The current edition of the “Diagnostic and Statistical Manual of Mental Disorders,” or DSM-5, introduced a new term and definition for ‘mental retardation,’ renaming the condition ‘intellectual disability.’ Although the change took place five years ago, we still lack empirical findings to evaluate its reliability, validity and impact.
Intellectual disability is a common condition, affecting about 1 percent of the general population in the United States.
How the condition is defined and diagnosed has implications for treatment and public health. It also has repercussions for the education, employment and legal status of people with the disability. It can even play a role in life-or-death decisions in the courtroom.
The diagnosis of mental retardation mostly relied upon intelligence quotient (IQ) tests, which are imprecise. The standard error of IQ measurements is 5 points, meaning that the intellectual disability cutoff of 70 on a 100-point scale should really be a range from 65 to 75.
The DSM-5 introduced a more realistic approach to the diagnosis of intellectual disability. It emphasizes adaptive functioning, which are intellectual abilities in daily life, more than the DSM-IV did. These include an understanding of time and money, and the ability to interpret social cues and attend to personal needs, such as eating and dressing.
The DSM-IV-TR required impairment in two or more adaptive-skill areas for a diagnosis. In the DSM-5, the criteria changed to impairment in one or more adaptive domains, each of which includes three or more adaptive skills. As a result, individuals would need more adaptive impairments to qualify for an intellectual disability diagnosis under DSM-5 criteria.
At the same time, however, the DSM-5 does not require a specific IQ cutoff for the diagnosis. As a result, if a person has an IQ above 70, she could still receive an intellectual disability diagnosis if her adaptive skills are poor. Assessments of these skills can better delineate the relatively blurry cognitive boundaries of intellectual disability than IQ tests can.
When the DSM-5 included the term in 2013, I, along with others, assumed this new emphasis on adaptive skills would result in an increase in intellectual disability diagnoses. Five years later, however, the consequences of the change are still unclear.
Dearth of data:
Relatively few publications have addressed the effects of switching from mental retardation to intellectual disability. A literature search for ‘DSM-5’ and ‘intellectual disability’ results in only 38 publications, mostly reviews and commentaries.
By contrast, a parallel search for ‘DSM-5’ and ‘autism’ produces nearly 300 articles.
In fact, only one empirical study has examined the impact of the new adaptive-functioning criteria on the likelihood of being diagnosed with intellectual disability. In this study, my colleagues and I used both the DSM-IV-TR and DSM-5 criteria to evaluate 884 children with neurodevelopmental conditions who had been referred for assessment.
We found that, contrary to earlier predictions, the DSM-5 criteria resulted in a 9 percent decline in intellectual disability diagnoses1. It is not clear why the diagnoses declined. Nevertheless, our findings indicate that the DSM-5’s increased emphasis on adaptive functioning, along with its decreased reliance on IQ, matters for diagnosis.
Given the implications of this single study, further research to determine the impact of the DSM-5 guidelines is important.
In line with DSM-5 guidelines, the U.S. Supreme Court has also opined that state courts must not rely solely on IQ scores to determine whether the death penalty is permissible.
In Hall v. Florida in 2014, the Supreme Court for the first time ruled on a death penalty case based on DSM-5 criteria for intellectual disability. The ruling instructed courts to consider the standard error in IQ scores in determining intellectual disability, although it did not recommend what other measures to use.
In their thoughtful analysis of Hall v. Florida, forensic psychologist Leigh Hagan and his colleagues provide recommendations that extend to clinical practice2. Adaptive functioning should be evaluated comprehensively, including abilities and impairments, they write. The evaluation should incorporate standardized tests as well as other sources of information and consider the potential biases of the individuals providing the information.
Replacing the term mental retardation is the counterpart to a legal process that reflects the public goal of upholding the dignity of those with developmental disabilities. In addition, the DSM-5 diagnostic criteria have been praised by groups such as the World Health Organization Working Group on Mental Retardation for providing a model that is not based on strict psychometric methods3.
Still, evidence for the reliability and validity of the new criteria is still lacking. Intellectual disability is a common and severe condition of high social impact. It deserves more research on its foundations.
Walter Kaufmann is Ravenel Boykin Curry Chair in Genetic Therapeutics at the Greenwood Genetic Center in South Carolina.