This book reports on a study of 850 pairs of twins who were tested to determine the influence of heredity and environment on individual differences in personality, academic ability, and interests. It presents the background, research design, and procedure of the study, tabulations of the test results, and extensive analyses of the findings. Subjects of the study were chosen from participants in the National Merit Scholarship Qualifying Test of 1962 and were mailed a battery of personality and interest questionnaires. Additionally, parents of the twins were sent questionnaires requesting information about the twins' early experiences. A similar sample of "nontwin" students who had taken the same examination provided the comparison group. Questions investigated included: how twins are similar to or different from nontwins; how identical twins are similar to or different from fraternal twins; how the personalities and interests of twins reflect genetic factors; how the personalities and interests of twins reflect early environmental factors; and what implications these questions have for the general issue of how heredity and environment influence the development of psychological characteristics. (Author)