[Page 1834]Intellectual disability (ID) is a condition diagnosed prior to age 18 years that includes limitations in intellectual functioning (e.g., reading, learning, problem solving) and a decrease in adaptive behavior, including conceptual, social, and practical skills. ID is diagnosed utilizing standardized tests of intelligence and adaptive behavior. ID affects between 2% and 3% of the U.S. population. There are numerous biological factors involved in identified causes of ID; however, for up to 60% of cases, there is no identifiable cause. The biological factors can be categorized utilizing the following:

• Infectious disease
• Chromosomal abnormalities
• Biomedical conditions
• Prenatal, perinatal, and postnatal issues