Neurosteroids play an important role in the development of the cerebellum. In particular, estradiol and progesterone appear capable of inducing increases in dendritic spine density during development, and there is evidence that both are synthesized de novo in the cerebellum during critical developmental periods. In normal neonates and adults, there are few differences in the cerebellum between the sexes and most studies indicate that hormone and receptor levels also do not differ significantly during development. However, the sexes do differ significantly in risk of neuropsychological diseases associated with cerebellar pathology, and in animal models there are noticeable sex differences in the response to insult and genetic mutation. In both humans and animals, males tend to fare worse. Boys are more at risk for autism and Attention Deficit Hyperactivity Disorder than girls, and schizophrenia manifests at an earlier age in men. In rats males fare worse than females after perinatal exposure to polychlorinated biphenyls, and male mice heterozygous for the staggerer and reeler mutation show a more severe phenotype. Although very recent evidence suggests that differences in neurosteroid levels between the sexes in diseased animals may play a role in generating different disease phenotypes, the reason this hormonal difference occurs in diseased but not normal animals is currently unknown.