Relative risk for many psychiatric disorders differs dramatically in males and females. Early-onset disorders, such as autism, occur more often in males; other conditions, such as schizophrenia, occur at similar rates in males and females, but the sexes differ in expression. It has been hypothesized that the prevalence and expression of these disorders is related to sex differences in brain development. X-chromosome effects and early exposure to gonadal hormones are strong candidates for a causal role. The aims of the research are (1) to characterize sex differences in brain development from birth to age 2; (2) to test whether brain development is altered in infants with Turner syndrome, a well-defined genetic disorder resulting from the partial or complete loss of one of the sex chromosomes. To address aim 1, high resolution MRI, including diffusion tensor imaging (DTI), will be used to characterize sex differences in brain development from birth to age 2 in a longitudinal cohort of 250 children. To address aim 2, high resolution MRI, including DTI, will be used to compare brain development in 30 infants with Turner syndrome (X monosomy) to 30 matched controls. The investigators hypothesize that sex differences in gray and white matter development and in white matter maturation as assessed by DTI will be present during the first 2 years of life and that children with TS will exhibit abnormal gray and white matter development in the neonatal period.