The Science of Sexual Romance

Why and how the sexes are attracted to each other — a book by Nigel Barber offers science-based insights.

review and notes by M.P. Shiel

NIGEL BARBER’S The Science of Romance is so thought-provoking that I had a hard time sleeping (no, not for that reason!) after finishing it last night. It puts what was once in the realm of mystery and romance and speculation and dogma into the bright light of biological science. For example, have you ever wondered why the younger woman / older man couple is so common, and why the reverse is relatively rare? Have you ever wondered why Huxley’s couplet — Higgamus hoggamus, woman’s monagamous / Hoggamus huggamus, man is polygamous — rings so true? And where does the double standard come from that sets such a severe judgement on female infidelity, but is more forgiving toward the male “sowing his wild oats”?

A few excerpts will show you the kind of insights Barber provides on nearly every page:


Men’s physical attractiveness to women declines with age, but the decline is generally less steep than that of women to men. In what might be called the second cardinal rule of dating, men want partners who are a year or two younger than they are, while women, in general, want to date older men. As men age, they want women who are increasingly younger than they are. A man of forty, for example, is likely to want a partner who is ten years younger. Why?

The most fundamental reason relates to limitation of women’s ability to conceive children with advancing years. Fertility reaches a high point in the early twenties and stays on a plateau until the age of thirty-five, after which it declines sharply. Natural selection would have caused men to select fertile women as wives since those who were attracted to women over fifty would have left no offspring to carry on their unusual taste. However, men see women as more attractive at twenty than at forty. This is right at the beginning of their most fertile phase in the life span.

Men are thus most attracted to women who are at the beginning of their reproductive career. If a man marries a woman of this age, then he has the potential of giving her all of her children and thereby hitting the reproductive jackpot. Natural selection has thus favored men who are attracted to younger fertile women rather than older fertile women. For this reason, the perception of youthfulness is critical to the physical attractiveness of women. This helps explain the success of the cosmetics industry, as women attempt to conceal signs of aging and try to appear younger and more attractive.

Men reach the peak of their physical attractiveness to women in the late teens or early twenties. However, as they grow older, they acquire social status and wealth, which enhances the value of the overall package as far as a marriage partner is concerned. Although men deteriorate with age, their physical appearance is less critical to their overall attractiveness. One important cue to feminine youthfulness that plays an important role in women’s physical attractiveness is their bodily shape….


Understanding courtship in other species illuminates differences between the sexes and explains why there are conflicts of interest between men and women in courtship and marriage. The conclusion that women are sexier in appearance than men suggests that women are competing among themselves for access to men. Yet this conclusion does not ring true in the real world. According to anthropologist Don Symons, of the University of California at Santa Barbara, sexual intercourse is everywhere a female favor granted to men. Any woman, whatever her looks, can succeed in becoming pregnant.

Why then do people of both sexes consistently rate women as more physically attractive? They do not need to be physically attractive to have sex with men. They are obviously not competing over opportunities for sexual intercourse. Instead, they are competing to marry desirable husbands who can help them to raise children.

Men’s bargaining position is based on social status and wealth. In subsistence hunter-gatherer societies, such as the Siriono of Brazil, a man’s sexual attractiveness to women is based largely on his reputation as a successful hunter. In modern societies, women are more interested in a man’s education and income level than his hunting ability, but this concern represents the same underlying need to find a mate who will be a good provider of food and other economic goods. Even though modern women sometimes earn more than most men, their evolved psychology has not changed. They are still attracted to successful men….

In early subsistence societies, as well as more recent ones, women were constantly pregnant, breast-feeding, or caring for children or all three. This would have interfered with their ability to work and acquire surplus food or property. Today, the birth rate is much lower due to the use of effective birth control techniques. Moreover, children spend the day in daycare or with babysitters, which frees their mothers for full-time occupations. Successful rearing of children among our hunter-gatherer ancestors was a cooperative enterprise in which men contributed to feeding, sheltering, carrying, protecting, and caring for their offspring. The critical importance of fathers for the survival of their children is demonstrated by the Ache of Paraguay, who are more than twice as likely to die during childhood if they lose their father. Women, in general, have evolved to compete for husbands with social status and wealth because these are reliable cues to the ability to protect and care for children.

Physically attractive women (as assessed from high school yearbook photographs) are much more likely to marry. They also marry up the social ladder, finding husbands that are wealthier than their parents; the same, however, is not true of men. Physically attractive women move up into wealthy elites through marriage, while physically attractive men do not.


Men tend to be very upset by the sexual infidelity of their wives. This fits with evolutionary logic because their reproductive interests are seriously threatened. When a wife is unfaithful to her husband, there is a good chance that the lover may father her child. British biologists Robin Baker and Mark Bellis found that women often timed their infidelities to coincide with ovulation and that they were less likely to use contraception with the lover than with their husband. This meant that the lover was more likely to sire any children conceived at the time of the affair.

A cuckolded husband risks not raising a child of his own and also wastes his paternal investment on the offspring of another man. It would be surprising if our male ancestors had been indifferent to this possibility. If they were, they would have left few offspring of their own to carry on their lackadaisical habits. This helps to explain why sexual infidelity is more distressing to husbands and why even the suspicion of infidelity can, unfortunately, provoke deadly aggression by men against their wives. Note that the emotional response of sexual jealousy would protect paternity even in societies where people did not grasp the connection between intercourse and pregnancy….

Although difficult to quantify because it is often concealed by the victim, abuse by husbands and lovers is a leading cause of injury for women between the ages of fifteen and forty-four years. It is estimated that one woman in four will be a victim of physical assault by a partner or ex-partner during her lifetime. American women, too, often use physical aggression against their husbands, and at least one researcher has found that women are violent more often than men. Yet husband battering is a far less severe social problem because fewer women have a murderous intent and because of the obvious fact that men are larger, stronger, and better able to defend themselves.

Violence against women the world over is produced by a psychology, derived from evolution, of male possessiveness and sexual jealousy. Male sexual possessiveness is accommodated by the legal codes of many countries. Marriage, in many countries, is considered to make a woman the sexual property of her husband. Under British law, a husband could sue his wife’s lover for “criminal conversation” and be awarded damages. The idea was that the lover had infringed a property right of the husband and was therefore liable for damages. Archaic though the law may seem, it has actually been invoked in the twentieth century to curtail the activities of adulterous male lovers.

By the same logic, adultery was considered an unbearable insult to the husband. If he caught his wife in the act of adultery, according to the in flagrante delicto principle, he was justified in killing both the spouse and her lover. This principle was the law of the land in the state of Texas until 1960. In contrast, serious aggression by wives against husbands is most often a response to being threatened, or feeling threatened, in an abusive relationship.


The conflicting interests of men and women are as old as sexual reproduction itself. Going all the way back in evolutionary time to the first sexually reproducing animals on earth, females are the ones that produced the largest sex cells. Eggs are much more energetically expensive to produce than sperms, which are tiny and produced in large numbers. Females thus invest more in offspring from the very beginning. Males therefore compete among themselves for access to the greater investment of the females.

It’s also worth thinking about this in terms of group evolution. We don’t evolve — or even survive — as isolated individuals. In addition to being a social and necessarily tribal species in general, we also must evolve in groups.

Now you could lose 50 or 75 per cent. of your men in war, and the remaining men of your tribe or race could still father just as many children with your women as would have been fathered without the war. Similarly, your men could “sow wild oats” for a few years while away from home, looting another people or searching for the golden fleece, and the same would hold true.

But your ethnic group or people can only survive if enough of its females give birth to the next generation of your people. That’s the only way for that generation to come into the world — through their wombs. And the percentage of your population which is female and also of childbearing age is very limited. So you can see how devastating it can be if a conquering tribe takes or impregnates all or half or even ten per cent. of your women. You would be on your way to extinction. Hence the general principle which underlies the instinctive protection of women’s fidelity to their husbands and to the group: eggs are infinitely precious, far more precious than sperm.

While female investment in offspring is almost always very large, male investment is highly variable. At a minimum, a male might contribute only his sperm. At a maximum, he might take responsibility for all care of the young. Surprising as it may seem, the latter happens. For example, among jacana birds, it is the males that incubate the eggs and take care of the hatchlings. Among most monogamous birds, like grebes, there is a fairly equitable distribution of the work of caring for the chicks.

When both sexes cooperate to care for the young, this leads to the establishment of very strong bonds of mutual affection that are extraordinarily persistent. For grebes, swans, and human beings, the pair bond can last a lifetime. Despite the strong bonds of love and companionship, ancient conflicts of interest linger in the shadows, one of the most important being conflict over sexual infidelity….

Male psychology evolved to become less choosy in the selection of a sexual partner. The cost to a man of being promiscuous was not great, whereas for a woman a single sexual encounter could result in the huge commitment of pregnancy and child rearing, Women who mated indiscriminately would leave fewer surviving offspring than women who were more cautious in the selection of a partner. The more selective would choose men who were physically attractive, and thus had good genetic characteristics, or contributed to nurturing the child, or both. In either case children of the discriminating woman would have a greater chance of surviving whether this was due to superior genes, better nutrition, or better protection from enemies and predators. This means that women would have evolved a high degree of selectiveness in their choice of sexual partners.

By contrast, the male partner in casual sexual encounters would have increased his probability of leaving more children without paying any of the costs of child rearing. Our male ancestors who enjoyed casual sexual relationships with several women would have been more reproductively successful than men who were faithful to a single partner. Their male children would have inherited the genetic basis for having a roving eye through the father’s genes. This explains why modern men are more interested in casual sexual relationships than women are. The reproductive benefits of philandering exceed the reproductive costs for men, whereas the costs exceed the benefits for women. Needless to say, ancestral men were not necessarily interested in having babies, and may not even have made a connection between sexual intercourse and reproduction. Natural selection designed them to be interested in casual sex for its own sake and this tended to increase their reproductive success. The same argument can be made about the greater eagerness to mate of males of other species.

Like males of all species, men compete for the reproductive resources of women. At the same time, women compete over help from men in raising their children. Given that the sexes have evolved differences in their needs and objectives, there is a potential for conflict. Anthropologists see the marriage contract, in different societies around the world, as one solution to this conflict. If men agree to support the children of the marriage, in return they are given an opportunity to father them.

Fathers in most societies perform some childcare services, but women everywhere do the lion’s share of this work. Paternal support revolves around helping to provide food, shelter, and protection, each of which would have been critical for survival in the evolutionary past. In one of the few remaining subsistence societies, the forest-dwelling Ache of Paraguay, support of fathers is crucial. In fact, 45 percent of children who lost their fathers to death or divorce at any point before the age of fifteen died before themselves reaching the age of fifteen years, compared to a 20 percent mortality rate for children who had fathers present in their families. The absence of a father triples the risk of death due to illness, and doubles the risk of being killed by other Ache. Reading between the lines, one can see that if Ache women did not marry, their chances of raising children to maturity alone would be fairly bleak.

Given that it is difficult to raise children, that a mother’s very great investment in her offspring is lost if the children die before reaching maturity, and that the presence of the father provides considerable insurance against the hostile forces of nature, the case for bonding with a particular man and obtaining masculine support must have been compelling for our female ancestors.

Whereas women have no choice but to make a very great investment in each of their children, if only because they must carry the fetus during pregnancy, men can theoretically pursue different reproductive strategies. If they abandoned their children, far more would have died before reaching maturity. Yet they could have made up for high infant mortality by siring many children through many women. Instead of being dads, they could behave like cads.

The behavior of modern men reveals a certain amount of emotional ambiguity about whether to be cads or dads. (Of course women may also feel ambiguous about marriage, but this usually seems to take the form of asking whether their current partner is worthy of them rather than dreams of sexual gratification with a limitless supply of attractive men; female fantasies are more likely to involve being wanted by men.) Some men who are happily married and love their wives are perfectly capable of having extramarital affairs without feeling much guilt or remorse, at least while they are getting away with it. Yet there are only a handful of preindustrial societies in which children grow up apart from their fathers and in some of these the absence of their father is temporary. From the point of view of the evolutionary dance between the sexes, men have been pulled away from their cad tendencies and turned into dads. Instead of constantly wandering around trying to impregnate women, ancestral men at some point began to stay at home and take responsibility for their children. They became domesticated, in the sense of organizing their hunting activities around a home base.

Barber doesn’t tread deeply enough in politically dangerous waters to tell us that biologically different human groups — races — have notably different reproductive strategies; that some groups have a much higher cad-to-dad ratio, and these tend also to be groups with higher birthrates generally, and who arrive at sexual maturity more quickly. He continues:

The notion of women domesticating men is slightly misleading, however. Men stayed at home and behaved like dads presumably because this maximized the number of children they could raise to maturity. Why could they not leave more offspring by behaving like cads all of the time? One possible reason is that the male relatives of their female love interest protected her, which would have made being a cad quite risky. Another plausible reason is that impregnating a woman takes a surprising amount of time. Whatever the reason, there is no need to see the sexual behavior of men as driven by conscious reproductive intent. Other species manage fine without any such intentions and so, in all probability, did our ancestors.

Modern couples deciding to have a child often experience several months, or even years, of steady sexual intercourse before the woman conceives. Unlike most other mammals, including the primate order to which we belong, women do not have a distinct period of sexual heat, or estrus, which induces mating at a time when impregnation is most likely. There is a very slight increase in women’s sexual motivation around the time of ovulation, but the probability of having sexual intercourse is roughly the same at all stages of the menstrual cycle.

Among humans, ovulation is cryptic, or concealed. This phenomenon could reflect the evolutionary dance of the sexes. According to University of Michigan biologists Richard Alexander and Katherine Noonan, concealed ovulation is a means by which our female ancestors got fathers to invest in their children. If men cannot detect when women are ovulating, then a cad strategy may not produce as many surviving children as a dad strategy. If one assumes that Alexander and Noonan are correct, concealment of ovulation is an effective technique by which women have reined in men’s polygynous tendencies, inducing them to stay at home with their wives, fathering most or all of a woman’s children and helping them to survive to maturity.

Stated from the woman’s perspective, women who had a very high sex drive around the time of ovulation, and who clearly advertised their reproductive condition to strange men, as female chimpanzees do through conspicuous sexual swellings, for example, would have been vulnerable to cads. They would have received less paternal investment for their children and would have raised fewer children to maturity. Concealed ovulation undermines a cad strategy and promotes a dad strategy because a man cannot time intercourse to coincide with a woman’s time of highest fertility. For this reason he may be more reproductively successful by staying with a single woman and siring all of her children, rather than pursuing many different women with a low probability of impregnating any of them. Once again, these “strategies” are unconscious products of natural selection.

One thing I’ve long believed is that humans, especially more highly evolved human groups, were gradually evolving away from a low-investment in children (“cad”) spread-your-seed-everywhere strategy toward a high-investment strategy that involves mating for life — and that romantic love, celebrated in poetry and art all through Western civilization from Ovid to the Provencal balladeers of courtly love to Keats and Poe, is evidence of our as-yet imperfect evolution in that direction.

A long evolutionary history of living with men who were physically stronger likely promoted the more finely tuned interpersonal skills of women. Women made up for a relative lack of bodily strength by developing aptitudes for predicting and controlling the behavior of men.

Absolutely fascinating, and likely to set the reader thinking about many things — not just sex, but human societal structures, morality, parenting, politics, and racial matters — from a biological perspective instead of a dogmatic one. This is exactly the kind of thinking we need among the decision-makers of our civilization. Thank you, Nigel Barber. Every intelligent man and woman should read this book and consider its implications.

The Science of Romance – by Nigel Barber

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