Hemispheric specialization refers to lateralized motor, perceptual and cognitive functions to the left and right cerebral hemispheres. Studies have shown that individual variation in patterns of neuroanatomical and behavioral asymmetries are associated with various clinical problems such as major psychoses (schizophrenia), neurodevelopmental disorders such as William's syndrome, autism and stuttering and learning disabilities such as dyslexia and specific language impairment. The long-term goals of the proposed studies are to identify the genetic and non-genetic mechanisms that influence the development of hemispheric specialization in primates. One aspect of the proposal will assess the independent and potential interactive effects of genetic and early social rearing factors on the development of brain asymmetries in primates. Notably, heritability estimates in brain asymmetries will be computed in genetically related individuals that have been reared together or apart during the first three years life. In a second part of the proposed studies, post-mortem brains will be obtained from primates and cytoarchitectonic analyses will be performed for 8 distinct Brodmann's regions. Individual cytoarchitectonic maps will be derived for each ape and these will be registered to a template brain. Subsequently probabilistic maps of these regions will be created and applied to a large sample of in vivo MRI scans (N > 290) registered to the same template in order to estimate grey matter volumes of each cytoarchtectonic region. Heritabilty estimates of the grey matter volumes will then computed in the entire sample in relation to known pedigrees. Lastly, analyses will be performed to examine whether variation in asymmetries in both in vivo and post-mortem brains are associated with oro- facial motor control among related and unrelated individuals. In particular, these analyses aim to assess whether developing oro-facial motor control associated with the production of intentional vocalizations results in cortical reorganization within the primary and premotor cortex. Collectively the results of this study will provide critical information on the influence of genetic and early rearing factors on both the development and evolution of hemispheric specialization in primates.