Why We Get Sick (31 page)

To succeed, an individual must predict the prospective mate’s future behavior, an iffy task at best. Both sexes look for indicators of loyalty and willingness to invest in offspring. Amotz Zahavi, an Israeli biologist, has suggested that these pressures might explain some otherwise mysterious conflicts by a mechanism he has called “testing of the bond.” By provoking the prospective partner, he suggests, one can assess his or her willingness to continue to deliver resources and loyalty in the face of future difficulties. Do lovers have spats to test each other? Zahavi provides examples from the world of courting birds to support his theory. Female cardinals, for example, peck and chase wooing males and allow mating only after long persecution. Their subsequent bond lasts for season after season. No one has yet looked in detail at human courtship to see whether we do the same.

Now we return to look at the strongest finding in the Buss study. Despite their differences, both sexes from cultures across the globe consistently agreed on the two most important characteristics they would look for in a mate: (1) kindness and understanding and (2) intelligence. Why do both sexes most of all want a caring and capable partner? For an answer we need to understand why there is such an institution as marriage. Why do men and women in every culture form long-lasting sexual and parenting associations while most other primates have very different kinds of mating systems? This question cannot be answered with certainty, but human patterns of food gathering and child rearing are certainly important parts of the explanation. In the natural environment, one caretaker cannot easily raise a child. Children are, for many years, too helpless and heavy to be taken on long foraging trips. In order to succeed, they need instruction in the ways of their culture and help in negotiating the group hierarchy. In short, each child is so expensive that it may take more than one individual to raise it. To the extent that both parents have all their children in common, they should have minimal conflicts of interest—except, that is, those conflicts that arise from obligations to other relatives. Problems with in-laws are entirely expectable, because helping in-laws directly benefits the genes only of the spouse, not one’s own.

M
ating without caring for the offspring benefits men’s reproductive interests more than women’s. This is consistent with some other aspects of human sexual patterns. First, prostitution is mainly a female profession. While erotic pleasures are possible for both sexes, the balance of supply and demand is such that everywhere men are willing to pay for sex while women rarely have difficulty finding willing sex partners. Second, the strategies that characterize the singles bar scene begin to make sense. In order to get women into bed, men brag about their ability to protect and provide, exaggerating their exploits and flashing their fake Rolex watches as they swear that they are in love forever. Experienced women are rarely completely taken in by this charade, but these patterns of male deception nonetheless seem to work. Men often accuse women of using the converse deceptive strategy, receiving expensive gifts with excited sexual interest and then, later, indignantly expressing surprise that he could have imagined her to be “that kind of woman.” For thousands of years, physicians have called this kind of emotional behavior pattern “hysteria.” This name arose because commonly associated physical symptoms such as abdominal pain and psychogenic paralysis were thought to result from the wanderings of the womb through the body. Had physicians usually been women, they might never have invented the dubious diagnosis of “hysteria.” Instead, women doctors, observing the deceptive mating strategies of men, might have attributed the characteristics of cads to an overly mobile prostate gland and called it “prostateteria.”

T
he human female’s reproductive cycles are quite different from those of other primates. Many female primates advertise their fertile periods with odors, bright patches of skin, and changed behavior. These advertisements are useful communications that increase competition and courtship by males during the females’ fertile period and discourage sexual harassment at other
times. In human females, ovulation is not only unadvertised, it seems to be carefully concealed. The scheduling in women is also different, with human ovulation regularly repeated at about twenty-eight-day intervals, while most primates ovulate only once or twice a year, often in synchrony with the cycles of other females they are associated with. At the end of the cycle, if there is no pregnancy, the human female loses a considerable amount of blood in the menstrual flow. Human sexual activity is not confined to brief fertile periods but occurs throughout the cycle, with substantial time and energy spent on frequent sexual intercourse. Female orgasm in most primates is either absent or brief and inconspicuous, but in humans it
is
common and may be intense.

While the details remain very much at issue, there is a growing consensus that all these facts fit together. The key is that the woman and her mate both benefit if he is frequently present instead of away for weeks and months at a time. If her cycles were obvious, he could maximize his reproduction by inseminating her only at fertile times, but because he cannot tell when she is fertile, he must stay nearby and copulate at frequent intervals. If early Stone Age women, with their enlarging mental capacities, could know when they were fertile and connect sex with the pain of childbirth, they might avoid their partners at those times and thus decrease their reproductive success. Here is a possibility, first suggested by ornithologist Nancy Burley, where
not
knowing something may be good for one’s fitness. Concealed ovulation also protects the woman somewhat from being impregnated by men more powerful than her mate since such men cannot know when she is fertile and take advantage of her only at that time.

The average frequency of human intercourse, every three days or so, is high enough to make it likely that an ovulation will result in a pregnancy. As we noted before, however, this continuous sexual activity could also mean that bacteria and viruses can hitch regular free rides deep into the woman’s reproductive tract. One defense against such infection is the plug of mucus at the cervix that blocks sperm from ascending except during two or three fertile days a month, when the fibrils in the mucus align to make channels just wide enough for the sperm to swim up into the uterus. As suggested by Margie Profet, menstruation may be another defense to kill pathogens and sweep away the beginnings of infection (see
Chapter 3
). In the natural environment, of course, most women would experience far fewer menstrual cycles, since they would not cycle while pregnant or lactating, which would be most of the time. Anemia from loss of menstrual blood is another of
the many problems that result largely from novel aspects of our environment, such as celibacy and effective contraception.

Men are also different from some other male mammals in having testicles permanently lodged in a scrotal sac outside the body proper. This is a precarious location for organs of such vital importance, so there must be a good reason for it. One clue is the infertility that many men experience from wearing tight underwear, which increases the temperature of the testicles. Anatomic examination shows that the veins bringing blood back into the body from the testicles wrap around the artery in a way that provides an effective countercurrent heat exchange mechanism to keep the testes cool. Why sperm cannot be formed at regular body temperature is an unsolved mystery. Men must keep their testicles cool and functioning all the time because fertile women may be available at any time.

The testicles of different primates vary greatly in size, and much of this variation can be explained by differences in mating patterns. A female chimp mates with several males, while female gorillas and orangutans mate with only one male. Because the reproductive success of the male chimpanzee depends not only on inseminating many females but also on the success of his sperm in competing with other sperm to fertilize the egg, natural selection has increased the number of sperm chimp males make as well as the size of their testicles. Gorillas, despite their large size and fearsomeness, have testicles that are about one-fourth the weight of the average chimpanzee testicles. In general, the relative testis weight is high for species in which females often mate with multiple males and low in those with little sperm competition. Where do humans fall? In between but toward the side of less sperm competition. It appears that multiple matings have, however, occurred often enough during human evolution to select for testicles slightly larger than those of species with reliably monogamous mating patterns.

Two British researchers, Robin Baker and Robert Bellis, have taken this topic of sperm competition much further. They note that human sperm in a single ejaculate are of several different kinds, some of which are incapable of fertilizing an egg. Many of these sperm are designed, they argue, specifically to find and destroy any sperm from other men. They have also shown that the volume of ejaculate collected in condoms from monogamous couples increases not merely with the amount of time since the last ejaculation but also with the amount of time the couple have been apart. This suggests an adaptation to increase sperm output when it may be needed to compete
with sperm from another man. If confirmed, this will demonstrate that selection has designed our sexual machinery to compete in many different ways and at very close quarters.

H
owever understandable jealousy may be, either in the theory of natural selection or in our intuitions, it has surely been responsible for a large part of the world’s miseries. Perhaps the ill will and bloodshed caused by Helen’s desertion of Agamemnon for Paris, as described by Homer, need not be taken literally, but it is not an implausible account of the emotions such an event could arouse. Canadian psychologists Martin Daly and Margo Wilson have convincingly demonstrated that a large proportion of the murders of women arise out of male jealousy. Othello’s lethal frenzy and Desdemona’s tragic death have all too many parallels in real life. More commonly, jealousy merely fuels marital battles that stop short of murder but lead to traumatic divorces and all their tragic consequences. In a few individuals the extremity of these feelings and false beliefs that the partner is unfaithful justify the clinical diagnosis of pathological jealousy. To make sense of all this, we must understand the evolutionary origins and functions of the capacity for sexual jealousy.

Maternity is a certainty, but paternity is always a matter of opinion. A man runs the risk of spending years providing for a woman who is having other men’s children and of unwittingly caring for children not his own, while women always know who their children are. A man incapable of jealousy would have a greater risk for being cuckolded, with a resulting decrease in reproductive success. Men who threaten potential interlopers and try to prevent their wives from mating with other men have an evolutionary advantage. Genes that predispose to male sexual jealousy will thus be maintained in the gene pool.

While women do not face the same risk, they face others. A husband’s wandering affections can lead to a drain of resources and time, to potential loss of the husband, and to the risk of sexually transmitted diseases. Cross-cultural data show enormous diversity in sexual mores, from cultures where extramarital liaisons are tolerated to those where any infidelity is punished by death. However, sexual jealousy is consistently reported to be more intense for men than women.

Sexual jealousy is such a strong influence on human life that it is institutionalized and regulated by custom or formal law in almost all societies. In technologically advanced Western countries men often treat women as property and try to control their sexuality, but in many traditional societies the control may be even more blatant and institutionalized. In some Mediterranean societies, women must demonstrate their virginity with blood on the marital sheet and then are cloistered so they can associate with no men but their husband. In some Muslim societies women must wear robes and veils that make them unrecognizable by men outside the home. In China, women’s feet were bound from early childhood to discourage straying. In many areas of Africa it remains routine for girls at puberty to have the clitoris excised and the labia sewn shut. Everywhere men create social institutions to control female sexuality.

What would be the attitude, in our own society, toward a woman who is faithful to her husband 90 percent of the time, but who has another lover for the remaining 10 percent of her sex life? Her husband would have a 90 percent probability of being the father of her next child, and so, from a strictly evolutionary perspective, we would expect him to be 90 percent as good a father to that child as he would if his wife had been perfectly monogamous. Yet in many cultures a single instance of adultery by a woman may be legally considered a justification for total cancellation of the marriage and abandonment of any ensuing child by the woman’s husband. Many people seem to think that culture opposes such biological tendencies, but with jealousy, culture and the legal system exaggerate a biological tendency. People who think that laws should oppose our more destructive biological tendencies would presumably want to change the social system in ways that would discourage divorces based on infidelity. What do you think the world will be like if someone invents a pill that cures jealousy?

P
eople are, to put it mildly, very interested in the quality of their sex lives. This is ultimately because genes that result in behaviors that increase reproduction have been selected for, while genes that make people uninterested in sex have been eliminated. But from this point on, sex becomes more problematic.
The ubiquity of sexual problems is confirmed by a visit to any bookstore. The very existence of rows of sex therapy books documents the unfortunate truth. Sex is a problem not just for a few people some of the time, but for many people much of the time. The books contain strong hints that these problems are not genetic defects, not results of an abnormal environment, but direct products of evolution. Each book has a chapter about premature orgasm in men and another on delayed or absent orgasm in women. There are no chapters about too-rapid orgasms in women or too-slow orgasms in men and no explanations for why men and women differ in this regard. There are chapters on men with fetishes but no mention of similar problems in women, and again, no comment on why the sexes differ in this susceptibility. Some difficulties the sexes share: both are troubled, on occasion, by lack of sexual desire and difficulty getting aroused. And both sexes (but especially men) are troubled by boredom with the same sexual partner. Here, at the heart of reproduction, we find a biological system that seems haphazard at best. Why should men and women have so many and such different complaints?