If indeed a man's great Darwinian peril is cuckoldry, and a woman's is desertion, then male and female jealousy should differ. Male jealousy should focus on sexual infidelity, and males should be quite unforgiving of it; a female, though she'll hardly applaud a partner's extracurricular activities, since they consume time and divert resources, should be more concerned with emotional infidelity -- the sort of magnetic commitment to another woman that could eventually lead to a much larger diversion of resources.

Another study, by the British biologists R. Robin Baker and Mark Bellis, found that women who cheat on their mates are more likely to do so around ovulation. This suggests that often the secret lover's genes, not just his resources, are indeed what they're after.

Is it possible "that females adjust their reproductive strategies in adolescence to their own assets"? In other words, maybe adolescent girls who get early social feedback affirming their beauty make the most of it, becoming sexually reserved and thus encouraging long-term investment by high-status males who are looking for pretty Madonnas. Less attractive women, with less chance to hit the jackpot via sexual reserve, become more promiscuous, extracting small chunks of resources from a series of males.

Women surrounded by men who are unwilling or unable to serve as devoted fathers may simply feel a deepened attraction to sex without commitment -- feel, in other words, a relaxation of "moral" constraint. And perhaps if market conditions suddenly improve -- if the male to female ratio rises, or if men for some other reason shift toward a high-investment strategy -- women's sexual attractions, and moral sensibilities, shift accordingly.

Emotions are just evolution's executioners.

All told, then, institutionalized monogamy, though often viewed as a big victory for egalitarianism and for women, is emphatically not egalitarian in its effects on women. Polygyny would much more evenly distribute the assets of males among them. It is easy -- and wise -- for beautiful, vivacious wives of charming, athletic corporate titans to dismiss polygyny as a violation of the basic rights of women. But married women living in poverty -- or women without a husband or child, and desirous of both -- could be excused for wondering just which women's rights are protected by monogamy. The only underprivileged citizens who should favor monogamy are men. It is what gives them access to a supply of women that would otherwise drift up the social scale.

Lasting love is something a person has to decide to experience. Lifelong monogamous devotion is just not natural -- not for women even, and emphatically not for men. It requires what, for lack of a better term, we can call an act of will.

For families at the upper end of the socioeconomic scale, sons are a better investment than daughters.

We should probe our commonsense reactions to evolutionary theories carefully before concluding that the common sense itself isn't a cognitive distortion created by evolution.

Gratitude, by reflecting the value of the benefit received, calibrates the repayment that's in order. Gratitude is an I.O.U., so naturally it records what's owed. For the benefactor, the moral of the story is clear: the more desperate the plight of the beneficiary, the larger the I.O.U.

Human beings aren't "fitness maximizers" but rather "adaptation executers."

Men seem loath to concede the superiority of another human being, even in such trivial realms as municipal geography. The reason, perhaps, is that during human evolution males who too readily sought reconciliation after a fight, or otherwise needlessly submitted to others, saw their status drop, and with it their inclusive fitness.

Once you get past stereotyped acts such as walking, eating, and suckling, "behaviors" don't get transmitted biologically. Mental organs do, and they're usually limber enough to yield lots of different behaviors, depending on circumstance.

Indeed, in the social psychology laboratory, people not only tend to attribute success to skill and failure to circumstance; they tend to reverse the pattern when evaluating others. Luck is the thing that makes you fail and other people succeed; ability works the other way around.

If advertisement is so deeply ingrained in people, why are there self-deprecators? One answer is that self-deprecation is without cost when everyone knows better, and can actually have some benefit; a reputation for humility boosts the credibility of subtle boasting.

In fact, you could say that low self-esteem evolved as a way to reconcile people to subordinate status when reconciliation is in their genetic interest. Don't expect people with low self-esteem to hide it. It may be in their genetic interest not only to accept low status, but, in at least some circumstances, to convey their acceptance of it -- to behave submissively so that they aren't erroneously perceived as a threat and treated as such.

It is as if the mind were programmed with a simple rule: so long as accounts are settled, no special rationalization is in order; the symmetry of exchange is sufficient defense of your behavior. But if you cheat or abuse another person who doesn't cheat or abuse you, you should concoct reasons why he deserved it. Either way, you'll be prepared to defend your behavior if challenged; either way, you'll be prepared to fight with indignation any allegations that you're a bad person, a person unworthy of trust.