A twin-based analysis of personality inventory scales yields information relevant to construct validity. If scales validly measure dimensions influenced developmentally either by environmental factors common to cotwins or by genetic similarities, these influences should be apparent in patterns of intraclass correlations and within-pair variance ratios for monozygotic and dizygotic pairs. Criteria for construct validity derived from adolescent twin statistics are illustrated with Wiggins' MMPI Content Scales. A type of discriminant validity indicates that some scales are not distinct in terms of the influences of familial similarity. Sex differences are examined.