Failure to adapt normally and grow intellectually may become apparent early in life or, in the case of mild retardation, not become recognizable until school age or later. An assessment of age-appropriate adaptive behaviors can be made by the use of developmental screening tests. The failure to achieve developmental milestones is suggestive of mental retardation.

A family may suspect mental retardation if motor skills, language skills, and self-help skills do not seem to be developing in a child or are developing at a far slower rate than the child's peers. The degree of impairment from mental retardation has a wide range from profoundly impaired (5%) to mild or borderline retardation (80-90%). Less emphasis is now placed on degree of retardation and more on the amount of intervention and care required for daily life.

Risk factors are related to the causes. Mental retardation affects about 1 to 3% of the population.

Causes of mental retardation can be roughly broken down into several categories:

• unexplained (this category is the largest and a catchall for undiagnosed incidences of mental retardation.)
• trauma (prenatal and postnatal)
  • intracranial hemorrhage before or after birth (for example, periventricular hemorrhage)
  • lack of oxygen to the brain before, during or after birth
  • severe head injury
• infectious (congenital and postnatal)
  • congenital rubella
  • meningitis
  • congenital CMV
  • encephalitis
  • congenital toxoplasmosis
  • listeriosis
  • HIV infection
• chromosomal abnormalities
  • errors of chromosome numbers (Down's syndrome)
  • defects in the chromosome or chromosomal inheritance (fragile X syndrome, Angelman syndrome, Prader-Willi syndrome)
  • chromosomal translocations (a gene is located in an unusual spot on a chromosome, or location on a different chromosome than usual) and deletions (cri du chat syndrome)
• genetic abnormalities and inherited metabolic disorders
  • galactosemia
  • Tay-Sachs disease
  • phenylketonuria
  • Hunter syndrome
  • Hurler syndrome
  • Sanfilippo syndrome
  • metachromatic leukodystrophy
  • adrenoleukodystrophy
  • Lesch-Nyhan's syndrome
  • Rett syndrome
  • tuberous sclerosis
• metabolic
  • Reye's syndrome
  • congenital hypothyroid
  • very high bilirubin levels in babies 
  • hypoglycemia (poorly regulated diabetes mellitus)
• toxic
  • intrauterine exposure to alcohol, cocaine, amphetamines, and other drugs
  • methylmercury poisoning
  • lead poisoning
• nutritional
  • malnutrition
• environmental
  • poverty
  • low socioeconomic status
  • deprivation syndrome

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