Classical twin models (44) that draw data from monozygotic (MZ) and dizygotic (DZ) twin pairs can be used to disentangle the extent to which additive genetic (A), shared environmental (C) and non-shared environmental (E) factors influence individual differences in PSI and RSI. Additive genetic influences refer to the aggregate variance attributable to segregating genes – members of MZ twin pairs share 100% of their genes identical-by-descent while, on average, members of DZ twin pairs share 50% of their genes identical-by-descent. Under the equal environments assumptions, shared environmental factors are shared equally, at 100%, across members of MZ and DZ twin pairs. Non-shared environmental factors are uncorrelated across members of twin pairs.
