Abstract
Correlations on scales of the Thurstone Temperament Schedule are examined in two twin studies (Michigan and Veterans twin samples), an adoption study (Texas), and an unpublished twin-family study. It is concluded that the joint evidence suggests (1) an appreciable effect of additive genes, differing across scales; (2) a negligible influence of shared environments, except possibly for monozygotic (MZ) twins; (3) a possible contrast effect among dizygotic (DZ) twins on temperament and personality traits, but shared interests; and (4) a causally ambiguous elevation of MZ twin correlations, which could be due either to nonadditive genetic effects or to a special sharing of environments (or self-concepts) in this group.
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