What is the approximate distribution of IDD severity across Mild, Moderate, Severe, and Profound?

• A. Mild: 85%, Moderate: 10%, Severe: 3-4%, Profound: 1-2%
• B. Mild: 50%, Moderate: 30%, Severe: 15%, Profound: 5%
• C. Mild: 30%, Moderate: 50%, Severe: 15%, Profound: 5%
• D. Mild: 60%, Moderate: 25%, Severe: 10%, Profound: 5%

Answer: (A)

Most people with intellectual and developmental disabilities (IDD) fall into the milder range of impairment, with the numbers thinning as severity increases. In commonly cited prevalence estimates, about 85% have mild IDD, around 10% have moderate IDD, roughly 3–4% have severe IDD, and about 1–2% have profound IDD. This pattern reflects how IQ thresholds translate into functional outcomes: mild IDD often allows greater independence with needed supports, while fewer individuals reach the deeper levels of impairment. Therefore, the distribution that assigns the largest share to mild, with progressively smaller shares for moderate, severe, and profound, best matches typical data. Other distributions would imply more people in the moderate or severe categories than is usually observed, which is why this pattern is considered the best approximation.