A long term goal in genetics research is to clarify the complex genetic and environmental origins of human emotional, behavioral, and social dysfunctions. Tracing the sources of individual differences in behavioral development will contribute to this goal. The objective of this research is to conduct a comprehensive search for the approximate chromosomal locations of genes that contribute to developmental variations in human temperament, personality, and cognition. Until now, this type of research has been hindered by the fact that each of these genes probably exerts only a small effect on the overall development of behavior. Exciting and dramatically swift discoveries in recent molecular genetics research have made it possible to locate genes of small effect with subject samples of attainable size. The project will draw from the families of a large sample of twins whose development has been studied longitudinally during infancy and childhood. Behavioral testing will be completed or extended for twins, siblings and their parents in Louisville Twin Study multi-sib families. Blood samples will be collected from family members for DNA analysis of highly polymorphic markers. Behavioral similarity in fraternal twin or sibling pairs will be compared to the proportion of genes shared by pairs at specific chromosomal markers or in intervals flanked by such markers. Multi-point and multi-interval genetic mapping strategies will be combined with the methods of maximum-likelihood analysis of twin-family resemblance. The method includes estimation of the effects of residual genotype, phenotypic assortative mating, shared and non-shared environments, non-genetic parental influence on offspring environment, genetic-environmental correlation, and sibling and twin interaction effects that lead to changes in similarity.