Sex hormones begin to influence our personality and sexuality well before birth. All foetuses begin development along a female course, but the male has testes which produce testosterone, and this hormone is the one that most strikingly alters the development of all the baby’s bodily structures, including the brain.

Particularly impressive evidence for this effect has been provided by June Reinisch of Rutgers University, New Jersey. She investigated the personality development of boys and girls whose mothers had been treated with steroid hormones during pregnancy (a procedure which used to be followed to maintain a pregnancy thought to be at risk). Children whose mothers had been given progestines (synthetic progesterones that simulate the action of male androgens in many ways) were found to be more independent, individualistic, self-assured and self-sufficient than those whose mothers had been treated with oestrogens (female hormones). When children born under these unusual chemical circumstances of the womb were compared with ‘untreated’ siblings, the same personality differences were observed (Reinisch, 1977).

Indications are that the middle third of pregnancy is critical with respect to sex differentiation in the brain, at least in humans (Ellis and Ames, 1987). In lower mammals such as rats sexual differentiation of the brain may continue until after birth.