A gestational hormonal wash creates many of the differences seen (e.g., boys are superior at spatial tasks, girls are superior at speech). If a male fetus does not get the appropriate pre-birth wash, his brain is likely to remain more typically female; if a female fetus gets exposed to a male-pattern hormonal wash, she is likely to be more typically masculine. (Carter, Rita, Ed. Mapping the Mind. CA: University of California Press, 1998, p 21)
Refer to Children and the Brain for additional information.