This is an individual National Research Service Award for post-doctoral research training, which provides support for promising Fellowship Applicants with the potential to become productive, independent investigators in scientific health-related research fields. The amygdala is a brain structure that plays a critical role in the modulation of social behavior and in the recognition and production of emotion. Because these functions are among the most impaired in autism, the amygdala has long been considered a potential locus of pathology. The amygdala is often enlarged in people with autism, and a decrease in the number of neurons in the area is also associated with the disorder. To understand the mechanism of the changes in volume, this study will analyze microglial cells in human subjects to relate the amount of neuron loss to changes in microglia, microglia-neuron clustering, and neuron-neuron clustering. Another possible explanation of the changes in amygdala volume involves dendritic arbors and axons. The postnatal development of the dendritic arbors of amygdala neurons in a rhesus monkey model will be assessed and correlated with the degree of neuron-neuron clustering. The description of cellular abnormalities will advance understanding of the neuropathology underlying autism, pointing the way toward improved animal models and the development of better biomarkers. The primate study will serve as a reference point for all future human studies of abnormal neuronal and neuropil development in the amygdala in multiple neuropsychiatric disorders.