A healthy brain has various differences of anatomy and function between its left and right sides. For example, most people have left-hemisphere dominance for language functions, and are also right-handed. In this thesis, Merel Postema found that asymmetries of grey matter in the brain are slightly reduced in people with autism, but less so in people with attention deficit/hyperactivity disorder. Altered development of the brain’s left-right axis may therefore predispose to autism. Genetic contributions to the development of brain asymmetry were also studied, first in data from thousands of healthy adults, and then in people with a rare condition in which the internal body organs are reversed on the left-right axis. Although there was evidence that some aspects of brain asymmetry are partly heritable, most of the variation between people could not be linked to specific genes, and probably originates randomly during development of the fetus.