In quantitative behavior genetic studies, researchers decompose variability in observed measures, typically referred to as phenotypes, into subcomponents. Why do people differ in how tall they are? Why are some people more outgoing than others? Classical behavior genetic studies attempt to answer these types of questions by partitioning variability into separate sources attributable to genetic, shared environmental, and nonshared environmental influences. This goal is accomplished by comparing the phenotypic similarity of pairs of individuals who have known genetic (e.g., identical and fraternal twins) or environmental (e.g., adoptive siblings) relatedness. The focus of this entry is on the two environmental components, which are much more complicated than they may first appear to be. The shared environment represents between-family effects that make siblings reared in the same ...

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