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Intellectual Disability

There are prenatal, perinatal, and postnatal causes of ID. Some prenatal causes (e.g., environmental influences) are preventable. Genetic causes account for 45% of ID (Batshaw, Roizen, & Lotrecchiano, 2013). Down syndrome is the largest genetic cause of ID, and Fragile X syndrome is the largest inherited cause of ID. Fetal alcohol syndrome is the largest environmental cause of ID.


  • Genetic syndromes (e.g., Down syndrome and Fragile X syndrome)
  • Inborn errors of metabolism
  • Brain malformation (e.g., microcephaly)
  • Maternal disease (e.g., placental disease)
  • Environmental influences (e.g., alcohol, other drugs, toxins, teratogens)


  • Labor and delivery–related events (leading to neonatal encephalopathy)
  • Anoxia at birth


  • Hypoxic ischemic injury
  • Traumatic brain injury
  • Infections
  • Demyelinating disorders
  • Seizure disorders (e.g., infantile spasms)
  • Severe and chronic social deprivation
  • Toxic metabolic syndromes and intoxications (e.g., lead, mercury)

Cross-cultural attitudes and beliefs may influence some individuals perceptions about ID and its causes (Allison & Strydom, 2009; Scior, 2011).

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