Onset and Prevalence of Intellectual Disabilities
Intellectual disability (intellectual developmental disorder, formerly mental retardation) is not a specific disease, but rather a condition that can have a variety of causes; therefore, there is no specific time of onset for intellectual disability. However, various classification systems, including both the American Psychiatric Association, and the American Association on Intellectual and Developmental Disabilities, require childhood onset, specifically before the age of eighteen. The cutoff point of age eighteen contained within these definitions of intellectual disability implies that cognitive development continues up until age eighteen. If the symptom onset is after the age of eighteen, the condition would be classified as neurocognitive disorder (formerly dementia) even though the symptoms may be identical to those that would qualify a younger person for an intellectual disability. The difference between intellectual disability and neurocognitive disorder, besides the age requirement, is whether or not someone has lost a prior level of functioning (as is the case with neurocognitive disorder/dementia) or whether someone never developed this level of functioning in the first place (as is the case with intellectual disability/intellectual developmental disorder). Because of this distinction, a person with an intellectual disability may also receive a diagnosis of neurocognitive disorder/dementia if they experience a loss of prior functioning.
According to the President's Committee for People with Intellectual Disabilities, an estimated seven to eight million Americans of all ages experience an intellectual disability. Intellectual disabilities (intellectual developmental disorder, formerly mental retardation) affect about one in ten families in the United States.
According to the American Psychiatric Association's Diagnostic and Statistical Manuel (DSM), about 2-3% of people meet the criteria for intellectual disability. This is because the diagnostic criterion for intellectual disability (intellectual developmental disorder, formerly mental retardation) requires an IQ score of approximately 70 or below, which represents two standard deviations below the mean (or below the average score for all people). Statistically, two standard deviations below the mean equates to 2.5 percent of the entire population.
IQ test results of seventy (70) or below are rare by test design. There are basic statistical reasons for why this is so. If everyone took an IQ test and you lined up all their scores, there would be a wide variation in scores. This is called a distribution of scores. The average score (the mean) is the score that is the smallest distance away from all the other scores in the distribution. The actual mean score may not have actually been observed - it may instead be merely an imaginary score which nevertheless is the smallest distance away from all the other real (observed) scores. The mean is not necessarily the most commonly observed score, (called the modal score), or located exactly in the middle of all the actual test scores (called the median score).