This study of nine-month-old twins is designed to investigate issues common to the areas of emotional development and temperament during infancy. Our behavior-genetic methodology will allow estimation of the degree to which genotypic factors account for individual differences in temperament. We will explore the relationship between parental report of infant fearfulness, activity level, and positive emotionality, (as assessed by Rothbart's Infant Behavior Questionnaire) and multiple laboratory measures (stranger distress, aviodance of a visual cliff, various "games" with mother and an experimenter, and a free play episode) of the same constructs. Other issues to be investigated include the short-term stability of laboratory assessment of emotionality, the cross-situational generalizability of temperament measures, and possible sources of bias in parental perception of infant temperament (including biases attributable to parental personality).