Supporting this are studies of the finger digit ratio of the right hand, which is a robust marker of prenatal testosterone exposure. In women, it is hypothesized that high levels of exposure to testosterone in key regions may increase likelihood of same sex attraction. It has been hypothesized that gay men may have been exposed to little testosterone in key regions of the brain, or had different levels of receptivity to its masculinizing effects, or experienced fluctuations at critical times. This masculinizing effect pushes males towards male typical brain structures, and most of the time, attraction to females. The presence of the Y-chromosome in males prompts the development of testes, which release testosterone, the primary androgen receptor-activating hormone, to masculinize the fetus and fetal brain. In simple terms, the developing fetal brain begins in a 'female' typical state. The influence of hormones on the developing fetus has been the most influential causal hypothesis of the development of sexual orientation. Main article: Prenatal hormones and sexual orientation
