The phenotypic expression of autism spectrum disorders varies widely in severity and characteristics and it is, therefore, likely that a number of etiological factors are involved. However, one finding which has been found consistently is that there is a greater incidence of autism in boys than girls. Recently, attention has been given to the extreme male hypothesis-that is that autism behaviors are an extreme form of typical male behaviors, including lack of empathy and language deficits but an increase in so-called systemizing behaviors, such as attention to detail and collecting. This points to the possibility that an alteration during sexual differentiation of the brain may occur in autism. During sexual differentiation of the brain, two brain regions are highly sexually dimorphicthe amygdala and the hypothalamus. Both of these regions are also implicated in the neuroendocrine hypothesis of autism, wherein a balance between oxytocin and cortisol may contribute to the disorder. We are thus proposing that the extreme male hypothesis and the neuroendocrine hypothesis are in fact compatible in that sexual differentiation of the brain towards an extreme male phenotype would result in the neuroendocrine changes proposed in autism. We have preliminary data, treating developing rat pups with the differentiating hormone 17-b estradiol during a critical time and showing changes in social behaviors and oxytocin, to support this hypothesis. Further studies should be undertaken to confirm the role of extremes of normal sexual differentiation in producing the neuroendocrine changes associated with autism. Anat Rec, 294:1663-1670, 2011. V V C 2011 Wiley-Liss, Inc.Key words: autism; oxytocin; cortisol; sexual differentiation; extreme male TWO HYPOTHESESSimply stated, the hypothesis which we are proposing in this article, and the work currently ongoing in our lab, is that enhanced sexual differentiation of the brain, towards the male phenotype, leads to changes in the neuroendocrine factors, oxytocin and cortisol. These factors are involved in the processing and evaluation of social bonding experiences and any disruption in their typical function can lead to the primary defining characteristic of the autism phenotype-inadequate social bonding. The Extreme Male Hypothesis of AutismAutism is a disorder of brain development and thus any factors which regulate brain development and are known to be altered in autism should be considered as possibly contributing to the phenotype. For example, much of our previous work and work of others, has focused on the known role of serotonin in brain development as well as the well known hyperserotonemia which occurs in autism (Whitaker-Azmitia, 2005). More recently, researchers have proposed a role for sexual differentiation of the brain in the phenotypic behaviors of