Twins: The finest natural experiment

Twin studies have moved beyond the classic design described by Galton (1875). Natural experiments involving twins and twin-like individuals occur constantly, driven largely by artificial reproductive technology and increased adoption rates. These events have produced exotic twin and sibling relations, e.g., virtual twins (same-age unrelated siblings) and twins reared apart (Chinese twin girls, abandoned and adopted separately). Research designs utilizing the unique genetic and social relatedness of such individuals can inform numerous behavioral domains. Many studies to be described were inspired by my association with the Minnesota Study of Twins Reared Apart (1982–1991), directed by Bouchard.