DESCRIPTION (provided by applicant): The primary objective of is to assess genetic and environmental influences on individual differences in behavioral development. The study includes 245 adoptive families, including biological parents, adoptive parents, and their children (probands and siblings), as well as 245 matched "control" (nonadoptive) families. The children are studied in their homes at ages 1, 2, 3, and 4; in the laboratory at 7, 12, and 16; and via telephone at 9, 10, 11, 13, 14 and 15. By the end of the current funding period we will have completed testing of adopted and non-adopted probands and their siblings through age 16. In addition, we have been testing a longitudinal sample of 385 twin pairs at the same ages. The proposed continuation of HD-10333 will complete the assessment of these twin pairs through age 16. The unique combination of the longitudinal adoption design and the longitudinal study of twins greatly strengthens the scope and power of inference regarding developmental processes, exploiting the advantages of each approach and minimizing their limitations. This final proposed continuation of HD-10333 will reap the harvest from this landmark study of child development, with important implications for education, mental health, and childrearing practices.