Why Beautiful People Have More Daughters: From Dating, Shopping, and Praying to Going to War and Becoming a Billionaire–Two Evolutionary Psychologists Explain Why We Do What We Do
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Q. Why Are There Virtually No Polyandrous Societies?
First, let’s get the terminology straight. As we discussed before, monogamy is the marriage of one man to one woman, polygyny is the marriage of one man to more than one woman, and polyandry is the marriage of one woman to more than one man. Polygamy, even though it is often used in common discourse as a synonym for polygyny, refers to both polygyny and polyandry. We will not use this ambiguous word in this book.
A comprehensive survey of traditional societies in the world shows that 83.39 percent of them practice polygyny, 16.14 percent practice monogamy, and 0.47 percent practice polyandry.2 Almost all of the few polyandrous societies practice what anthropologists call fraternal polyandry, where a group of brothers share a wife. Nonfraternal polyandry, where a group of unrelated men share a wife, is virtually non existent in human society.3 Why is nonfraternal polyandry so rare?
As we discussed in chapter 2, paternity certainty is low enough in a monogamous marriage, where the woman is “supposed to” be mating with only one man; the estimates of cuckoldry (where the man unknowingly raises another man’s genetic child) in monogamous societies range from 13–20 percent in the United States, 10–14 percent in Mexico, and 9–17 percent in Germany.4 When multiple men are officially married to one woman, who is “supposed to” mate with all of them, the co-husbands have very little reason to believe that a given child of hers is genetically his, and will therefore not be very motivated to invest in it. If the children receive insufficient paternal investment, they will not survive long enough to become adults and continue the society. Nonfraternal polyandry therefore contains the seeds of its own extinction.
In contrast, fraternal polyandry, where the co-husbands are brothers, can survive as a marriage institution because even when a given husband is not the genetic father of a given child (sharing half of his genes), he is at least the genetic uncle (sharing a quarter of his genes). The child of a fraternal polyandrous marriage could never be completely genetically unrelated to any of the co-husbands (assuming, of course, that the wife has not mated with anyone outside of the polyandrous marriage), so all the co-husbands are motivated to invest in all the children.
By the same token, the most successful type of polygyny is the sororal polygyny, where all the co-wives are sisters (although, unlike nonfraternal polyandry, nonsororal polygyny is very common). While a woman, when given a choice between marrying an unmarried man and marrying a married man, might under some circumstances rationally choose to marry polygynously (see the section “Why (and How) Are Contemporary Westerners Polygynous?” later in this chapter), it is never in the existing wife’s material interest for her husband to acquire another wife. Every senior wife who is already married to the man suffers from the addition of each new wife to the house hold, because each additional wife takes away the husband’s resources, otherwise available to her and her children. Thus, conflict among co-wives in polygynous marriages is very common, and for this reason polygynous men in many traditional societies maintain a separate house hold for each wife.5 However, the conflict and competition for the limited resources of the husband are somewhat alleviated when the co-wives are sisters because then they will not object so strongly to the diversion of the resources to the new wife and her children, to whom the senior wife is genetically related.6
If You Want to Know What Women Have Been Up to, Look at Men’s Genitals
Now, the fact that polyandry is very rare in human society decidedly does not mean that married women have always been faithful to their husbands and mated with only one man. On the contrary, human females have been promiscuous throughout their evolutionary history. (Recall the dangers of moralistic fallacy from the introduction. The fact that marital fidelity is a virtue means neither that it is natural for us nor that we are always faithful to our spouses. Promiscuity may be morally good or bad, but its evolutionary naturalness has no bearing on the question.) How do we know? There are several pieces of evidence that support this conclusion. First, the very high rates of cuckoldry (men unwittingly raising another man’s genetic children) in many contemporary societies cited above strongly suggest that extra-pair copulation (mating with sex partners to whom one is not formally married) has been an evolved strategy for females (both human and other species).7
Second, it turns out that we can measure the degree of female promiscuity rather precisely by the relative size of testes on the male body. Across species, the more promiscuous the females are, the larger the size of the testes relative to the male’s body weight. This is because when a female copulates with multiple males within a short period of time—in other words, when they are promiscuous—sperm from different males must compete with each other to reach the egg to inseminate it. One good way to out-compete others is to outnumber them. Male gorillas, whose females live in a harem controlled by one silverback male and therefore do not have many opportunities for extra-pair copulations, have relatively small testes (0.02 percent of body weight) and produce a very small number of sperm per ejaculate (5 ´ 107). On the other extreme, male chimpanzees, whose females are highly promiscuous and do not attach themselves to any single male, have relatively large testes (0.3 percent of body weight) and produce a very large number of sperm per ejaculate (60 × 107).8 On this scale, humans lie somewhere between the gorilla and the chimpanzee, but closer to the former than the latter. Men’s testes are about 0.04–0.08 percent of their body weight, and the approximate number of sperm per ejaculate is 25 × 107. So human females have been more promiscuous than gorilla females in their evolutionary history, but not nearly as promiscuous as chimpanzee females. The evidence of women’s promiscuity throughout evolutionary history is in the relative size of men’s testicles. Men would not have such large testicles and produce so many sperm per ejaculate had women not been so promiscuous.
Finally, according to the pioneer biopsychologist Gordon G. Gallup and his collaborators, another piece of physiological evidence of promiscuity among human females in the evolutionary past is the precise shape of the human penis. The shape of the human penis is quite distinct from that of many other primate species. In particular, the glans (“head”) of the human penis is shaped like a wedge. “The diameter of the posterior glans is larger than the penis shaft itself, and the coronal ridge, which rises at the interface between the glans and the shaft, is positioned perpendicular to the shaft.”9
In addition, the human male during copulation engages in repeated thrusting motions before he ejaculates. The combined effect of the particular shape of the penis glans and the repeated thrusting motions “would be to draw foreign semen back away from the cervix…. If a female copulated with more than one male within a short period of time, this would allow subsequent males to “scoop out” semen left by others before ejaculating.”10 In other words, the human penis is a “semen displacement device.”11 If human females did not engage in extensive extra-pair copulations throughout human evolutionary history, the human penis would not be shaped as it is, and the human male would not engage in repeated thrusting motions before ejaculating.12 Clear evidence of women’s promiscuity throughout evolutionary history is in the size and shape of men’s genitals and what men do with them.
Q. Why (and How) Are Contemporary Westerners Polygynous?
Polyandry (a marriage of one woman to many men) is very rare in human society (see “Why Are There Virtually No Polyandrous Societies?” above). This means that almost all human societies practice either monogamy or polygyny, which is the reason why the term polygamy is often used synonymously with polygyny. Polygyny is the only form of polygamy widely practiced in human societies, and a vast majority of human societies practice polygyny. Even though those of us in Western industrial societies tend to think of monogamy as both natural and normal, and even though Judeo-Christian religious traditions tell us that monogamy is the only natural form of marriage, monogamous societies are a small minority throughout the world. Why is this?
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This is because, contrary to the Judeo-Christian tradition, humans are naturally polygynous.13 By naturally, we mean that humans have been polygynous throughout most of their evolutionary history. (Recall the danger of naturalistic fallacy from our introduction. “Natural” means neither good nor desirable.) Strict and socially imposed monogamy is a recent invention in human evolutionary history. Unlike physical artifacts, however, human practices (like the institution of marriage) do not leave fossil records. How, then, do we know that our ancestors practiced polygyny more than ten thousand years ago in the ancestral environment?
It turns out that the clear evidence of our ancestors’ polygyny is embodied in each of us. Both among primate and nonprimate species, the species-typical degree of polygyny (how polygynous members of a given species are on average) highly correlates with the degree of sexual dimorphism in size (the extent to which males of a species are larger than females).14 The more polygynous the species, the greater the size disparity between the sexes. For example, among the completely monogamous gibbons, there is no sexual dimorphism in size; both by height and by weight males are about the same size as females. In contrast, among the extremely polygynous gorillas, males are 1.3 times as large by height and twice as large by weight as females.15
On this scale, humans are somewhere in the middle, but closer to the gibbons’ end than that of the gorillas. Typically, human males are 1.1 times as large by height and 1.2 times as large by weight as human females.16 This suggests that, throughout evolutionary history, humans have been mildly polygynous, not as polygynous as gorillas but not completely monogamous like gibbons either. This is how we know that humans are naturally polygynous.
Why Is Polygyny Related to Sex Differences in Body Size?
However, this begs the question: Why does the degree of sexual dimorphism in size correlate with the degree of polygyny? There are two possible explanations of this correlation. The first, more established theory posits that males have become larger throughout evolutionary history; the second, newer theory argues that females have become smaller.
Did Men Become Bigger…
The proponents of the first theory17 point out that 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 by allowing a few males to monopolize all females in the group. (See chapter 2, “Why Are Men and Women So Different?”) The greater fitness variance among males creates greater pressure for men to compete with each other for mates. Under such severe physical competition, only big and tall males can emerge victorious and get mating opportunities, while small and short males are left out of the reproductive opportunities altogether. At the same time, among pair-bonding species like humans, where males and females stay together to raise their children, females prefer to mate with big and tall males who can provide better physical protection for themselves and for their children against predators and other males. Thus, through both competition among men and preference by women, only big and tall males can reproduce and pass on their “big and tall” genes to their sons, while most or all females of all sizes reproduce and pass on their full range of sizes to their daughters. (Remember, the “fitness floor”—the worst one can do—is relatively high for women.) Over many generations, males will get bigger and taller, while females will retain the same distributions of height and weight in each generation.
Recent critics18 point out that this theory assumes that body size (height and weight) is transmitted exclusively or largely along the sex lines, from fathers to sons, and from mothers to daughters. The theory assumes that tall men married to short women have tall sons but short daughters. The critics use Finnish data on twins19 to demonstrate that this assumption is false. The data show that sons are just as likely to inherit their height from mothers as from fathers, and daughters are just as likely to inherit their height from fathers as from mothers. So a tall father will have both tall sons and tall daughters, and a short mother will have both short sons and short daughters. What gives?
…or Did Women Become Smaller?
The critics then point out that under polygyny, there is an evolutionary pressure for girls to mature earlier (see “Why Do Girls of Divorced Parents Experience Puberty Earlier Than Girls Whose Parents Remain Married?” in chapter 5). Under monogamy, most adult males are already married and cannot marry again, so there are no incentives for prepubescent girls to mature earlier; prepubescent boys in their age group are in no position to marry them. In contrast, under polygyny, married adult males can acquire additional wives. So girls who mature early can become a junior wife of a wealthy village chief while their prepubescent age mates cannot. Because girls who mature early attain smaller adult height than girls who mature late throughout the world (because girls essentially stop growing when they reach puberty),20 this suggests that height differences between the sexes should be greater in polygynous societies as a result of girls undergoing earlier puberty and becoming shorter. Cross-cultural data show that this is indeed the case. Girls in polygynous societies are shorter than girls in monogamous societies, whereas boys from both types of societies are about the same height.[21]
Whichever theory is correct, it appears to be the case that polygyny and sex differences in height are closely related. This is how we know that humans are naturally polygynous: because men are taller than women.
What Women Want
If humans are naturally polygynous, why, then, do many human societies in the world today practice monogamy (even though a large majority still practices polygyny)? One theory suggests that it is because that is what women want. In any species for which the female makes a greater investment in children than does the male (including humans), sex and mating is a female choice. Sexual intercourse occurs if and when the female wants it; the male has very little choice (outside of forcible rape).22 (See “What Do Bill Gates and Paul McCartney Have in Common with Criminals?” in chapter 6.) Humans are no exception. Monogamy emerges as the institution of marriage in the society when many or most women choose to marry monogamously, and polygyny similarly emerges as the institution of marriage when many or most women choose to marry polygynously.23
What, then, would lead women to choose to marry monogamously or polygynously? One important determinant of the institution of marriage is the degree of resource in equality among men (the difference between the richest men and the poorest men). In societies with a high degree of resource in equality, where rich men are very much richer than poor men, women (and their children) are better off sharing the few wealthy men, because one-half, one-quarter, or even one-tenth of a wealthy man is still better than a whole of a poor man when resource in equality is extreme. Or, 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.”24
In contrast, in societies with a low degree of resource in equality, where rich men are not much richer than poor men, women (and their children) are better off monopolizing a poor man than sharing a rich man, because one-half of a rich man will not be as good as a whole of a poor man.25 Thus, polygyny emerges as the institution of marriage in societies characterized by greater resource in equality among men, while monogamy emerges in societies characterized by lesser resource in equality. This theory is an extension to human society of what is known as the polygyny threshold model in biology, originally formulated to explain the mating systems of birds,[26] thus once again illustrating the fundamental principle of evolutionary psychology that humans are no different from other species. (See “The Evolutionary Psychological Perspective” in chapter 1.)
The reason most Western industrial societies are monogamous, despite the fact that humans are naturally polygynous, is that men in such societies tend to be more or less equal in their resources, compared to their ancestors in medieval times. The degree of in equality tends to increase as societies become more complex, from hunter-gather
er and pastoral societies, to horticultural and agrarian societies, and typically reaches its maximum in advanced agrarian societies.27 Industrialization tends to decrease the level of in equality in society.
Individual decisions of women to marry monogamously rather than polygynously combine to produce social institution and norms.28 If many or most women choose to marry monogamously, then the society becomes monogamous. However, the true polygynous nature of humans is never too far beneath the surface, even in nominally monogamous societies such as ours.
All Human Societies Are Polygynous, Simultaneously or Serially
Wealthy and powerful men throughout history, even while monogamously married, have always mated polygynously by having mistresses, concubines, and other extramarital affairs.29 (See “What Do Bill Gates and Paul McCartney Have in Common with Criminals?” in chapter 6.) And it is true even today. Whether married or not, wealthier men in the United States and Canada have more sex partners and have sex more frequently than less wealthy men.30 This is not because wealthy men can afford the ser vices of prostitutes; wealthy men are no more likely to have sex with a prostitute than are poorer men. They do not have to. Wealthy men have more sex partners and have sex more frequently because women seek them out.
Most nominally monogamous societies also allow people to get a divorce, and in many societies, such as the United States, divorce is both very easy and very common. Liberal divorce laws allow men in these societies to practice serial polygyny (a man having multiple wives, not simultaneously but sequentially, through a series of divorce and remarriage). In the United States, the strongest predictor of remarriage after divorce is sex (male vs. female): men typically remarry, women typically do not. As we discuss in chapter 3, this is because men become more desirable with age to potential mates (thanks to the greater income and higher status that typically accompany age), while women become less desirable with age due to declining reproductive value and fertility. While some women do remarry after divorce and thus practice serial polyandry, a far greater number of men practice serial polygyny through divorce and remarriage. Contemporary Westerners who live in nominally monogamous societies that nonetheless permit divorce are therefore in effect polygynous; they practice serial polygyny.