Intellectual Disability (previously known as Mental Retardation) is characterized by:

  • Deficits in intellectual functioning as evidenced by an IQ of 70 or lower on standardized intelligence tests (Wechsler Intelligence Scales, Stanford-Binet). IQ numbers can help specify:
    • Mild ID: 55-70 IQ; 85% of all cases
    • Moderate ID: 40-55 IQ; 10% of all affected
    • Severe ID: 25-40 IQ; 3-5% of affected
    • Profound ID: <25 IQ; 1-2% of affected cases
  • Deficits in adaptive functioning in one or more of the following indicated through assessment (Vineland, Adaptive Behavior Assessment System):
    • Communication
    • Daily Living Skills
    • Socialization
    • Motor Skills
  • Onset of deficits during the developmental period


Most causes are undetermined/unknown. Most identified causes are from before birth (Prenatal) including brain malformations, maternal diseases, exposure to teratogens. Sometimes, problems at the time of delivery or immediately after birth (Perinatal) can also lead to Intellectual Disabilities, such as hypoxia (lack of oxygen), Traumatic Brain Injury, Infections, and exposure to toxins such as lead or mercury. 

Co-occuring Disorders

  • Attention-Deficit/Hyperactivity Disorder
  • Pervasive Developmental Disorders
  • Neurocognitive Disorder


There’s no known cure or effective treatment. Goal of intervention is to teach daily living skills, work with family members, and treat co-morbid disorders.