David Byrne tells us a little about Neurochemicals. My favorite part:

Left on our own we tend to select partners who have a set of genes known as the MHC complex that are dissimilar to our own. Pairing these different gene sets produces healthier offspring. It is thought we do this by scent. (Here's to sensory abilities we didn't know we had.) However, women on the pill tend to select men whose MHC is the same as their own. Something gets blocked or short-circuited. They make what is not necessarily the best choice…but, they won't have a child anyway, so it all evens out.