Sex and Symmetry

Randy Thornhill first made his mark with research showing that female scorpion flies choose their mates based on the size of the “nuptial gifts”--dead insects and the like--with which males present them. It was the first field study to test the role of female choice in Darwin’s theory of sexual selection.

Later, Thornhill joined several other biologists in studying wild jungle fowl, the ancestors of domesticated chickens. They found that hens prefer roosters with bright red combs, which signal their overall health, more evidence of female choice.

Thornhill and a colleague, evolutionary psychologist Steve Gangestad, recently studied 105 undergraduate couples to see whether principles of attractiveness based on evolutionary models apply to people.

They took detailed medical and sexual histories and measured seven body traits in the men intended to gauge how symmetrical they were. They also scanned photos of men’s and women’s faces into a computer and precisely measured their features.

“What we’re finding is the best predictor of female orgasm in a mateship is male body symmetry, and that’s exactly what we expected to find,” Thornhill says. “More symmetrical people have more partners and begin sex at an earlier age.”

Thornhill thinks women with attractive mates reach orgasm more easily because they want the use of their genes. He cites research by English scientists suggesting that when a woman experiences orgasm, she retains more of her partner’s sperm, presumably increasing the likelihood of fertilization.