Women who take the pill may choose mates who are more genetically similar to them than those who don't — which is not a good thing, evolutionarily speaking, according to a new study.

When it comes to choosing a mate, the saying "opposites attract" is generally true. Because the immune system is so vitally important to survival, women typically — and perhaps unconsciously — choose men who have different immune structures from their own. This gives the offspring an advantage by having broader array of genes in their immune response, which is determined by genes in the Major Histocompatability Complex (MHC). But how do women makes this choice? Past research has suggested that it is largely due to odor: the genes in the MHC also code for body odor in the way they interact with bacteria on the skin. So by choosing a man who smells good to them, a woman is actually seeking an immune system whose make-up is different from her own.

Although the difference was not significant, single women tended to prefer the scents of MHC-similar men, while women in relationships picked MHC-dissimilar men.

Poorly matched couples — those who are genetically similar — have been reported to have trouble conceiving and higher incidents of miscarriage.

Based on earlier reports, researchers wondered what taking oral contraceptives might do to these choices. Researcher Craig Roberts and his team studied 97 women who were not initially on the pill and asked them to rate their reactions to six different male scents. Although the difference was not significant, single women tended to prefer the scents of MHC-similar men, while women in relationships picked MHC-dissimilar men. This finding makes sense since the latter group would typically be more geared towards having offspring. The study is published in the Proceedings of the Royal Society, Series B. Biological Sciences.

Two months after 40 of the subjects began taking the pill, however, their preferences shifted towards the odors of MHC-similar men.

Roberts suggests that this shift may be because the pill tricks the body into thinking it is pregnant, a state during which point there is no evolutionary pressure to mate. Any pressure to cultivate relationships might be towards relatives, who would help raise the baby, and these individuals would be MHC-similar to the female.

Roberts points out that in the extreme, "[n]ot only could MHC similarity in couples lead to infertility problems but it could ultimately lead to the breakdown of relationships when women stop using the Pill, as odour perception.