Autism Spectrum Conditions (ASC) are much more common in males, a bias that may offer clues to the etiology of this condition. We argue that it occurs because ASC are an extreme manifestation of the male brain. The extreme male brain (EMB) theory, first proposed in 1997, is an extension of the Empathizing-Systemizing (E-S) theory of typical sex differences that proposes that females on average have a stronger drive to empathize while males on average have a stronger drive to systemize. In this chapter, we describe some of the evidence relating to the EMB theory of ASC and consider how typical sex differences in brain structure may be relevant to ASC. One possible biological mechanism to account for the male bias is the effect of fetal testosterone (fT). The weight of evidence in favor of the fT theory is growing from converging sources (longitudinal studies from gestation to age 10 years old based on fT in amniotic fluid, current hormone studies, and genetic association studies of SNPs in genes involved in the sex steroid biosynthetic pathways). The strong test of the fT theory will come from the large collaborative study with the Danish biobank that is adequately powered to test if fT and the related fetal steroids in the ?4 pathway are elevated in children who go on to develop ASC.
