Testing hypotheses on specific environmental causal effects on behavior

There have been strong critiques of the notion that environmental influence s can have an important effect on psychological functioning. The substance of these criticisms is considered in order to infer the methodological chal lenges that have to be met. Concepts of cause and of the testing of causal effects are discussed with a particular focus on the need to consider sampl e selection and the value (and limitations) of longitudinal data. The desig ns that may be used to test hypotheses on specific environmental risk mecha nisms for psychopathology are discussed in relation to a range of adoption strategies, twin designs, various types of "natural experiments," migration designs, the study of secular change, and intervention designs. In each ca se. consideration is given to the need for samples that "pull-apart" variab les that ordinarily go together, specific hypotheses on possible causal pro cesses. and the specification and testing of key assumptions. It is conclud ed that environmental risk hypotheses can be (and have been) put to the tes t but that it is usually necessary to use a combination of research strateg ies.