Unlucky in love: "T-shirt sniffing" studies have shown that the birth control pill causes women to select partners who are genetically similar to themselves.
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CANBERRA: Oral contraceptive pills may adversely affect a woman's choice of partner, says a new British study.
Subtle differences in odour are an important factor in controlling both who we find sexually attractive and who we later choose as a partner.
Now, researchers led by Craig Roberts, an evolutionary biologist at the University of Newcastle in England, have confirmed that women prefer different body odours when they take the contraceptive pill. The findings are reported today in the U.K. journal the Proceedings of the Royal Society B
MHC genes
Many studies in humans and other mammals have found that females are more attracted to odours of genetically dissimilar males.
To probe the effect of the pill on this phenomenon, Roberts compared the odour preferences of 60 women who didn't take the pill to those of 37 women who started taking the pill part way through the study.
As both body odour and body odour preferences are partly controlled by a group of genes known as the major histocompatibility complex (MHC), the researchers took blood samples from all of the female and male participants and analysed their DNA.
"We [then] captured men's body odour by asking them to wear T-shirts to bed for two consecutive nights," said Roberts. "We tried to control as far as we could for non-body smells, so our men were non-smokers, used non-perfumed soap to wash and avoided smoky bars, strong-smelling foods and deodorants."
For each female participant, the researchers selected T-shirts from three genetically similar and three genetically dissimilar men. The women were asked to rate the six different T-shirts based on odour pleasantness, intensity and desirability.
Though Roberts found no significant preference for the smell of genetically dissimilar males at the start of the study, he did find that the women who started to take the pill part way through appeared to develop a preference for males who were genetically similar to themselves.
A 1995 study, led by Claus Wedekind, now at the University of Lausanne in Switzerland, found a similar result, but as it was based on just 18 participants, it was difficult to draw definitive conclusions from it. Wedekind is the pioneer of so-called "T-shirt sniffing" studies that found a link between mate preference and MHC genes in the 1990s.
Increased chance of divorce
"Our study shows for the first time that the pill [actually] shifts preferences towards genetically similar men," said Roberts. He speculates that this may not only increase the chance of separation or divorce if the female partner stops taking the pill, but could also reduce a couple's chances of conception.
"If pill use increases the chance of selecting a more genetically-similar partner than a woman would otherwise choose, then, when the woman comes off the pill to start a family, she may find it harder to conceive." Children of genetically similar parents are more likely to have birth defects that cause them to be naturally aborted in the womb.
Alexei Maklakov, an evolutionary biologist from the University of New South Wales in Sydney, Australia, thought the study was interesting, but said that it's difficult to extrapolate findings like this to the population at large, because the study subjects used are not a random sample of the population.
"Unfortunately, it is impossible to use true randomised trials when analysing human reproductive behaviour," said Maklakov. "For this reason, it is always difficult to get around the fact that subjects are not a random sample from the population."
The study group was not representative of the population at large because a financial incentive was offered to encourage women to participate and because the researchers had no control of who would start taking the pill during the study.
MHC genes are an important part of the immune system, and experts think variety within the complex is important to offer the best protection against disease.
The theory is that new combinations of genes slow the rate at which disease pathogens can evolve to break through our defences, and this is why we have evolved to favour partners with different MHC genes to our own.
