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 (11 page)

Why do these problems happen? Why are men more likely to infer sexual interest in a neutral encounter than women are? Two evolutionary psychologists, Martie G. Haselton and David M. Buss, offer an explanation in their Error Management Theory.
⁷²

The Cost of Misreading the Signals

This theory begins with an observation, made earlier by others,
⁷³
that decision making under uncertainty often results in erroneous inferences, but some errors are more costly in their consequences than others. Natural and sexual selection should then favor the evolution of inference systems that minimize the total
cost
of errors rather than their total
numbers
. For instance, if a man must infer the sexual interest of a woman whom he encounters, he can make two types of errors: He can infer that she is sexually interested when she is not (false positive), or he can infer that she is not sexually interested when she is (false negative). What are the consequences of each type of error?
^[74]

The consequence of a false positive, thinking that she is interested when she is not, is that he would be turned down, maybe laughed at, possibly slapped in the face. The consequence of a false negative, thinking that she is not interested when she is, is a missed opportunity for sexual intercourse and to increase his reproductive success. The latter cost is far greater than the former. Thus, men should be selected to possess a cognitive bias that constantly leads them to overestimate a woman's sexual interest.

Haselton and Buss's Error Management Theory not only explains previously known phenomena, such as the results of the laboratory experiment mentioned above or the Safeway fiasco, but also leads to two novel predictions. First, women should underestimate a man's romantic commitment to them, because the cost of a false positive (thinking that a man is romantically committed to her when he is not, getting pregnant by him, and then having him desert her) is far greater than the cost of a false negative (thinking that he is not romantically committed to her when he is, and missing an opportunity to form a committed romantic relationship). If a woman misses an opportunity to form a long-term committed relationship with one man, she can soon get an opportunity to form one with another man; there are other fish in the sea. In contrast, one mistake with a wrong man can burden the woman with a child and ruin her future romantic prospects for many years to come.

Second, the tendency of men to overestimate a woman's sexual interest should not apply to their sisters' interest in other men, because men need to perceive their sisters' sexual interest in other men accurately so that they can protect them from unwelcome sexual advances from those they are not interested in. In other words, the cognitive bias of men to overestimate women's sexual interest is not blind or unqualified; it is only activated in encounters with women with whom they might conceivably have sex, which exclude their sisters. Haselton and Buss's studies confirm both of these novel predictions.

While Haselton and Buss apply their Error Management Theory exclusively to the area of mind reading between men and women (inference about sexual interest of potential mates), their insight may be applied to human behavior in other areas as well.
⁷⁵
For example, evolutionary social psychologist Toshio Yamagishi and his colleagues suggest that people in social exchange situations make similar (and similarly unconscious) calculations when they decide whether or not to cooperate with each other.
⁷⁶
They face the possibility of making two different types of errors: thinking that freeriding on others is possible without detection or punishment when it is not (false positive), or thinking that freeriding on others is not possible when it is (false negative). The cost of the former error is ostracism from the group, while the cost of the latter is the foregone benefit of exploiting others. Yamagishi and his colleagues suggest that people are less likely to commit the error of false positive when they are highly dependent on the group and cannot risk ostracism and expulsion from it.

As another example, Stewart Elliott Guthrie
⁷⁷
and Pascal Boyer
⁷⁸
use the same principle of cognitive bias to explain the emergence of religion. When something either good or bad happens to you, it might be the result of a purposeful, intentional act of someone (in other words, you have a friend or a foe that you may not know about), or it might be the result of a random event (“luck”). In deciding which it is, you can once again make two types of errors: thinking that it is a purposeful, intentional act when it is random (false positive), or thinking that it is random when it is a purposeful, intentional act (false negative). The consequence of the first type of error (ignoring a potential friend or foe) is far greater than the consequence of the second type of error (being a bit too paranoid or anthropomorphic). Guthrie and Boyer argue that religion and the human tendency to believe in God are byproducts of an evolved cognitive bias toward anthropomorphism. (See the section “Where Does Religion Come From?” in chapter 8 for further details.) In other words, at the most abstract level, it may be that we believe in God for the same reason that men constantly think that women are coming on to them: Because getting it wrong the other way would be much worse.

THE EVOLUTIONARY PSYCHOLOGY OF MARRIAGE

It is a mistake to think that marriage is unique to the human species. While, of course, some of the specific accoutrements of human marriage—such as the wedding ceremony—is unique to humans, the institution of marriage itself—the predictable and regulated patterns of matings between a male and a female—is shared by many other species, particularly birds.
¹
Further, some of the specifics of a Western marriage—the church wedding, marriage certificates—are not even human universals.

Because marriage is closely related to sex and mating, this is another area where evolutionary psychology has produced a large number of fascinating studies. Perhaps two of the most surprising findings of evolutionary psychology and biology (to be discussed in greater detail in this chapter) are about polygyny (marriage of one man to many women). First, despite the impression you might get from the history of Western civilization in the last millennium, humans are naturally polygynous, not monogamous, and as a result,
all
human societies (including the United States) are polygynous to various degrees. Second, contrary to what you might think, most women benefit from polygyny, while, conversely, most men benefit from monogamy.

Intrigued? Then read on….

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 ´ 10
⁷
). 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 × 10
⁷
).
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 × 10
⁷
. 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?

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.