Mental retardation affects about 1 - 3% of the population. There are many causes of mental retardation, but doctors find a specific reason in only 25% of cases.

A family may suspect mental retardation if the child's motor skills, language skills, and self-help skills do not seem to be developing, or are developing at a far slower rate than the child's peers. Failure to adapt (adjust to new situations) normally and grow intellectually may become apparent early in a child's life. In the case of mild retardation, these failures may not become recognizable until school age or later.

An assessment of age-appropriate adaptive behaviors can be made using developmental screening tests. The failure to achieve developmental milestones suggests mental retardation.

The degree of impairment from mental retardation varies widely, from profoundly impaired to mild or borderline retardation. Less emphasis is now placed on the degree of retardation and more on the amount of intervention and care needed for daily life.

Risk factors are related to the causes. Causes of mental retardation can be roughly broken down into several categories:

• Infections (present at birth or occurring after birth)
  • Congenital CMV
  • Congenital rubella
  • Congenital toxoplasmosis
  • Encephalitis
  • HIV infection
  • Listeriosis
  • Meningitis
• Chromosomal abnormalities
  • Chromosome deletions (cri du chat syndrome)
  • Chromosomal translocations (a gene is located in an unusual spot on a chromosome, or located on a different chromosome than usual)
  • Defects in the chromosome or chromosomal inheritance (for example, fragile X syndrome, Angelman syndrome, Prader-Willi syndrome)
  • Errors of chromosome numbers (such as Down syndrome)
• Environmental
  • Deprivation syndrome
• Genetic abnormalities and inherited metabolic disorders
  • Adrenoleukodystrophy
  • Galactosemia
  • Hunter syndrome
  • Hurler syndrome
  • Lesch-Nyhan syndrome
  • Phenylketonuria
  • Rett syndrome
  • Sanfilippo syndrome
  • Tay-Sachs disease
  • Tuberous sclerosis
• Metabolic
  • Congenital hypothyroid
  • Hypoglycemia (poorly regulated diabetes mellitus)
  • Reye syndrome
  • Hyperbilirubinemia (very high bilirubin levels in babies)
• Nutritional
  • Malnutrition
• Toxic
  • Intrauterine exposure to alcohol, cocaine, amphetamines, and other drugs
  • Lead poisoning
  • Methylmercury poisoning
• Trauma (before and after birth)
  • Intracranial hemorrhage before or after birth
  • Lack of oxygen to the brain before, during, or after birth
  • Severe head injury
• Unexplained (this largest category is for unexplained occurrences of mental retardation)