Pleasure and procreation

“Lie back and think of England.”

Queen Victoria, by most accounts, did not enjoy sex. And yet, she had nine children. It’s a reminder that pleasure and procreation may not be closely correlated.

Yet there is an hypothesis that is (dare I say) seductive in its simplicity. Orgasms make sex feel nice for women, causing women to have more sex, and therefore more babies. Therefore, female orgasms provide a fitness advantage, and are adaptive.

Whether female orgasm is adaptive or not has been an ongoing scientific controversy, because it it very difficult to provide evidence for it. After all, people generally don’t track their orgasms very closely over the long term, even if they did like talking about their orgasms with scientists. Now, for a feature like female orgasm to be an adaptation shaped by by natural selection, a few things have to be true: female orgasm would have to be:

• Variable across the population;
• Heritable;
• Make a difference to reproductive success. 

A short new paper provides presents more evidence about the last of these three.

Zietsch and Santtila used survey information from female twins in Finland, both identical (same genes) and non-identical (50% of gene shared). They reproduced some previous research the propensity for women to have orgasms has a genetic component.

They found a “weak but significant” correlation between orgasm rate and fertility. How weak? Correlation scores vary from 1 (perfect) to 0 (none), and the score here was closer to 0. Specifically, it was 0.06. This means whether woman have orgasms explains about one third of one percent of the variation in number of children (correlation score squared is how much variation is explained).

Furthermore, the correlation was weaker in identical twins, which was the opposite of what you would expect if orgasm rate caused the change in number of births. The authors found that how long the women had been in a relationship, and how often she had sex, were confounding most of the initial (tiny) relationship between female orgasm and number of children.

So even though there is variation and heritability, it is highly unlikely female orgasms make any difference to reproduction. Thus, this evidence suggests female orgasm is not adaptive: instead, it may just be a happy accident (very happy).

The body’s reaction to sex is not a sign of an increased chance of getting pregnant. This may be good news for women who are highly orgasmic: it means they do not need to double down on their birth control. No need for both the pill and a condom.

Reference

Zietsch BP, Santtila P. 2013. No direct relationship between human female orgasm rate and number of offspring. Animal Behaviour: in press. DOI: 10.1016/j.anbehav.2013.05.011