Women on birth-control pills are more likely to pick an incompatible partner because they cannot smell out someone compatible, according to a study by British researchers.
The study by researchers at the University of Liverpool and the University of Newcastle found that the pills interfere with the women's capability to sniff partners that are genetically dissimilar and hence compatible, LiveScience.com reported.
The researchers said that besides other factors body odour plays a critical role in a woman's choice of a mate.
The human body releases aromatic molecules that also indicate genetic compatibility. The study found that when women are on the pill they prefer men with similar Major histocompatibility complex (MHC) odours.
MHC genes play an important role in the human body's immune system and the best mates for someone are those with different MHC odour.
When individuals with different MHC genes mate, their offspring's immune systems can recognise a broader range of foreign cells and are therefore healthier.
Past studies have also found that couples with dissimilar MHC genes are more satisfied and more likely to be faithful to a mate and the opposite is true for those with matching-MHC genes.
"Not only could MHC-similarity in couples lead to fertility problems but it could ultimately lead to the breakdown of relationships when women stop using the contraceptive pill, as odour perception plays a significant role in maintaining attraction to partners," said Stewart Craig Roberts, an evolutionary psychologist at the University of Newcastle.
The study comprised about 100 women, aged 18 to 35. They were tested at the beginning of the study when none was taking contraceptive pills and three months later after 40 women had started taking the pill about two months earlier.
The study showed that when women started taking birth control pills, their odour preferences changed and they were more likely than non-pill users to prefer MHC-similar odours compared to non-pill users.
"The results showed that the preferences of women who began using the contraceptive pill shifted towards men with genetically similar odours," Roberts said.
She also suggested a likely reason for the pill's effect on women's odour preferences. The pill puts a woman's body into a hormonally pregnant state (thereby stopping ovulation) and during such state there is no reason to find a mate.
"When women are pregnant there's no selection pressure, evolutionarily speaking, for having a preference for genetically dissimilar odours," Roberts said.
"And if there is any pressure at all it would be towards relatives, who would be more genetically similar, because the relatives would help those individuals rear the baby."
So the pill puts a woman's body into a post-mating state, even though she might be still in the game.
"The pill is in effect mirroring a natural shift but at an inappropriate time," Roberts added.
The results of the study were published in the journal Proceedings of the Royal Society B: Biological Sciences.