MHC-correlated odour preferences in humans and the use of oral contraceptives

Abstract

Previous studies in animals and humans show that genes in the major histocompatibility complex (MHC) influence individual odours and that females often prefer odour of MHC-dissimilar males, perhaps to increase offspring heterozygosity or reduce inbreeding. Women using oral hormonal contraceptives have been reported to have the opposite preference, raising the possibility that oral contraceptives alter female preference towards MHC similarity, with possible fertility costs. Here we test directly whether contraceptive pill use alters odour preferences using a longitudinal design in which women were tested before and after initiating pill use; a control group of non-users were tested with a comparable interval between test sessions. In contrast to some previous studies, there was no significant difference in ratings between odours of MHC-dissimilar and MHC-similar men among women during the follicular cycle phase. However, single women preferred odours of MHC-similar men, while women in relationships preferred odours of MHC-dissimilar men, a result consistent with studies in other species, suggesting that paired females may seek to improve offspring quality through extra-pair partnerships. Across tests, we found a significant preference shift towards MHC similarity associated with pill use, which was not evident in the control group. If odour plays a role in human mate choice, our results suggest that contraceptive pill use could disrupt disassortative mate preferences.

Figure 1

Mean (±s.e.) scores for odour pleasantness (white bars), intensity (light grey bars) and desirability (dark grey bars) and mean preference rank (black bars) according to the length of frozen storage (n=42).

Figure 2

Mean difference in odour ratings for MHC-similar and MHC-dissimilar men by pill-using and non-pill-using women in two rating sessions (open bar, first test; filled bar, second test). Positive scores indicate preference for MHC-dissimilar odours. (a) odour pleasantness ratings, (b) odour intensity and (c) odour desirability.

Figure 3

Effects of relationship status on MHC-correlated odour preferences. (a) Differences in partnership desirability ratings of MHC-similar and MHC-dissimilar male body odours by 42 single and 43 paired women. The interaction is significant (p=0.033). (b) Effect of the frequency with which women fantasize about sex with other men (p=0.080). Filled bar, MHC-similar male body odour; open bar, MHC-dissimilar male body odour.