A model for sibling effects in man.

A model is developed to specify the environmental effect of one sibling on another for a polygenic trait. Such effects are detectable in priniciple and the approach is illustrated with twin data relating to psychoticism. The relationship between the model and those employed in the treatment of kin selection is indicated.

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.

Name-printing as a test of developmental maturity.

An attempt to derive a practical scoring technique for name-printing by children in kinderten (age-range 5 1/2 to 6 1/2 years) as a quick test of skills and maturity is reported. The relationship of the Print-Your-Name test scores to other established tools of developmental assessment is examined. Teachers' assessments of the study children's readiness for academic promotion and of those children with school problems tended to correlate with the test scores.

The nature of extraversion: a genetical analysis.

A biometrical-genetical analysis of twin data to elucidate the determinants of variation in extraversion and its components, sociability and impulsiveness, revealed that both genetical and environmental factors contributed to variation in extraversion, to the variation and covariation of its component scales, and to the interaction between subjects and scales. A large environmental correlation between the scales suggested that environmental factors may predominate in determining the unitary nature of extraversion. The interaction between subjects and scales depended more on genetical factors, which suggests that the dual nature of extraversion has a strong genetical basis.

Testing models for variation in intelligence.

A biometrical-genetical model for human variation adequately predicts observed correlations for measured intelligence. The estimation procedure is outlined.