Genetic analyses were conducted using the structural equation modelling programme OpenMx (Boker et al. 2011). The twin method involves comparing intra-familial similarity in MZ and DZ twin pairs. MZ twins share all genetic effects, whereas DZ twins share on average 50 % of their segregating genes. Analyses involve decomposing variance/covariance into influences due to additive genetic (A), shared environment (C environmental factors that make members of a twin pair alike) and non-shared environment factors (E environmental factors that make members of a twin pair different to one another).
