The evolution of the female orgasm, which has remained a mystery among evolutionary biologists, has been explored in a new study.
For the last 40 years biologists have been debating whether female orgasm evolved to give women a reproductive boost, or whether it is simply a by-product of male orgasm evolution.
Brendan Zietsch at the University of Queensland, Australia, and Pekka Santtila at Abo Akademi University in Turku, Finland, think they can help to answer the question.
If female orgasm is a simple by-product of male orgasm, they 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.
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.
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.
"This indicates that the genes that influence orgasmic function in men are not the same as those in women," New Scientist quoted Zietsch as saying.
In other words, male and female orgasm evolved through different genetic routes, and the by-product hypothesis is incorrect.