The Nature of the Shared Environment.

While a standard part of twin modeling, the magnitude of shared environment (c2 ) is rarely examined by comparing estimates obtained using other methods. To clarify these effects on familial resemblance, we estimated c2 for 20 diverse phenotypes in: (i) monozygotic and dizygotic twins, (ii) all step-siblings, and (iii) reared together and apart half-siblings, ascertained from the Swedish general population. The mean c2 estimates (± 95% CIs) differed across methods and were higher from twins (0.18; 0.13-0.23) than from the step (0.12; 0.09-0.14) and half-sibs (0.09; 0.06-0.13). c2 estimates correlated moderately across these three methods (ICC = + 0.28). When step-siblings from blended (each sib biologically related to one parent) and adoption-like families (one sib offspring of both parents and one of neither), were examined separately, resemblance was much lower in the latter. We need to clarify the range of environmental processes now considered together under the term "shared environment."