D.F. Swaab
Netherlands Institute for Neuroscience, an Institute of the Royal Netherlands Academy of Arts and Sciences, Meibergdreef 47, 1105 BA Amsterdam, The Netherlands

Addictive substances, medication, and environmental substances can permanently disrupt fetal brain development, which might ultimately lead to learning difficulties as well as to many types of behavioral disorders in later life. Congenital defects of this kind are known as functional or behavioral-teratological defects. Women who wish to become pregnant and require drug treatment, e.g. for epilepsy or depression, must be made aware of the potential problems at an early stage so that the safest drug or alternative therapy is prescribed. The sexual differentiation of the brain may also be influenced by chemicals during intra-uterine development. Variability has always been the motor of evolution and is still present – as evidenced in, e.g., sexual orientation and gender identity. There can be little doubt that our gender identity and sexual orientation are programmed in the structure of the brain while we are still in the womb and will last us for the rest of our lives. Our sex organs are differentiated in the first months of pregnancy, while the sexual differentiation of the brain takes place in the second half of pregnancy. Since these processes occur at different times, what may happen, as it does in transsexuals, is that female structures are present in male brains and vice versa. The long-established idea that we are completely free to choose our gender-identity (i.e. the feeling to be male or female) or our sexual orientation (hetero-, bi- or homosexuality) is not only incorrect but has also been the cause of a great deal of misery.
human brain, chemicals differentiation, sexual differentiation