The multiple regression analysis of twin data in which a cotwin's score is predicted from a proband's score and the coefficient of relationship (the basic model) provides a statistically powerful test of genetic etiology. When an augmented model that also contains an interaction term is fltted to the same data set, direct estimates of heritability (h 2) and the proportion of variance due to shared environmental influences (e 2) are obtained. A simple transformation of selected twin data prior

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... regression analysis facilitates direct estimates of h 2 (an index of the extent to which the difference between the mean of probands and that of the unselected population is heritable) and a test of the hypothesis that the etiology of deviant scores differs from that of variation within the normal range.