labor force and working side by side with men is the increasing number of sexual harassment cases. Why must sexual harassment be a necessary consequence of the sexual integration of the workplace?
Psychologist Kingsley R. Browne identifies two types of sexual harassment cases: the quid pro quo ("You must sleep with me if you want to keep your job or be promoted") and the "hostile environment" (the workplace is deemed too sexualized for workers to feel safe and comfortable). While feminists and social scientists tend to explain sexual harassment in terms of "patriarchy" and other ideologies, Browne locates the ultimate cause of both types of sexual harassment in sex differences in mating strategies.
Studies demonstrate unequivocally that men are far more interested in short-term casual sex than women. In one now-classic study, 75 percent of undergraduate men approached by an attractive female stranger agreed to have sex with her; none of the women approached by an attractive male stranger did. Many men who would not date the stranger nonetheless agreed to have sex with her.
The quid pro quo types of harassment are manifestations of men's greater desire for short-term casual sex and their willingness to use any available means to achieve that goal. Feminists often claim that sexual harassment is "not about sex but about power;" Browne contends it is both—men using power to get sex. "To say that it is only about power makes no more sense than saying that bank robbery is only about guns, not about money."
Sexual harassment cases of the hostile-environment variety result from sex differences in what men and women perceive as "overly sexual" or "hostile" behavior. Many women legitimately complain that they have been subjected to abusive, intimidating, and degrading treatment by their male coworkers. Browne points out that long before women entered the labor force, men subjected each other to such abusive, intimidating, and degrading treatment.
Abuse, intimidation, and degradation are all part of men's repertoire of tactics employed in competitive situations. In other words, men are not treating women differently from men—the definition of discrimination, under which sexual harassment legally falls—but the opposite: Men harass women precisely because they are not discriminating between men and women.