It is my hypothesis that increased testosterone in female hominids / humans caused phenomena which differentiates humans from the (other) apes. One of these phenomena is the enlarged breasts of humans compared to the apes. Estradiol is approximately the same in human and chimpanzee females, so, what causes the difference? It is increased testosterone in human females. Human females produce more testosterone than chimpanzee females.


Testosterone was significantly positively associated with %DBV [ percentage of dense breast volume] and ADBV [absolute dense breast volume].” Cancer Epidemiol Biomarkers Prev. 2014”\: “Endogenous sex hormones and breast density in young women.,” Jung, et al.,


Voilà, increased female testosterone in humans causes the increased breast volume.