GENES behind the immune system may explain why some women have more sexual partners than others.

The major histocompatibility complex (MHC) is a region of DNA vital to the immune system. The more diverse the genes of the MHC, the more effective it is at conferring disease resistance, and in a variety of animals individuals with diverse MHCs are more likely to be chosen as a sexual partner. Its effect in humans, however, is unclear.

Hanne Lie of the University of Western Australia in Perth measured the diversity of the MHC in 74 heterosexual female students and asked them about the number of sexual partners they had had. After controlling for attitudes towards sex and age of first intercourse, she found that the greater a woman's MHC diversity, the more sexual partners she had had. There was no correlation with overall genetic diversity (Animal Behaviour, DOI: 10.1016/j.anbehav.2009.12.040).

Whether this means women with diverse MHCs have greater evolutionary success is debatable. Boguslaw Pawlowski, an anthropologist at the University of Wroclaw, Poland, points out that contraception means there may not be a strong correlation between number of sexual partners and reproductive success.