John Manning and his colleagues set out to test the cyclic asymmetry of several traits including women’s ears, digit ratios, and breasts (this last one being a sexually selected trait). Of relevance to today’s post, the researchers used cranio-caudal mammograms of 280 premenopausal women to calculate their breast asymmetry, with thirty-four of the women having used oral contraceptives within the two months prior the mammography. The data contained the necessary information to establish on which day of a woman’s menstrual cycle the mammogram was taken. Accordingly, Manning et al. could test whether scores of breast asymmetry varied across the cycle.