Birth Control Confuses the Nose
A woman's preference for men with MHC genes that are immunologically different from her own ensures that her offspring will be healthy. But once women are pregnant -- or on the pill -- they prefer men who smell similar rather than different. Researchers theorize that women who are pregnant -- or whose hormones mimic pregnancy because of birth control pills -- are no longer looking for a mate. Instead, they need the support of close family, which they find by sniffing out those with similar genes.
We have long wondered if -- and how -- females respond to males' scents. Although there's a lot of debate about the role of pheromones in human attraction, some studies have shown that these chemical signals you're sending out are providing information to potential mates about your immune system [source: PBS].
When isolated, men's sex hormones -- like testosterone -- don't seem to have any effect on women, regardless of ovulation.
However, it seems that chemical markers detectable through scent provide data about certain genes within our DNA. Major histocompatibility complex (MHC) genes are responsible for some of the coding related to fighting disease with the immune system.
There are dozens of variations of these genes, and women are most attracted to the scents of men with whom they have very dissimilar MHC genes. The more similar the genes, the less attracted women are to a particular man's smell.
Regardless, the stronger the actual scent was overall, the less attracted they were. So if your pheromones indicate you're a perfect match for a particular woman, she'll be less interested after her nose detects you've saved up a month's worth of pheromones on your T-shirt in an effort to impress her.