Gad Saad Ph.D.

Homo Consumericus

Breast Symmetry and the Menstrual Cycle.

Is she ovulating? Watch her breasts.

Posted Aug 02, 2010


An ongoing debate amongst evolutionists is the extent to which human females experience cryptic ovulation. Of course, if compared to some of our animal cousins, women experience less conspicuous advertisements of estrus. However, there are certain subtle morphological features that change as a function of where a woman is in her menstrual cycle. One such trait is the extent to which a woman's breasts are symmetric. Whereas there are clear inter-individual differences in breast asymmetry, this metric also varies within women as a function of their ovulatory cycle. On a more somber note, several studies have revealed a link between breast asymmetry and breast cancer (see here and here).

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.

As expected, they found that the greatest symmetry occurred during the most fertile phase of a woman's menstrual cycle. Of equal interest, shortly prior to this fertile period (no pun intended), breast asymmetry was quite high. In a sense, this creates a contrast effect such that it increases the likelihood of a male partner picking up the morphological change (and hence alerting him in a subtle manner to the woman's ovulatory status).

Hence, when women state that they feel and look sexier during the fertile phase of their cycles, they are perhaps in part responding to these subtle symmetry changes.

On a lighter note, see my earlier posts on the relationship between a waitress's tips and her bust size (see here), and the likelihood of a man picking up a female hitchhiker and her bust size (see here).

Ciao for now.

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