It is clear that there are both genetic and environmental causal influences on neuroticism (Rettew et al., 2006; Viken et al., 1994). In discussing this topic, it useful to distinguish two kinds of environments. Shared environments are common to members of a family and make them more similar on a trait. In contrast, non-shared environments are not experienced by all members of a family (e.g., an automobile accident involving only one sibling) and make members of a family less similar. Estimates of the magnitude of shared environmental influences on neuroticism from several large twin studies are essentially zero, but there appear to be substantial non-shared environmental influences on neuroticism (Fullerton, 2006; Lake et al., 2000). It would be a mistake to conclude that only nonshared environments influence neuroticism, however (Rutter, 2007a). It is possible that environments that siblings share, such 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
