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Study: Low IQ Scores Do Not Reflect Low Intelligence in Adults with ADHD

Adults with ADHD test just as well as their peers on measures of IQ — once working memory and processing speed are taken out of the equation.

October 5, 2016

Lower IQ scores among adults with ADHD do not necessarily reflect lower intelligence, but rather comparatively poor working memory and decreased processing speed, according to a 2014 study.

The study, conducted by researchers from the University of Bremen, in Germany, tested the intelligence of 116 adults with ADHD and 116 controls using a battery of tests. The primary one was the Wechsler Adult Intelligence Scale–IV (WAIS-IV), a comprehensive test of cognitive ability that is widely regarded as the “gold standard” of intelligence measures. Researchers also utilized the Full Scale Intelligence Quotient (FSIQ) — an estimate of overall intelligence functions — as well as the General Ability Index (GAI), which measures intelligence and ability without accounting for working memory and processing speed, two areas that are generally deficient in adults with a mental health condition like ADHD.

The researchers were primarily interested in finding out if adults with ADHD would show the same deficiencies — working memory, processing speed, and perceptual reasoning — on the WAIS-IV as they did on previous versions of the test. They did. Adults with ADHD also tested lower than the controls on FSIQ, which the researchers expected, too. But on top of that, the researchers noticed that results of the GAI differed significantly from the FSIQ results for the ADHD group — nearly 60 percent of adults with ADHD scored significantly higher on the GAI than they did on the FSIQ. This means that when working memory and processing speed — their weakest areas on the WAIS-IV — weren’t factored in, adults with ADHD tested just as well (and in some cases, better) than control subjects.

Societal stigma and rigid academic demands may have led many adults with ADHD to think of themselves as “stupid” or “slow” — though they often demonstrate high levels of creativity and ingenuity. And, unfortunately, past research on the relationship between IQ and ADHD is mixed. Thomas Brown, Ph.D., conducted a study several years ago that had similar results to this one; despite impairments in working memory and processing speed, his subjects — all of whom met diagnostic criteria for ADHD — had IQs of 120 or above. That placed them in the top 9 percent of the population. But a 2006 meta-analysis of several studies about ADHD and IQ produced different results: in it, adults with ADHD were found to have an IQ that was, on average, 2.94 points lower than that of neurotypical controls.

This University of Bremen study, however, adds further support for Brown’s results. Differences in IQ scores might not be a result of lower overall intelligence, it seems; rather, adults with ADHD may be as smart or smarter than control subjects — though they might be held back by poor working memory and weaknesses in processing speed.

Still, the authors cautioned that the results deserve further exploration and research. Measures of working memory and processing speed are critical for an accurate overall picture of someone’s IQ, they write, and the WAIS-IV and FSIQ results are more widely accepted than the GAI.

However, the GAI accounts for the effects of mental health on our IQs in ways that the other two tests cannot. If used appropriately by clinicians during the diagnostic process, the GAI can help paint a more accurate picture of each individual with ADHD — potentially allowing for more personalized treatment plans and, ideally, reduced stigma.

“Even though the GAI may not be interpretable in some cases, experts on this topic suggested the GAI can still be used as a sensible estimate of a patient’s intelligence, and for certain disorders, it may be clinically informative in ways that the FSIQ is not,” the authors write. “It should not be used to make a diagnosis but rather to differentiate patients from each other, to use it as a source for neuropsychological interpretation, and to target strengths and weaknesses within treatments.”

The study was conducted in 2014, but will be published in the upcoming November 2016 issue of the Journal of Attention Disorders. The reason for the publication delay is unclear.