The central fact that controls the the preferences of both sexes is that bearing children is difficult and dangerous for women, but fertilizing a woman is almost trivially easy for a man. Furthermore, the female investment in childbearing is front-loaded (proportionally more of the risk is before and at birth) while the male investment is back-loaded (proportionately more of the risks and costs are incurred after birth).

Moderns living in a largely disease-free environment seldom realize how cruel and pressing these differences were over most of our species history. But before modern sanitation, death in childbirth was so common that men wealthy enough to afford it expected to have several wives during their lifetimes, losing many of them to childbed fever and other complications.

Also relevant is the extremely high rate of childhood death from infectious diseases and parasites that was characteristic of premodern societies. Disease resistance in humans is highly variable and generally increases with genetic mixing (the same reason a mongrel puppy or kitten is less likely to catch a disease than a purebreed). Thus, both men and women have instincts intended to maximize genetic variety in their offspring in order to maximize the chances that some will survive to reproductive age.

Our instincts evolved to cope with these patterns of life and death. The next piece we need to understand those instincts is what physical beauty means. Recent anthropology revealing strong cross-cultural patterns in the perception of pulchritude is helpful here.

In both sexes, the most important beauty indicators include symmetrical features and a good complexion (clear skin without blemishes, warts, etc.). It turns out these are indicators of resistance to infection and parasites, especially resistance in childhood and during adolescent growth. Good hair is also a health indicator.

In men, physical signs of strength, dexterity, and agility are also favored; this reflects the value female instinctive wiring puts on male specializations in burst exertion, hunting, and warfare. In women, signs of fertility and fitness to bear are favored (healthy and generous breasts, a certain range of hip-to-waist ratios).

Men fixate on physical beauty and youth because under primitive conditions it is a leading indicator of the ability to bear and suckle children. Through most of history, plain or ugly women were bad risks for the next round of infectious diseases — and their children, carrying their genes, were too.

The last piece of the puzzle is that men and women have asymmetrical information about the parentage of their children. A woman is seldom in doubt about which children are the issue of her womb; a man, by contrast, can never be as sure which are the fruit of his seed. Thus, genetic selfishness motivates the woman in a mated pair to sacrifice more for her children than it does the man. This is why women abandon their children far less often than men do.

While women do respond to male good looks, it’s not the agenda-topper for them that it is for men. To understand why this is, it helps to know that the optimal mating strategy for a woman begins with hooking a good provider, a man who will stick around to support the kids in spite of not being as sure that he’s their father as the woman is of being their mother. Where men look for fitness to bear children, women seek the capability and willingness to raise them.

Thus, robust health and infection resistance, while desirable in a potential husband, are not the be-all and end-all. Behavior traits indicating attachment, loyalty, nurturance, and kindness are more important than a tight six-pack. Men instinctively worry about these things less because they know women are more certain of parentage and thus more tightly bonded to their children. Fitness-to-raise also means that indicators of success and social status count for more in men. Men marry health and beauty, women marry security and good prospects.

There is, however, one important exception — one circumstance under which women are just as physical, beauty-oriented, and “shallow” in their mating preferences as men. That’s when they’re cheating.

Both sexes have a genetic-diversity incentive to screw around, but it manifests in different ways. Again, the reason is parentage uncertainty. For a man, diversity tactics are simple — boff as many hot babes as possible, accepting that you don’t know which of their kids are yours and counting on stronger maternal bonding to ensure they will have at least one devoted parent around. Because a woman can be more sure of who her offspring are, her most effective diversity tactic is different — get married to a good provider and then cheat on him.

Under those circumstances, she doesn’t have to value good character in a mating partner as much; hubby, who can’t tell the kids aren’t his, will supply that. Thus the relative value of handsomeness goes up when a woman is taking a lover on the sly. Marrying the lord and screwing the gardener is an old game, and from a genetic-selfishness point of view a very effective one.

Eric S. Raymond, “A Unified Theory of Male Slobbishness and Female Preening”, Armed and Dangerous, 2005-01- 06.