It was an experiment that involved sweat, love... and could have ended in tears. When 100 women were asked to sniff 100 men's sweaty shirts in the interests of discovering the secrets of sexual attraction, they found the contraceptive Pill disrupted their ability to select the ideal partner.
Body odour plays an important part in mate selection but the Pill appears to interfere with a woman's sense of smell, undermining her capacity to make the best choice, researchers said.
If confirmed, the finding suggests that, by disturbing a woman's natural olfactory preferences, the contraceptive Pill could be to blame for fertility problems, relationship dissatisfaction and marital breakdown. It could even lead to problems in the next generation, resulting in children with weaker immune systems.
Opposites attract – in gender and in genes. But the study revealed that instead of going for genetically dissimilar mates, as human beings are instinctively inclined to do, women on the Pill tended to select men more genetically similar to themselves.
This is evolutionary suicide because the survival of the species depends on genetic diversity. Going for genetically similar men, detected from body odour, may increase a woman's risk of difficulties trying to conceive, miscarriage and of long intervals between pregnancies.
Whether the "armpit effect" is sufficiently strong to trump those traditionally desirable marriage traits – a square jaw, razor wit and a house in Antibes – is a matter of conjecture. But researchers at the University of Liverpool insist body odour is a powerful determinant of mate selection.
Craig Roberts, a lecturer in evolutionary psychology who led the study, published in Proceedings of the Royal Society, said: "When eyes meet across a crowded room, looks obviously count first in determining who we find attractive. But smell is an important sense for us. We are animals after all. Smell kicks in more in maintaining a relationship – it may not affect who you are attracted to in a crowded bar, but it may determine how you feel about them next morning."
According to evolutionary theory, humans choose partners through their body odour and tend to be attracted to those with a dissimilar genetic make-up to maintain genetic diversity.
Human diversity is nowhere more apparent than in our immune systems. Our immune response is determined by genes in the major histocompatibility complex (MHC), which also controls body odour through the immune system's interaction with skin bacteria. Genes in the MHC thus help determine which individuals find us attractive.
Appearance affects who we pick up, but it is body odour that determines who we stay with.
For the study, women were asked to sniff six male body odour samples, obtained by shredding unwashed T-shirts which the men had worn for two nights. For each woman, three samples were selected that were genetically similar and three that were genetically dissimilar. The experiment was repeated before and after they started taking the Pill.
Dr Roberts, who carried out the study in collaboration with colleagues at the University of Newcastle, said: "The preferences of women who began using the contraceptive Pill shifted towards men with genetically similar odours. Not 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 plays a significant role in maintaining attraction."
The nose is the pathway to love
* It is through the baby's bond with its mother that the power of body odour is established, scientists believe.
* When we are attracted to someone we unconsciously register their odour, because it triggers ancient memories of being cuddled in the first few minutes of life.
* The sexually compatible enjoy one another's odour – that is one meaning of the sexual chemistry between them.
* For women, body odour is important in mate selection because it is a way of avoiding in-breeding.
* For men, spraying themselves with aftershave could be a mistake: studies show this can inhibit female arousal.
* Women often say that the most important sex organ is the brain, but the nose may run a close second.