by Patrick Appel

One of Christopher Ryan's readers asks:

My question is, could there be a natural selective pressure in post-agricultural societies to favor monogamy, such as if offspring raised by monogamous parents would be more likely to "succeed" (have higher fitness, or reproductive success) than those raised by single parents?

Ryan's answer:

What's the genetic correlate to "monogamy?" In other words, assuming there are no genes specifically devoted to making one more or less prone to long-term sexual monogamy, how would the very significant selective pressures you describe affect the genome?

I have to say up front that neither I nor my co-author are experts in genetics, so I may well be missing something, but while I see how monogamy could have been promoted by very strong familial, cultural, and economic pressures (see centuries of arranged marriage among the wealthy and powerful of Europe, for example), I don't see how that would be replicated at a genetic level...There may be some association with genes that affect novelty-seeking behavior perhaps, or overall libido, but I can't imagine it getting more specific than that.

In addition to the selective pressures Ryan's reader notes, what about disease?

In her book on HIV, Elizabeth Pisani argues that Western nations haven't seen the same sort of heterosexual HIV epidemics as some African nations partially because of differing sexual practices. Citizens of nations such as America are more likely to practice serial monogamy, i.e. one sex partner after another. HIV is most infectious in the early stages, so monogamy limits its spread and makes it more likely that those with HIV will become aware of their condition before passing the disease to multiple partners. According to Pisani, in several African nations individuals are much more likely to have several sexual partners at the same time and they are therefore more likely to both contract and spread HIV through these networks. It seems possible that as population density increased, which caused tremendous death and poverty, sexually transmitted diseases made monogamy evolutionarily advantageous, in some communities at least. The Sex At Dawn authors get half-way there on page 208:

While there were no doubt occasional outbreaks of infectious disease in prehistory, it's unlikely they spread far, even with high levels of sexual promiscuity. It would have been nearly impossible for pathogens to take hold in widely dispersed groups of foragers with infrequent contact between groups. The conditions necessary for devastating epidemics or pandemics didn't exist until the agricultural revolution.

Any other theories? Or problems with this one?

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