Please note: This is a mini-project that Anna Guyatt (Wellcome 4 year phd student) and Bernice Knight (clinical lecturer) will be working on. DR has direct access and will liaise with Kate Northstone to anonymise the IDs (this has already been discussed with Kate).

Background and Aims: Autism spectrum disorders and associated traits are over-represented in males. A popular hypothesis to explain this gender bias is the 'extreme male brain' theory (EMB), which is an extension of the empathising-systemising theory of sex differences in cognitive styles (Baron-Cohen 2002; Baron- Cohen and Hammer 1997). It suggests that male brains are hardwired for the drive to understand and construct systems based on if-then rules (i.e., systemize) while the female brain is programmed for the drive to understand the mental state of others and experience appropriate reactions (i.e., empathizing). The EMB hypothesis suggests that beyond the biological sex, higher levels of fetal testosterone underlies the male brain (Auyeung and Baron-Cohen 2008). Baron-Cohen has suggested that the deficits observed in autism of extreme systemising and less empathising may suggest that higher exposure to testosterone during fetal life may be aetiologically linked to autistic traits. The ratio of the index finger (2nd digit) to the ring finger (4th digit), commonly referred to as 2D:4D is a commonly used putative marker of fetal testosterone relative to fetal estrogen activity. Although a number of studies have tried to test the relationship between 2D:4D and autism/traits, the samples have been relatively small and selected; and the results are inconclusive. There is a lack of population based studies.To address this gap in the literature, we aim to study the association between 2D:4D and autism and its component traits, testing the extreme male brain hypothesis.