Psychology Today: The Smell of Love: "A team led by Claus Wedekind at the University of Bern in Switzerland decided to see whether MHC differences in men's apocrine gland secretions affected women's ratings on male smells. The team recruited just under 100 college students. Males and females were sought from different schools, to reduce the chances that they knew each other. The men were given untreated cotton T-shirts to wear as they slept alone for two consecutive nights. They were told not to eat spicy foods; not to use deodorants, cologne, or perfumed soaps; and to avoid smoking, drinking, and sex during the two-day experiment. During the day, their sweaty shirts were kept in sealed plastic containers.

And then came the big smell test. For two weeks prior, women had used a nasal spray to protect the delicate mucous membranes lining the nose. Around the time they were ovulating (when their sense of smell is enhanced), the women were put alone in a room and presented with boxes containing the male volunteers' shirts. First they sniffed a new, unworn shirt to control for the scent of the shirts themselves. Then the women were asked to rate each man's shirt for 'sexiness,' 'pleasantness,' and 'intensity of smell.'"