Cheating is a fact of life. Ostensibly monogamous relationships in the animal world are not really monogamous. About a quarter of the offspring raised in nests of socially monogamous birds, such as barn swallows are sired by males other than the mate.

In a previous post I argued that infidelity is a part of the human heritage also.
But why? If a man, or woman, is unhappy with some aspect of their live-in lover, or spouse, why did they agree to shack up with them in the first place?

Not long ago, this would have seemed an easy problem to researchers. For women, the main benefit of infidelity was better genes for her offspring with the ancillary benefit of gifts provided by lovers. For a man, the advantage was simply spreading his genes around or siring more children.

Goodbye wild oats theory
In recent years, both of these theories have been dashed on the rocks of skepticism and contradictory evidence. As to the male explanation, this works only if men differ widely from each other in the number of children they sire. Yet. recent data on subsistence human societies contradict this view: men's reproductive success is no more variable than that of women (1). The bottom line is that seeking to maximize the number of children sired was not be a viable (unconscious) reproductive strategy for men. One plausible reason is that fathers were much more important in providing nutrition for young children than anthropologists used to think.

The female rationale for infidelity was simpler. A woman who threw in her lot with the best man available had second thoughts when she beheld the sculpted figure of a natural athlete. She yielded to temptation and her progeny benefited from having better genes.

Goodbye good genes theory
The key problem with the good genes theory is that no one has ever been able to nail down exactly what makes genes good. If there is plenty of food, it is fine to look like Mr. Universe but most local ecologies would be far too poor to support that sort of physique and a much slighter build is found for men in most subsistence societies.

Any gene that is good for one environment may prove a liability if the environment changes. Perhaps that is why the evidence relevant to good genes is so bafflingly inconclusive as I pointed out in my post on the peacock's tale.

Even if there really were good genes, and if females preferred to mate with males having these good genes, then the bad alternatives would get bred out and all males would have to have good variants.

Parasite resistance
One aspect of the good genes theory that seems to hold up in animal research is the view that females select males who are resistant to local diseases and parasites.

This theory seems to hold up for women also. In societies where parasites are prevalent, women are more interested in physical attractiveness of a man. They are also more interested in casual sex, suggesting that one motive for infidelity is to acquire better immunity for their children.

Another fairly obvious benefit from infidelity for females is to look for a new permanent partner and women generally engage in extramarital relationships when they are dissatisfied with the emotional quality of their marriage.

Sauce for the gander
So much for women! Why do modern men engage in extramarital sex if this was not particularly likely to increase the number of offspring raised to maturity by their ancestors? There is little evidence that dissatisfaction with their spouse motivates men to cheat as a way of initiating a new marriage.

Men may be pursued by women intending to supplant their spouse, of course. Otherwise, there are two plausible masculine motives for cheating - the desire for sexual variety, and attraction to younger, more fertile, women. The first seems like reproductive opportunism but the second might reflect the psychology behind selection of a long term mate.

Men are interested in sex without strings attached but cheating alone is not a viable reproductive strategy. The reason is that the vast majority of women resist it.

(1) Brown, J., Laland, K. N., & Borgerhoff Mulder, M. (2009). Bateman's principles and human sex roles. Trends in Ecology and Evolution, 24, 297-304.

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