Saturday, November 19, 2011

Sex at Dawn: Ryan and Jethá

Christopher Ryan and Cacilda Jethá's Sex at Dawn argues convincingly that monogamy isn't particularly natural for humans. It certainly is one common choice, but many modern people have a lot of trouble sticking to the program despite a lot of exhortation and systemic incentives promoting the practice. Ryan and Jethá marshall evidence from anthropology, evolutionary evidence, comparisons with other primates, and examinations of current practices. Their main argument is that a reasonable definition of "naturally monogamous" would mean that most people pair up with someone from the opposite sex, and aren't tempted to stray. There are a few species that mostly act that way, but looking at the broad range of what humans do, we're not like that. It's an interesting question as to why sociologists, and anthropologists seem to want us to believe that it is natural in the face of all the evidence.

There are several places in the book where the authors don't seem to really understand how evolution works. When talking about male parental investment, they ridicule the notion that maximum reproductive productivity is anyone's goal. It's clear from context that they're misunderstanding a discussion in which individuals are described as acting as if maximizing fecundity is the goal. But the evolutionary reasoning is just that those individuals who produce more offspring end up predominating in subsequent generations, regardless of why they acted that way. But regardless of this, they still make a strong case.

When biologists compare anatomy and mating behavior across species, human genitalia and sexual cycles don't make sense for a species in which couples stick together over the long term and don't cheat on one another. The size of male Genitalia, timing and (lack of) visibility of ovulation, breast prominence, are all unnecessary if the pair bond is unshakeable. They make sense when you assume each individual normally mates with multiple individuals of the opposite sex.

Our close relatives the chimpanzees and bonobos don't restrict themselves to single partners and we look more like humans evolved in an environment where individuals didn't restrict their attention to a single partner. In this kind of environment, evolutionary pressures push toward the large penises (by body weight), external scrotum, long duration of intercourse, and large volume of ejaculate you see in humans. If our ancestors had had reliable access to a partner, they wouldn't have needed these (evolutionarily) expensive features.

Another myth they take on is that of the demure female, uninterested in sex. It certainly occurs, but it's not predominant, either in societies (like ours) that constantly promote the idea or in societies that don't. Ryan and Jethá also make it clear that, evolutionarily speaking, homosexuality is nothing to be ashamed of. Our nearest relatives and many other species engage in the practice, though seldom exclusively. Mainstream society's insistance that each person can be categorized as either heterosexual or homosexual, is just not consistent with our behavior or the evolutionary or anthropological evidence.

Anyway, if you're not sqeamish about these topics, it's a fun, eye-opening read. Not likely to change anyone's behavior, but maybe some people will feel less constrained about their choices. It'll probably also provide grist for some arguments, but that's a fine thing, too.

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