There was an experiment concerning Major Histocompability Complex (MHC), scent, and sexual selection. MHC controls immune response and effectiveness against pathogens. This can be detected by women based on the naturally occuring odor of a man.

The experiment goes, a few different men would wear a t-shirt for a few days and return it to be sealed in a plastic bag. The men were not to wear any deodorant, colognes, or other scents.

Women were asked to smell the t-shirts and rank the men based on their attractiveness.

The overwhelming majority of women thought the man who was the most attractive was the man who was had the most dissimilar MHC to themselves.

The only women who were incapable of making this judgement were the ones who were on birth control.

Over 60% of women in my generation (Millennial) are on birth control.