Birth control proven to alter which males' scents appeal to a woman.
Another study—this one from the University of Liverpool—shows that birth control pills do indeed lead a female to be attracted to the opposite kind of fellow she would be into without the presence of artificial hormones in her system.
A groundbreaking, much-referenced 1995 study revealed the correlation between the level of attractiveness a woman feels to a man's scent and the compatibility of their DNA were they to have kids. The "sweaty T-shirt study," as it's widely known, also revealed that women on hormonal birth control pills were attracted to the scents of men whose immune system DNA was most similar to their own. This makes for a bad reproductive match because mixing too-similar DNA is like mixing the DNA of kin.
So why the reverse attraction? Evolutionarily speaking, women would want to have family around them when pregnant—or simply not ovulating—for protection, and how better to remember your brother than by his scent? The whole situation seems completely harebrained and shocking, but—as the results of this study released today show—scientific evaluation proves it's true. The Liverpool crew replicated the original Swiss study and came to the same conclusions on the correlation.
Hard proof on what this means for the fertility and long-term compatibility of couples who met while the woman was on the pill is still yet to come, but experts point out there are many other factors besides smell attraction that make for a fertile, compatible coupling. Phew.