Why We Get Sick

by Randolph M. Nesse

  If there is internal fertilization, the young can presumably be released at the optimal stage. Optimal for whom? Mother? Baby? Father? We’ll come to that soon. Exactly how long the young are retained is a life history feature very much subject to natural selection. With the nine-month human pregnancy, in which an offspring grows from a microscopic mite to an infant of several kilograms, a mother’s investment in each baby is vastly larger than that of the father. On the other hand, she is sure the baby is hers, while her mate may well be uncertain. This uncertainty means that male expenditures of time and energy caring for the offspring will generally have a more dubious payoff than similar investments by females. The initial tiny difference in the cost of sperm versus the cost of an egg is greatly amplified by human reproductive physiology and leads, as we will see, to different reproductive strategies for males and females.

  Girls and boys are born in nearly equal numbers, as we explained in Chapter 2, because individuals of whichever sex is in excess will have lower reproductive success, on average. Selection therefore constantly shapes parents who have offspring of the scarcer sex, thereby equalizing the sex ratio in the long run. From the standpoint of maximizing collective reproduction, this is inefficient. It takes only a few men to keep a large number of women reproducing at whatever rate would maximize the women’s reproductive success. This is a clear illustration of the greater importance of lower levels of selection relative to higher (group) levels. If selection at the group level were at all important, the sex ratio would be biased toward females.

  This is not a matter of merely academic interest. In India, a cultural preference for males has combined with a proliferation of ultra-sound imaging machines, which allow the determination of the sex of a fetus, to severely distort the sex ratio. More than 90 percent of abortions in India are now of female fetuses, and the sex ratio in the general population is beginning to show an imbalance. Similarly, in many areas of China, where population limitation campaigns restrict a couple to one child, that child is a boy more than 60 percent of the time. In the long run such imbalances will be tempered by natural selection, but in the coming generation they will have unpredictable political and social consequences. Our guess is that the excess men will compete vigorously and the scarce women will gain social power with remarkable speed.

  CONFLICT AND COOPERATION BETWEEN THE SEXES

  Conflict between the sexes is not continuous. Men and women can get along, sometimes for whole days at time, even weeks. This harmony is, however, inevitably disrupted by conflicts that originate in the differing reproductive interests and strategies of men and women. From the original difference between the tiny sperm and the larger egg, whole separate worlds of conflicting strategies have emerged to ensnarl our lives. Women can have a limited number of babies, usually four to six, rarely even as many as twenty according to the record books. Men can, however, have hundreds of children and have done so in cultures where a combination of surplus resources and social stratification made it possible for some men to have harems of hundreds of women while many others lacked even a single mate. These exceptional cases are extreme examples of the principle that the number of offspring may vary more widely for men than women. This difference arises from a woman’s unavoidably high investment in both time and calories for a single baby, compared to a man’s minimal expense of a few minutes and a single ejaculate.

  These differences mean that men and women can and do use different kinds of strategies to maximize their Darwinian fitness. A woman can maximize the number of her genes in future generations by finding and keeping a man who will care and provide well for her and her children and who is disinclined to invest in other women. Men can use a similar strategy by finding and keeping a woman who is fertile, inclined to take good care of her children, and disinclined to mate with other men. Men also have another strategy not available to women, that of inseminating many women while providing little or no support for them and their babies. None of this implies that men and women think through their options in order to arrive at conscious strategies to maximize their reproductive success, and it certainly implies nothing about how people ought to act. Nonetheless, natural selection has inevitably shaped our emotional machinery in ways that maximize our reproduction—or that would have in Stone Age circumstances.

  MATE PREFERENCES

  The problems that result from these divergent strategies are evident in courtship choices. Females of all species do best if they can find a male who offers good genes and abundant resources. Thus, when females can choose, males compete to prove their abilities in contests that range from the familiar butting contests of deer and sheep to the deep braggadocio of the bullfrog. In other species the female mates with the male with the biggest nuptial gift, usually an insect or other source of protein, sometimes the male himself, as when the preying mantis male is eaten by the female even as he copulates with her. The male mantis might try harder to escape, but since he is unlikely to find another mate, he probably maximizes his own reproductive success by donating his bodily protein to the female, who can use it to give more to their offspring.

  Men, while notoriously less choosy than women, still have strong preferences. A man maximizes his reproductive success by mating with a woman who has been healthy and successful (indicating good genes) and who is maximally fertile (indicated mainly by being in the peak reproductive years), uncommitted (indicated by lack of prior offspring), and able and motivated for mothering. As University of Michigan psychologist David Buss puts it:

  Imagine a state in which human males had no mate preferences aside from species recognition and instead mated with females randomly. Under these conditions, males who happened to mate with females of ages falling outside the reproductive years would become no one’s ancestors. Males who happened to mate with females of peak fertility, in contrast, would enjoy relatively high reproductive success. Over thousands of generations, this selection pressure would, unless constrained, fashion a psychological mechanism that inclined males to mate with females of high fertility over those of low fertility.

  So both sexes can increase their fitness by choosing their mates carefully, but they choose different characteristics. Males are relatively more interested in fertility and sexual loyalty, females in good genes and resources. In a study of 10,047 people from diverse cultures and religions in thirty-seven countries, Buss has confirmed these generalizations. Earning capacity was significantly more important to women than to men in all but one of the thirty-seven samples. Youth and appearance were relatively more important to men, and in twenty-three of the thirty-seven samples, men valued chastity significantly more highly than women did, while there was no culture in which the reverse was true.

  Mate choice is especially complicated in the human species, where parents mate repeatedly and both provide care for the young. These circumstances mean that a woman faces the risk of being deserted and so must not only assess the current status of her mate but must also try to predict his ability and willingness to stay and provide for her and their offspring. An enduring bond and continuing investment by the man mean that he now also runs a new risk compared to most other primates, that of being cuckolded. He therefore must assess the likelihood that his prospective mate will mate with other men, thus exposing him to the possibility of unwittingly investing in a woman who may be carrying another man’s baby and, later, in the offspring of another man.

  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 th
eory. 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.

  DECEPTIVE MATING STRATEGIES

  Mating 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.”

  REPRODUCTIVE ANATOMY AND PHYSIOLOGY

  The 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. Ma
ny 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.

  JEALOUSY