Breast asymmetry, sexual selection, and human reproductive success

Abstract

Breasts of human females are large compared to those of closely related primate species, and they can thus be hypothesized recently or currently to have been subject to directional sexual selection. Here we show that (1) large breasts have higher levels of fluctuating asymmetry than small breasts, (2) breast fluctuating asymmetry is higher in women without children than in women with at least one child, (3) breast fluctuating symmetry is a reliable predictor of age-independent fecundity, and (4) breast fluctuating symmetry appears to be associated with sexual selection. These conclusions were similar in studies from two cultures differing in fecundity and breastfeeding traditions (Spain; New Mexico, U.S.A.). Choosy males that prefer females with symmetrical breasts may experience a direct fitness benefit in terms of increased fecundity and an indirect benefit in terms of attractive or fecund daughters.