Foetal testosterone plays a significant role in a wide range of sex-differences. In animal studies it has been found that foetal testosterone influences the development of the of the brain; the hypothalamus, limbic system, and neocortex (Arnold & Gorski, 1984; Breedlove, 1994; MacLusky & Naftolin, 1981). It has also been found to influence behaviours (Goy & McEwen, 1980) and cognitive abilities (Williams & Meck, 1991). Sex differences in behaviour and cognition are also apparent in humans (Baron-Cohen, 2003). Traditionally research in this area has focused on abilities of logic; spatial, mathematical, and verbal ability (Kimura, 1999). More recently investigations have looked at potential social sex differences. Geary (1998), suggested that women exceed men in particular aspects of socio-cognitive ability; non-verbal communication, language and theory of mind. Baron-Cohen (2002) proposed that women have a better ability to identify and understand another's state of mind, emotional state (empathy), and to respond appropriately.

Lower 2D:4D digit span ratio is a sexually dimorphic trait, thought to be indicative of higher testosterone exposure. On average males have a lower digit span ratio than females (Maning, 2002). Growth of the 4th digit, as promoted by testosterone, is thought to occur in a 'respective window' in utero and therefore 2D:4D ratio remains constant through out postnatal development. Subsequently 2D:4D has been deemed a useful biomarker for extent of prenatal testosterone exposure (Maning, 2002). There is an abundance of evidence in support of this view. For example, it has been found that women with congenital adrenal hyperplasia have more masculine span and males with the condition have smaller digit ratios than control males (Okten, Kalyoncu, & Yari?, 2002 ).

Whereas digit span ratio has been associated with numerous mental health disorders- Depression, Psychopathology, ADHD (Bailey & Hurd, 2005; Blanchard & Lyons, 2010; McFadden, Westhafer, Pasanen, Carlson, & Tucker, 2005), most frequently the literature has investigated links with social cognition and autism. Individuals with autism are thought to be subject to a hypermasculined brain, characterised by low empathising and high systemising traits. At the other end of the continuum the so-called feminised brain is characterised by high empathising and low systemising (Baron-Cohen, 2002). Deficits in social cognition and autistic traits have been found to be indirectly associated (via digit span ratio) with prenatal testosterone exposure (e.g. Honekopp, 2012). In support of these findings, more males have autism than females.

However, conclusions made by papers investigating digit pan ratio and social cognition are often based on small sample sizes (around 50 participants). Furthermore, a study by Voracek and Dressler (2006) which used a relatively large number of participants (N= 423), found a lack of correlation between 2D:4D and empathising, systemising and autistics traits, using measures developed by Baron-Cohen (2002). The purpose of this research is to better investigate whether a link between foetal testosterone and social cognition exists, specifically the ability to empathise. The aim of the study is, by using such a large data set, to overcome the experimental limitations to which previous studies may have succumb.

The exposure variable will be 2D:4D and outcomes variables will be social communication, non verbal information, theory of mind, friendships, and gender behaviour. If 2D:4D is indicative of in utero testosterone exposure, and this has organisational effects on brain development that are reflected in lateroutcomes, it is predicted that lower digit span ratio will be associated with poorer social communication, non- verbal information, theory of mind scores and friendships scores and more male typed gender behaviours.This hypotheses will be assessed using appropriate regression techniques.