Mental retardation: The condition of having an IQ measured as below 70 to 75 and significant delays or lacks in at least two areas of adaptive skills. Mental retardation is present from childhood. Between 2 and 3 percent of the general population meet the criteria for mental retardation. Causes of mental retardation include fetal alcohol syndrome and fetal alcohol effect; brain damage caused by the use of prescription or illegal drugs during pregnancy; brain injury and disease; and genetic disorders, such as Down syndrome and fragile X syndrome. Treatment of mental retardation depends on the underlying cause. In some cases, such as phenylketonuria and congenital hypothyroidism, special diets or medical treatments can help. In all cases, special education starting as early in infancy as possible can help people with mental retardation maximize their abilities.