PROJECT SUMMARY/ABSTRACT The goal of this project is to understand how quantity and quality of the language input directed to children by their caregivers is influenced by genetically-influenced child characteristics (language ability as measured by standard assessment as well as children's productions during the interactions, and temperament as measured by standard assessments). The overarching activity of this research project is transcription and analysis of naturalistic parent-child interactions that have already been collected as part of a longitudinal study of over 300 same-sex twin pairs and their parents. The data set consists of one-on-one play interactions between a parent with each twin separately, at each of three time points (ages 3, 4, and 5 years). This unique sample allows investigation of how the parent's linguistic input to each child differs. Because half of the sample consists of monozygotic (genetically identical) twins and the other half of dizygotic twins (sharing approximately 50% of their segregating genes), we can make inferences about genetic influences on traits by the degree of similarity between members of a monozygotic and dizygotic twin pair. In this investigation we focus on how genetically-influenced traits affect the properties of the parent's linguistic input. Quantitative genetic model- fitting analyses will reveal bidirectional relations between parental linguistic input and child characteristics at the level of etiology by informing about genetic and environmental influences on parental linguistic input at age 3, 4, and 5; stability and change in parental linguistic input across age; and sources of covariance between parental linguistic input and child characteristics both within and across age. This is a timely investigation: much attention is currently being given to the study of environmental or parental traits that affect linguistic input, such as the parent's level of education, and how interventions might train parents within high-risk groups to improve the quality of the input they provide. The proposed work adds another dimension to these efforts by investigating child-specific features and how these may influence parental linguistic input above and beyond environmental and parental traits. The knowledge gained will reveal how these child factors contribute to the parental linguistic input children receive and the longitudinal consequences of this relationship.