The key mental domains in which sex differences have traditionally been studied are verbal and spatial abilities. In this article I suggest that two neglected dimensions for understanding human sex differences are 'empathising' and 'systemising'. The male brain is a defined psychometrically as those individuals in whom systemising is significantly better than empathising, and the female brain is defined as the opposite cognitive profile. Using these definitions, autism can be considered as an extreme of the normal male profile. There is increasing psychological evidence for the extreme male brain theory of autism.