From the Proceedings of the Royal Society B: Biological Sciences :
MHC-correlated odour preferences in humans and the use of oral contraceptivesThe sense of smell - or absence of it - has been associated with various Intersex conditions, in particular Kallman Syndrome. We know (from the previous post) that transsexual woman have female-typical senses of smell when exposed to sex hormones. Now it seems that hormone levels may affect more complicated neurological functions in females, in particular, what kind of guys smell yummy.
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. Here we test directly whether contraceptive pill use alters odour preferences using a longitudinal design in which women were tested before and after initiating pill use; a control group of non-users were tested with a comparable interval between test sessions. In contrast to some previous studies, there was no significant difference in ratings between odours of MHC-dissimilar and MHC-similar men among women during the follicular cycle phase. However, single women preferred odours of MHC-similar men, while women in relationships preferred odours of MHC-dissimilar men, a result consistent with studies in other species, suggesting that paired females may seek to improve offspring quality through extra-pair partnerships. Across tests, we found a significant preference shift towards MHC similarity associated with pill use, which was not evident in the control group. If odour plays a role in human mate choice, our results suggest that contraceptive pill use could disrupt disassortative mate preferences.
In my own case, one of the first symptoms I noticed and recorded during the peculiar change was a drastically more sensitive sense of smell. The area was being re-wired, though all I could do was record the signs, without attempting to hypothesise about causation.
I really wish I could have had a series of fMRI scans at the time, just to see what was happening. An opportunity lost, though given my fragile mental state at the time, I was too busy trying to merely ride it out.
This part of the puzzle is still more gaps than pieces, with tantalising hints about connections between the functioning of the lymbic system, in particular the hypothalamus, ovulation, hormone levels and olefactory sensation. It's too early to even speculate, we need more data, but I can't help thinking that there's something there. But what, and how to design experiments to test hypotheses? I'm putting it in the "too hard" basket for now, anyway.
One thing we can do, once we adopt the theory (no longer a hypothesis) that gender determination resides in the lymbic nucleus, is to figure out what the implications of that are in other areas.