Analysis and interpretation of twin studies including measures of the shared environment.

Recent reports using a classical behavior genetic research design in which twin data are combined with a measured characteristic of their shared family environment have made striking claims about estimating environmental influences on behavior with genetic effects controlled. Such claims are overstated for two related reasons. First, when a variable is measured at the family level in a way that makes it necessarily equivalent for twins reared together, it is not possible to partition it into genetic and environmental components. Second, although structural equation modeling and DeFries-Fulker analysis are sound tools for the analysis of many types of twin data, they do not control for genetic or environmental confounds when estimating the effect of measured family-level variables.