Mental retardation is low intellectual capacity arising, not from mental illness, but from impairment of the normal development of the brain and nervous system. Causes include genetic defect (as in Down's syndrome); infection of the embryo or fetus; hydrocephalus or inherited metabolic defects (e.g., cretinism), and injury at birth including cerebral hemorrhage and fetal asphyxia. Disease in infancy such encephalitis may cause mental retardation in children with normal previous development. Retardation is initially recognized by slowness to develop normal patterns of social and learning behavior and confirmed through intelligence measurements.