How Does an Intellectual Disability Happen?Intellectual disability – formerly known as mental retardation – can be caused by injury, disease, or a problem in the brain. For many children, the cause of their intellectual disability is unknown.Some causes of intellectual disability—such as Down syndrome, Fetal Alcohol Syndrome, Fragile X syndrome, birth defects, and infections—can happen before birth. Some happen while a baby is being born or soon after birth.Other causes of intellectual disability do not occur until a child is older; these might include severe head injury, infections or stroke.