The term “GxE interaction” refers to instances where the joint effects of genetic and environmental risk factors are significantly greater (or significantly reduced, in the case of protective factors) than would be predicted from the sum of the separate effects.1 Researchers who study genetic influences on behavior among members of a family (e.g., sibling pairs) have long distinguished between environmental influences that are shared by siblings reared in the same family and environmental influences that differ for those siblings (Jinks and Fulker 1970). Shared environmental (SE) influences could include parental influences that affect all siblings, effects of the neighborhood or school that apply to all siblings, shared peer influences, and similar factors. Nonshared environmental (NSE) influences or exposures, in contrast, are unique to each sibling, such as trauma experienced by only one child in the family. Except for studies of twin pairs who are reared in the same household and who are likely to be highly similar in their environmental experiences, however, this classification is clearly an oversimplification. For example, changes in the family’s socioeconomic status, moves to a new
