Breast asymmetry and phenotypic quality in women

Breasts in humans are highly sexually dimorphic compared to other closely related primate species. One possible explanation for this increase in breast size is that breast asymmetry contains information regarding women's phenotypic quality. The purpose of this work was to consider the relationship between asymmetry and measures of body size, body composition, and fertility. We found significant associations between breast asymmetry and height, body mass, and breast volume (positive associations), breast “density” (negative association), and the woman's age and her age at first childbirth (positive associations). The relationship between asymmetry and breast volume showed evidence of negative allometry, that is, women with large breasts had smaller asymmetry than expected for their breast size. Asymmetry was lower in married women compared to unmarried women and negatively related to offspring number. A multiple regression analysis, with breast asymmetry as the dependent variable, showed a strong significant relationship with breast volume (positive association) and significant relationships with age at first childbirth (positive association) and number of offspring (negative association). Asymmetric women therefore had fewer children later in life that did symmetric women. Simple linear and multiple regression analysis showed that breast volume was a strong predictor of body size and composition (weight, height, and breast density) and a weak predictor of age and age at menarche. It was not related to offspring number or age at first childbirth. Our data support the thesis that mammotropic hormones, particularly estrogen, are harmful because they suppress the immune system. Heavy women with high levels of body fat produce more estrogen and, therefore, bigger breasts. However, more estrogen leads to an increase in breast asymmetry. Women with “good genes” are able to produce symmetric breasts despite the presence of large quantities of estrogen. Large and symmetric breasts are therefore honest signals of high phenotypic quality in women.