A forthcoming article (pdf) in the journal Hormones and Behaviour suggests that as women approach the fertile phase of the menstrual cycle, they are more likely to dress to look most attractive.
The research was led by Prof Martie Haselton who asked 30 women in committed romantic relationships to have a hormone tests to determine where they were in their monthly menstrual cycle, and have their photograph taken on two occasions.
One photograph was taken when the women were most fertile (the follicular phase) and another when they were least fertile (luteal phase).
The photographs were then shown to a panel of people (17 men and 25 women) who were asked “In which photo is the person trying to look more attractive?”.
The panel, who did not know what the rest of the study was about, tended to pick out photographs taken during the women’s most fertile time.
Importantly, the women who volunteered to have their photographs taken, did not know the exact purpose of the study either, so had no reason to dress especially differently for each of the two photographs.
Haselman and her colleagues suggest this may be a human version of an outward display that is common in female animals that signals to potential mates when they are most fertile.
Haselman has done a huge amount of research on sexuality, attraction and evolutionary psychology, most of which is freely available from her website.
pdf of full-text research paper.
Link to coverage from The Guardian.
Link to Prof Martie Haselman’s website.
Mind Hacks 
I’ve read that men have “menstrual” cycles, too, when they experience cramps and other symptoms associated with PMS. I wonder if men dress differently during these periods, the way women seem to?
I am wondering about the validity of taking measures when a woman is at her least fertile — it seems to me from anecdotal evidence that this is when women, on average, feel their worst. Most people when feeling under the weather tend to have less energy to put into aesthetic concerns. It seems that the effect could be about dressing down as an effect of discomfort rather than about dressing up as an effect of fertility.
Seems like one ought to either measure the confound of discomfort or take measures at a time when a woman is less fertile, rather than least fertile.
Perhaps you could shed some light on something I’ve overlooked?