A8. Testosterone is an anabolic-androgenic steroid. Anabolic steroids build body tissue, including but not limited to bone and muscle tissue and red blood cells. Androgenic steroids are responsible for male sex differentiation, i.e., for the development of male primary sex characteristics (in utero), and male secondary sex characteristics (in puberty). Because of its body building and sex differentiation effects, testosterone produced endogenously (naturally within the human body) is the primary driver of the sex differences in athletic performance, i.e., of the performance gap between male and female athletes. Beginning at puberty, at approximately age 11, the male testes begin producing significantly more testosterone than they did earlier in childhood, and also significantly more than is ever produced by female ovaries. This increased production triggers the onset of male puberty, and thereafter builds and sustains the male body in the respects that matter for sports performance: speed, strength, power, and endurance.
Pre-puberty, males also have substantial testosterone-based advantages, which begin before birth. In utero, male fetuses receive an infusion of testosterone that is later associated with young boys’ somewhat greater strength and somewhat greater propensity for aggression – the androgenic effects of testosterone. See question 3 for more detail.
The exogenous use of testosterone (doping) is banned by all national and international sports organizations because of these anabolic effects.