The link between attractiveness and facial symmetry seems to hold across both black and white faces, but also in non-human primates, according to a study just published in the open-access science journal PLoS One.

One of the most striking studies in sex and symmetry research isn’t mentioned, however. A 1995 study found that the likelihood of female orgasm during sex was related to the extent of bodily symmetry in the male partner.

The study was led by biologist Randy Thornhill and recruited 86 young couples who completed a number of relationship questionnaires, including one on how often the female partner orgasmed during sex. The males then had their bodies measured and assessed for how much one side differed compared to the other – a measure of bodily asymmetry.

In the final analysis neither the male’s age, wealth, social skills, physical attractiveness or relationship style predicted the frequency of female orgasm. Only male bodily symmetry was statistically associated with the chance of the women climaxing during sex.

The researchers thought that maybe women who have more orgasms, or who are just more sexual, simply get the more symmetrical (maybe hotter) guys. But when they looked at frequency of orgasm outside copulation (such as during oral sex or masturbation), the relationship to male symmetry disappeared, suggesting that this wasn’t the case.

This study, and the new study published in PLoS One, also suggested that symmetry was associated with more masculine features generally – a bigger body in the orgasm study, and a more typically male face in the PLoS research.

The evolutionary explanation suggested by the authors is that female orgasm during copulation may make pregnancy more likely, so it’s an adaptive strategy to increase fertility when making love to males with genes more likely to lead to healthy children.

How orgasm increases with body symmetry is not clearly understood, though. The authors speculate that female perception of a highly symmetrical male might psychologically prime sexual arousal, but the mechanism is left largely to guesswork.

Link to PLoS One study on attractiveness and symmetry (via Anthro).

Link to abstract of orgasm and symmetry study.