Contraceptives Alter Scent and Partner Selection

Summary
Human and animal studies show that the major histocompatibility complex (MHC) genes – a group of genes that influence immunity – influence odor. Studies have shown that females prefer the odor of mates with dissimilar MHC. This is reversed in women on oral contraceptive pills. Ninety-seven women completed this study, wherein they had to select favorable odors from men with either similar MHC or dissimilar MHC genes. It was found that single women preferred the odor of MHC-similar men, while women in relationships preferred the opposite. This shows that the use of contraceptive pills could influence mate preferences.

Introduction
Odor plays an important role in the selection of a mate. Like in animals, major histocompatibility complex-associated odor preferences for a mate are seen in humans, too. Various studies have been done on women, correlating the preference of odors and MHC similarity. Although the results are inconsistent, it was found that women preferred the odors of men with dissimilar MHC. This could be a process of natural selection to control genetic inbreeding. In one of the studies, it was observed that the women on oral contraceptive pills preferred the odor of men with similar MHC. Similar MHC between partners could result in recurrent miscarriages and longer birth periods. Hence, this study assessed whether the use of oral pills could alter the female preferences for male body odor.

Methodology

  • The researchers selected 193 females who had regular menstrual cycles, were not pregnant, and were not on oral pills prior to the study. They were divided into two groups – the group given oral contraceptives (after the first assessment) and the control group.
  • Blood samples were obtained from the participants, to assess their MHC gene type. It was matched with that of three men.
  • T-shirts supplied by the researchers were worn for two consecutive nights by three MHC-similar and three MHC-dissimilar men. The women were asked to rate the odor of these T-shirts for pleasantness, desirability, and intensity. It was done between days 10 and 14 of the women’s menstrual cycles and repeated at the same time of the cycle, after 3 months.

Results

  • The odor from men with dissimilar MHC had no significant effect on odor pleasantness or desirability scores in the women who were not using the pill during the first assessment. The significance dropped further when only British women were questioned.
  • The scores given to the odor of all the six T-shirts were higher in ratings given by women in the pill group.
  • The ratings of the odors shifted in favor of those from men with similar MHC on beginning the use of pills. This was, however, not seen in the control group.

Shortcomings
Although the men were asked to refrain from using perfumes, they were not monitored during the study. This could have brought about a conflict in the results. The women using oral pills had a higher scoring pattern, which could have introduced bias in the study. The reporting of odor also depended on the type of oral pills and this aspect needs further research.

Conclusion
This study showed that the response of women on oral contraceptive pills, to the odor of men, was contrary to the response of women not on pills. The women on pills preferred men with similar MHC genes. Studies indicate that, “women consider the olfactory domain to be an important factor in their assessment of potential partners.” Thus, due to serious alterations in odor preference, the use of oral pills could influence partner choice. It could lead to the selection of a less preferred partner. A previous study demonstrated that the current or previous partner of the woman could impact her odor preferences. This, in turn, may influence her choice of a particular odor.

For More Information:
Influence of Oral Contraceptives on Odor Preferences in Humans
Publication Journal: Proceedings of the Royal Society Biological Sciences, August 2008
By S. Craig Roberts; L. Morris Gosling
From the University of Newcastle, Newcastle-upon-Tyne, UK

*FYI Living Lab Reports Are Summaries of the Original Research.



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