This post initially appeared on Science Blogs

Anthony Weiner is an idiot. I think we can at least all agree that if you're going to use a social networking site to spread illicit photos of yourself, you damn well better learn the difference between direct messaging and displaying your crotch to all of your followers. That said, all of us are idiots sometimes. And if I had to put money on it, I'd wager dollars to donuts that the thing we are the most stupid about is sex. But why?

What's the point of sex? You may think I'm the idiot for asking the question - the answer is obvious, right? Well...

The answer to why humans have sex turns out to be a more complicated than you might think. Obviously, reproduction is one reason. But compare how many times you've had sex and how many children you have. Clearly, there must be something else going on. What about, "We have sex because it feels good." But why does it feel good? If your answer is "So that we'll have sex,"

It's not that science never asks questions about sex. It's just that a lot of the studies are bad, and a lot are weighed down by prejudice, squeamishness or both.

By way of example: Most people believe there is a difference between men and women regarding attitudes towards sex. Men are more promiscuous, and women are more selective. The standard explanation for this is as follows:

Men must invest relatively little time and resources to potential offspring, so having sex with as many partners as possible increases potential progeny without much cost. However, men can not be sure of paternity once a woman gets pregnant. Women, on the other hand, must invest huge amounts of time and resources in each child, but they can know that the child is theirs. This led to something called the "Sexual Strategies Theory," in which men and women have evolved different attitudes and behaviors towards sex that will maximize their evolutionary fitness (the number of children they produce that go on to have children of their own). Leaving aside the assumption that sex is only about reproduction, a lot of the "science" that purports to support this theory is circumstantial at best. Take one of the best known examples: At a college campus, an attractive female grad student would approach random men and say, "I've been noticing you around campus and find you very attractive. Would you go to bed with me tonight?" Most of the men said yes. When an attractive male graduate student asked the same of random women, most of them declined. Using this data, researchers concluded that this supported sexual strategies theory - men are promiscuous and women are selective, because they've evolved to maximize reproductive success. Never mind the rampant casual sex that occurs on college campuses, or the increased danger women face in casual sexual encounters. The data fits the theory, so the theory must be right (for the best demolition of this theory, check out this post about a follow-up study - it's long but fantastic)

But what if the theory is wrong? What if even the underlying assumptions about human sexuality are wrong? That is the premise of Sex at Dawn: The Prehistoric Origins of Modern Sexuality by the husband and wife team of Christopher Ryan and Cacilda Jethá.

The major thesis of the book is that humans did not evolve to be in long-term, sexually monogamous relationships. Far from it, Ryan and Jethá conclude that pre-agrarian human societies were exceedingly promiscuous. Drawing on archeological, behavioral and anatomical evidence, as well as analysis of extant pre-agrarian human societies and our closest primate relatives, the authors build an argument with page after page of clear, convincing and often humorous discussion. The book is not for the sexually repressed though. It was a little unsettling to read lengthy treatises on the size of primate testicles on the bus, but these authors clearly know their stuff, and know how to build their case. Speaking of testicle size, here's the argument in a nutshell: It turns out humans have huge huevos. Maybe not squirrel huge (NSFW), but still pretty big compared to related species. Large testicles in animals usually points to sperm competition. Sperm competition usually means that females mate with multiple males. Bonobos and chimps, our closest primate relatives and profligate polygamists, also have sizable scrotums. Bonobos have bigger balls (and more sex) while chimps' are slightly smaller. By contrast, gorillas have tiny testes - but male gorillas enforce their primacy by physically preventing other males from mating with their harem. Male gorillas have enormous muscles, so their sperm don't need to do as much work.

If giant gonads isn't enough to sway you, there's plenty more where that came from. My favorite aspect of this book is the extensive critiques of the science that supports human monogamy. For example, when looking for evidence of monogamous primates, scientists that want to uphold the standard view often turn to gibbons. Ryan and Jethá point out that our last common ancestor with gibbons was over 20 million years ago (as opposed to 6 million years for chimps and bonobos), gibbons rarely have sex unless it will lead to pregnancy, they don't live in social communities and of course, they have small testicles. They also cite lots of studies on primitive human cultures that appear to be monogamous, but upon closer scrutiny are anything but. There's a culture in the amazon that supposedly has "marriage," but in order to "divorce" and remarry, a woman just has to hang her hammock in another man's hut. Another supposedly monogamous tribe has hunting ceremonies where men are required to chose a mate other than their wife, and if the chosen woman's husband objects, he's ostracized. There's much more, but I fear I can't do it justice. Unfortunately, since I'm not trained as an anthropologist, I can't be certain that the authors aren't cherry-picking examples that support their conclusions. Thomas MacAulay Millar, who blogs at Yes Means Yes (and wrote the amazing critique of the sexual strategies theory research I mentioned above) is a bit more circumspect:

My take was that they did a wonderful job of undermining the arguments that pair-bonding, chimp-focused folks make, but a less good job actually making an affirmative case.

He notes that, as with most evo psych research, they're almost entirely dependent on circumstantial evidence. To me, it seems like a lot of evidence, and the weight of it all makes it more compelling. But maybe I just want to be persuaded. Besides that, Millar notes:

Christopher Ryan makes little attempt to hide that he's writing a brief for polyamory, so read him essentially as a polyvangelist, if you know what I mean.

Fair enough, but I definitely recommend you read him just the same.

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