as the stress of living in poverty, strongly influence neuroticism, but their causal influences do not appear in estimates of shared environmental influences in twin studies (Plomin, DeFries, & Loehlin, 1977). This can happen when the shared environment operates through gene-environment interactions (Jinks & Fulker, 1970; Johnson, 2007; Robinson, 2004). That is, if the impact of a shared environment is moderated by genes, it would not contribute to the estimate of shared environmental influences. This is because gene-environment interactions with shared environmental factors tend to make identical (monozygotic) twins, who share essentially 100% of their polymorphic genes, more similar to one another. In contrast, because fraternal (dizygotic) twins only share 50% of their polymorphic genes on average, interactions between shared environments and genes would only make fraternal twins more similar when they happen to share the same allele(s) of the relevant polymorphism(s). Otherwise, gene-environment interactions make dizygotic twins and other siblings less similar and contribute to the estimate of nonshared environmental influences in twin studies. Thus, it is possible that shared aspects of the environmental influences exert causal influences on neuroticism, but do so through gene-environment interactions.
