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The basic methodology of behavior genetics is described in the context of the twin and sibling/adoption design. Applying the models to cross-sectional data on general intelligence, genetic influences increase with age, while shared environmental influences decrease with age, a finding consistent with siblings leading increasingly separate lives. Upon examining language development in children during the first three years of life, shared environmental influences were, not surprisingly, found to be important for both expressive and receptive language development. For receptive language development, which is largely under the control of the parent, shared environmental influences were strong and genetic influences almost entirely absent, in contrast to the finding for expressive language. The models were also used to conduct longitudinal analyses of continuity and change in intelligence, as well as to examine genetic and environmental influences causing extremes of behavior, such as might be represented by psychopathology.