Sleep around with too many people, and your lover will probably leave you. Sexual choice is not just the power to initiate relationships, but the power to end them. Our capacity for sexual fidelity, imperfect though it may be, is a result of our ancestors favoring the faithful by breaking up with the unfaithful. As David Buss has emphasized, humans have evolved specialized emotions for detecting and punishing infidelity in sexual relationships, distinct from our instincts for detecting cheats in reciprocity relationships.
Evolutionary psychology has rightly stressed how pervasive human sexual infidelity is when compared with the cultural ideal of monogamous commitment. The greater tendency of males to philander is certainly consistent with the predictions of sexual selection theory. However, I also find it astonishing how faithful most humans are when compared with other mammals. Some male birds are relatively faithful, but most male primates never turn down an opportunity to copulate with a willing female. Other female primates show no sense of sexual commitment to a particular male. If a better male comes along and a female is not too afraid of a jealous beating (as can often happen to female chimpanzees), she may switch partners. Humans are different. We value sexual fidelity in others, and have the capacity to inhibit our own courtship and copulation behavior, even in the face of awesome temptation.
Fidelity could be viewed as an example of reciprocity, insofar as cheating by one individual tends to provoke punishment by the other. However, the punishment is usually implemented by sexual choice. The "punisher" ends the relationship, denies further sexual access, or chooses to have sex with someone else. It may not matter whether we view this as sexual choice in the service of reciprocity, or reciprocity in the service of sexual choice. In either case, sexual preferences favor the virtue.
Sexual selection produced a sort of two-stage defense against sexual infidelity: romantic love, and then companionate sexual commitment. Romantic love powerfully focuses all courtship effort on a single individual to the exclusion of others. For at least a few weeks or months, it inhibits infidelity. Needless to say, romantic love is sexually attractive. It may not increase the appeal of an otherwise unattractive individual enough to provoke mating, but, all else being equal, it is clearly valued in mate choice. Love evolved through sexual selection, not least because it signaled fidelity.
However, passionate sexual love, "being in love," rarely lasts more than a couple of years. That is not nearly long enough to keep a couple together to raise a toddler, which they may have a shared interest in doing. Much more important over the long term is the feeling of friendly, mutually respectful sexual commitment. This does not work by shutting off all sexual attraction to others, but by managing that attraction through flirtation and sexual fantasy. The human capacity for flirtation (sexually inhibited pseudo-courtship) is one of the modern world's most underrated virtues, the principal spice of adult social life throughout history Equally important is sexual fantasy the spice of adult mental life. It permits sexual infidelity in the virtual reality of the imagination, without offending one's real sexual partner as much as a real affair would.
Our sexual fidelity evolved as a compromise between two selection pressures. On the one hand there was sexual selection favoring high fidelity through romantic love and sexual commitment. On the other hand there were the potential reproductive benefits of philandering. Especially for males, those potential benefits made it maladaptive to completely turn off their sexual attraction to everyone other than their partners. Flirtation and fantasy sometimes escalated into real affairs, and those affairs sometimes gave our ancestors net reproductive benefits. Sexual choice could not reach into our minds and totally eliminate polygamous desires. It could only punish observable infidelity and blatantly wandering eyes. We are not always sexually faithful, but that does not mean that our capacity for fidelity is a flawed adaptation. It may be perfectly adapted to a Pleistocene world in which the highest reproductive success went to those who were almost always faithful, except when a significantly more attractive option arose.
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