Intellectual disability refers to abnormal cognitive functions from abnormal development of parts of the brain. This usually involves abnormal development of neurons diffusely throughout the cerebral cortex. This may occur from many causes, although for many individual cases the cause is unclear. A common cause is the genetic disorder called Down syndrome, where an extra chromosome causes intellectual disability, and often abnormalities of other systems as well. Vascular abnormalities involving insufficient blood flow to diffuse areas of the developing brain may cause the syndrome, as may some metabolic, infectious, nutritional, and toxic disorders. Intellectual disability, like many of the other neurodevelopmental disorders, may occur in isolation or, commonly, a patient may have overlapping syndromes with features of several disorders. For example, with extensive brain injury early during development from any pathology, many children will have cerebral palsy from injury to developing motor areas of the brain as well as intellectual disability from diffuse injury to the cerebral cortex. Preterm birth is an important risk factor for these types of mixed neurodevelopmental disorders.