http://observationsofanerd.blogspot.com/2009/01/oestradiol-makes-women-hot-and-hard-to.html

"The estrogen hormone oestradiol is associated with just about every aspect of female beauty. As well as serving as a growth hormone for female reproductive organs, it's associated with having a symmetrical face, large breasts and a low waist-to-hip ratio. Now, the hormone is credited with one more association: it makes women serial monogamists."

""Are birth control pills changing modern women's mating strategies?"

Research into the Major Histocompatibility Complex has demonstrated that women tend to be attracted to men with substantially different genes in that region. This helps insure that offspring have a more efficient immune system. During pregnancy, the attraction is reversed. Women tend to prefer intimacy with people with like MHC, probably because their families will be supportive and protective of them during pregnancy and it is preparation for bonding with the offspring.

But when women are on the contraceptive pill, the hormonal effects mimic pregnancy. Thus, when on the pill, you're more likely to be attracted to men with a similar MHC. So if you meet your partner while you're on the pill, then go off the pill when you've decided you want a family, serious trouble can brew. Indeed, there is research supporting the notion that it is at exactly this point that a substantial number of marriages fall apart.

It also raises the issue of those offspring in relationships which started when the woman was on the pill. Is a substantial proportion of the subsequent generation going to have less effective immune systems?
"

http://journals.royalsociety.org/content/f542428772r96x64/fulltext.pdf

Sounds like this should be linked up with http://www.face-and-emotion.com/dataface/facs/description.jsp
Paul Ekman's work I think - on microslicing video, and predicting the strength of a relationship.