Genetic and environmental components of inconsistency and unrepeatability in twins' responses to a neuroticism questionnaire.

Responses of twins to an 11-item neuroticism scale extracted from questionnaires administered on two widely separated occasions were subjected to a genetic analysis. The results confirm earlier findings concerning the genetic determination of neuroticism and reveal that there is a genetic component in the inconsistency of the test measured by the interaction of subjects and test items. Variation within subjects over the 2-year period between tests was due purely to environmental factors specific to individuals. When a genetic model was fitted to the raw mean squares, there was no evidence that genetic variation was other than additive and no indication of an environmental component common to members of the same family.