From Brendon Ellis:
http://psychologytoday.com/articles/pto-20071228-000001.xml
The author of this article suggests that smell plays a larger role in selecting a mate than previously understood. Despite our unawareness, the odor of a significant other has extremely significant biological effects. For instance, reproduction with another individual who one deems to "smell good" often allows for a more genetically diverse offspring. Claus Wedekind, of the University of Lausanne in Switzerland, demonstrated this through his experiment by having 44 men sleep in gray shirts for two nights in order to collect their body odor. He then had 49 women smell the shirts and indicate which ones they preferred. When analyzed, the data suggested that women, more often than not, chose the shirt of a man "immunologically dissimilar" to themselves. This data indicates that odor plays a far greater role in love than previously believed. Not only do wives smell the pillow cases of their absent husbands for memories sake or boyfriends lovingly sniff their girlfriends necks just to remember, but now, this experiment proves that odors and pheromones allow an individual to a find a biologically satisfying partner.
Elizabeth Svoboda continues her argument by supplying data that connects a higher miscarriage rate to a "higher degree of MHC similarity," and even longer intervals between conception for these couples. She also submits that the birth control pill (or "The Divorce Pill" as she likes to call it) can send off false pheromones; these mixed signals make it very difficult for both sexes. For instance, if a woman goes off the pill, then her boyfriend/husband might respond negatively to the new body odors. In fact, she might respond differently; the birth control could have acted on her reception of her significant others' odor.