[Page 1849]Intellectual disability (ID) is defined as a condition, diagnosed prior to age 18 years, that involves limitations in intellectual functioning (reading, learning, problem solving) and a decrease in adaptive behavior (conceptual, social, and practical skills). ID is diagnosed by utilizing standardized tests of intelligence and adaptive behavior and affects between 2% and 3% of the U.S. population. Many individuals have ID of unknown etiology, but with more recent scientific advances, there is an increasing number of identified causes. The known causes of ID (prenatal, perinatal, and postnatal) can be categorized in the following manner:

• Infectious diseases
• Biomedical conditions, chromosomal abnormalities, metabolic conditions
• Environmental causes
• Nutritional causes
• Toxic causes
• Traumatic brain injuries