All in the Family: Comparing Siblings to Test Causal Hypotheses Regarding Environmental Influences on Behavior

Psychologists in both basic and applied fields are keenly interested in the environmental influences that shape our lives. Therefore, researchers test causal hypotheses to construct models of environmental influences that can withstand attempts at refutation. Randomized experiments provide the strongest tests of causal hypotheses, but are not always feasible and their assumptions cannot always be met. In such cases, a number of quasi-experimental research designs can be used to substantially reduce confounding in tests of causal hypotheses. Sibling-comparison designs provide robust quasi-experimental tests of causal environmental hypotheses, but they are underused in psychology in spite of their power, feasibility, and convenience.