Beware the birth control pill
Sniffing Out Different Gene Pools
Beware that while a woman is on the pill she will be attracted to the opposite male than when she is off the pill. This is due to a woman's ability to smell a partner with a different gene pool than her own. The benefits of this through evolution is to prevent inbreeding and also to provide a child with a larger range of genes and thus a more diverse immune system. Studies have shown that the pill interferes with this ability and actually makes women prefer the smell of men with similar gene pools and immune systems. So while on the pill she may choose a partner based on this, if ever she goes off the pill, or becomes pregnant, her partner will become unattractive to her.*
* Razib Khan in Genetics (August 16, 2008). "Taking the pill might make your brother hawt?". Discover Magazine. Retrieved 2011-07-25. "Previous studies in animals and humans show that genes in the major histocompatibility complex (MHC) influence individual odours and that females often prefer odour of MHC-dissimilar males, perhaps to increase offspring heterozygosity or reduce inbreeding. Women using oral hormonal contraceptives have been reported to have the opposite preference, raising the possibility that oral contraceptives alter female preference towards MHC similarity, with possible fertility costs."