Shared Environment
Shared environmental influences refer to all nongenetic influences that make family members similar to one another. This can include a wide range of factors, includ- ing neighborhood, parental education, and family factors such as parenting behavior or the amount of conflict that occurs within the household. These factors will be shared environmental influences only if they result in greater similarity among indi- viduals living in the same household and if they do not vary as a function of genetic relatedness. In other words, if fraternal twins are as similar as identical twins, and this similarity is not negligible, then shared environmental influences are important. Similarly, if the similarity of “environmental” siblings is the same as that of “genetic” siblings, then shared environmental influences are indicated. The Appendix provides more detail about how shared environmental influences are estimated in twin, sibling, and combination designs.
There has been confusion about shared environmental influences. As will be de- scribed in Chapters 11 through 19, there is little evidence of shared environmental influences on many commonly studied behaviors such as personality and cognitive abilities; the modest shared environmental influences that have been found are often significant only during childhood and adolescence (Plomin, 2011; Plomin & Daniels, 1987). However, shared environmental influences do have some effect through child- hood and adolescence (Burt, 2009b), especially for certain types of behavior prob- lems, although they also become less important for explaining similarity for family members no longer living in the same household. In other words, residing in the same household does increase the similarity of family members, although these effects do not appear to persist once children have moved out of the home.
