Whence the female orgasm? After 40 years of debate evolutionary biologists are no closer to deciding whether it evolved to give women a reproductive boost, or whether it is simply a by-product of male orgasm evolution. The latest attempt to settle the dispute involves quizzing some 10,000 twins and pairs of siblings on their sexual habits.

Some evolutionary biologists reckon the female orgasm is adaptive and possibly influences mate choice, strengthens pair bonds or indirectly helps to suck sperm into the uterus. Others argue that women have orgasms for the same reason that men have nipples – being highly adaptive in one sex, the traits tag along for the ride in the other.

Brendan Zietsch at the University of Queensland, Australia, and Pekka Santtila at Abo Akademi University in Turku, Finland, think they can help to settle the question. If female orgasm is a simple by-product of male orgasm, the duo argue, then similar genes would underlie orgasmic function in both men and women. As a consequence, opposite-sex twins and siblings will share more similarities in their susceptibility to orgasm – “orgasmability” as Zietsch calls it – than pairs of unrelated people.

Timing’s everything

To measure this orgasmability, the researchers used survey data from just under 5000 sets of identical and non-identical twins and pairs of regular siblings. The questionnaire asked about the time to orgasm in men and the frequency and ease of orgasm in women.

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In keeping with previous findings, Zietsch and Santtila found that same-sex identical twins had more orgasmic similarity than same-sex non-identical twins and siblings, showing that genes do play a role in orgasmic function and apparently providing some evidence that the by-product scenario might be correct.

However, contrary to the expectations of the by-product scenario, the two researchers found that opposite-sex twins and siblings had virtually no correlation in orgasmability.

Premature extrapolation

“This indicates that the genes that influence orgasmic function in men are not the same as those in women,” says Zietsch. In other words, male and female orgasm evolved through different genetic routes, and the by-product hypothesis is incorrect.

Those who favour the by-product hypothesis think such a firm conclusion is premature. Kim Wallen at Emory University in Atlanta, Georgia, points out that the study measured different things in men and women – timing to orgasm versus likelihood of orgasm – and so a correlation would be unlikely. Zietsch counters that different measures were necessary because of the different nature of male and female orgasm.

“Of course, it’s possible that different questions would reveal different results,” says David Puts, a behavioural anthropologist at Pennsylvania State University in University Park, who places himself in the adaptive camp. “But this study certainly isn’t helping the by-product theory.”

Journal reference: Animal Behaviour, DOI: 10.1016/j.anbehav.2011.08.002