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Why Do We Orgasm At All?

02/17/2013 02:26 pm ET | Updated Apr 19, 2013

Click here to read an original op-ed from the TED speaker who inspired this post and watch the TEDTalk below.

Scientific study of the orgasm has revealed some fascinating results, as seen in Mary Roach's TEDTalk. Her presentation is informative, titillating and to some perhaps scandalous. As a scientist who specializes in the study of sexual behavior, I especially appreciated her talk. But what drew me to science is my constant need to ask 'why'. Why do people behave the way they do? What is the purpose or design in an anatomical or behavioral trait? Regarding orgasm, readers may be surprised to learn this is a hot debate.

Some argue that orgasm is a by-product of selection for orgasm in males. In brief, the argument is that like male nipples, women's ability to orgasm is a holdover of natural selection deriving from shared developmental pathways. In other words, the ability to orgasm is little more than a happy accident for women.

On the other hand, some have found evidence suggesting that orgasm has a specific purpose and is not simply a by-product. While Roach showed that 'upsuck' doesn't appear to work out, data suggests that somehow orgasm may bias the retention of some man's sperm. We see this in our primate cousins and other species. For example, Japanese macaques orgasm more often with dominant males than with subordinate males. Human women are of course generally not mating with multiple males in a short period of time, but research does show that more attractive men have partners who orgasm more.

The jury's still out on female orgasm, but what about males? Much more straightforward, right? Well, yes and no. Of course, without male orgasm and ejaculation, reproduction is unlikely at best. So it's clear that the orgasm has a very specific function. But, there's more. For example, most of what is actually produced is not sperm. It's mostly water, sugars, and compounds that make the vagina safer for sperm. And recent work shows that human ejaculate may contain anti-depressant substances, and even encourage ovulation. It gets even weirder.

As it turns out, males in most species can benefit from mating as often as possible with as many female partners as possible. Of course, females have a say in this and tend to slow the males down. But in many species, it's not uncommon for a female to mate with multiple males over a short period of time. Of course each male wants (subconsciously of course) to be the one to impregnate her. One solution to this problem is simply to get more sperm in the fight. Just like a raffle, the best way to win what scientists call sperm competition is to have as many chances to win as possible. This is, in part, why a given ejaculate can consist of millions of individual sperm. Of course natural selection has made this competition even more complex. Some males (rats, for example) produce copulatory plugs to try and block other males' semen. Some species will leave their penis itself in the reproductive tract! Of course, human men don't do anything quite so drastic, but there is evidence that sperm competition has impacted human evolution.

Human males have (somewhat) big balls. Relative to their body size, human testicle size falls somewhere between our cousins -- the mostly monogamous gorilla and its remarkably small testicles, and the astonishingly large testicles of the extremely promiscuous chimpanzee. This suggests that we do have sperm competition in our mating system, at what is at least a small to moderate frequency. When sperm competition is a bigger problem, selection leads to more tissue to produce sperm. Furthermore, men produce more sperm when sperm competition is more likely. When is it more likely? When the man isn't around. If a woman in a relationship were unfaithful, it's not going to be when her partner is around. (This is a particular problem for men, who provide lots of paternal care, relative to most species.) To be cuckolded into raising another man's child unwittingly is, genetically speaking, very bad news. Thus, we find that men produce much more sperm when they spend an increasing percentage of time apart from their partner. Perhaps there is a potential for fertility treatment in that finding? If a couple is having trouble, and the man has a lowered sperm count, maybe a short vacation apart is in order.

Behavior too, is affected by sperm competition. Just as sperm counts increase, men who've spent more time apart from their partner report she is more attractive, think other men find her more attractive, and want to have sex even sooner. Even the sex itself is more vigorous. Recent research even shows that men are more interested in making sure their partner has an orgasm when they've spent more time apart from her. So if the love life is disappointing, particularly for the woman, that vacation might not hurt there either.

The science of orgasm is fascinating, titillating, and sometimes controversial, perhaps like the idea of orgasm itself. Continued research in this field will only serve to better our understanding of this once-taboo topic.

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