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Based on the normal distribution of intelligence, only about 2.2% of the population has an IQ score below 70. A score of 70 represents two standard deviations below the population mean of 100. While such a score is a primary indicator for a potential Intellectual Disability diagnosis, it is not sufficient on its own. A formal diagnosis also requires the presence of significant deficits in adaptive functioning, with both issues appearing before the age of 18.

IQ Scores as an Indicator for Intellectual Disability

Note that the DSM-5 has abandoned specific IQ scores as a diagnostic criteria, however it still retains the general notice of possible suspicion of IDD with IQ scores two or more standard deviations below the general population. 

There has been a greater emphasis on adaptive functioning and ability for everyday function, pointing to the direction of observation of impairment in skills relating to social or conceptual-thinking domains. 

De-emphasis of IQ scores in IDD testing, and Emphasis on Adaptive Functioning and Everyday Function

The landmark U.S. Supreme Court case of Atkins v. Virginia originated from an August 1996 crime in which Daryl Atkins and an accomplice robbed, kidnapped, and murdered a local airman. A central element during the subsequent trial was the testimony of a clinical psychologist, who evaluated Atkins and stated that his IQ score was 59—substantially below the mean IQ score of 100. Based on this assessment, the psychologist concluded that Atkins possessed an intellectual disability.

Atkins v. Virginia

A 28-year-old man is referred for psychological testing and receives an IQ score of 67. He lives in his own apartment, works a steady job, and manages his personal finances without assistance. Based on the standard diagnostic criteria, which of the following is the most accurate conclusion regarding an intellectual disability diagnosis?

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