Mental retardation is described as a condition that is diagnosed before age 18, and includes below-average general intellectual function, accompanied by impairment in the person’s ability to acquire the skills necessary for daily living.

Causes of mental retardation are numerous, but a specific reason for mental retardation is determined in only 25% of the cases.

Failure to adapt (adjust to new situations) normally and grow intellectually may become apparent early in 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 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 to mild or borderline retardation. Less emphasis is now placed on degree of retardation and more on the amount of intervention and care required for daily life.

Mental retardation affects about 1 to 3% of the population.

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

• unexplained (This category is the largest and a catchall for unexplained occurrences of mental retardation.)
• 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
• infections (present at birth or occurring after birth)
  • congenital rubella
  • meningitis
  • congenital CMV
  • encephalitis
  • congenital toxoplasmosis
  • listeriosis
  • HIV infection
• chromosomal abnormalities
  • errors of chromosome numbers (such as Down's syndrome)
  • defects in the chromosome or chromosomal inheritance (for example, fragile X syndrome, Angelman syndrome, Prader-Willi syndrome)
  • chromosomal translocations (a gene is located in an unusual spot on a chromosome, or located on a different chromosome than usual)
  • chromosome 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 (bilirubin is a byproduct of normal breakdown of red blood cells) 
  • 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

• failure to meet intellectual developmental markers
• persistence of infantile behavior
• lack of curiosity
• decreased learning ability
• inability to meet educational demands of school

Note: Variations in normal adaptive behaviors depend on the severity of the condition. Mild retardation may be associated with a lack of curiosity and quiet behavior. Severe mental retardation is associated with infantile behavior throughout life.

• development significantly less than that of peers
• an intelligence quotient (IQ) score below 70 on a standardized IQ test
• adaptive behavior score below average
• abnormal Denver developmental screening test