IQ and ADHD: Why the Score Understates You

If you have ADHD and an IQ score that feels lower than it should be, there is a good chance the score is not measuring what you think it is. This is not reassurance. It is a documented property of how the test is constructed, and there is an established clinical correction for it.

Your IQ is four scores, not one

A Full Scale IQ is not a measurement in its own right. It is a composite, averaged from index scores that measure quite different things. On the WAIS-IV and WISC-V those are:

Look at the third column. The two indexes ADHD most reliably affects are the two that have the least to do with reasoning: how much you can hold in mind at once, and how fast you get through simple repetitive work. Those are real cognitive functions and genuinely worth measuring. They are not what most people mean when they ask how clever someone is.

What ADHD does to the profile

The pattern is consistent enough across studies to be described as characteristic.

In adults, Theiling and Petermann’s (2016) WAIS-IV study of 116 adults with ADHD against matched controls found significant decrements on subtests with working memory and processing speed demands, with moderate to large effect sizes, alongside relatively preserved verbal and perceptual reasoning. In children the shape is similar and often more pronounced, with working memory in some samples running around a full standard deviation below comparison groups.

The consequence is arithmetic rather than mysterious. Average four numbers where two have been pulled down and two have not, and the average lands below the two that were left alone. Your Full Scale IQ ends up describing neither your reasoning nor your attention, but a blend of the two that corresponds to nothing in particular.

This is why clinicians have a working rule about it. If the gap between your highest and lowest index exceeds roughly 23 points, about one and a half standard deviations, the guidance is not to interpret the Full Scale IQ as a single summary of ability at all. The number stays arithmetically correct and stops describing a person.

The General Ability Index

This is the part most people have never been told, and it is the reason this page exists.

The General Ability Index, usually written GAI, is an alternative composite that uses only verbal comprehension and perceptual reasoning. Working memory and processing speed are left out entirely. It is not a workaround someone invented on a forum: it is a published, normed index that comes with the test, and it exists precisely because those two indexes are selectively suppressed in several conditions, ADHD among them.

The size of the difference it makes is substantial. In the WAIS-IV work on adults with ADHD, close to six in ten scored significantly higher on the GAI than on their Full Scale IQ. Not marginally higher. Significantly, by the test’s own statistical criteria.

If you have had a full assessment, the GAI may already be in your report. It is worth looking for, because in this situation it is usually the more representative figure of the two.

The “average IQ with ADHD” figure

You will find a figure quoted for this, and it needs handling carefully.

Meta-analytic work puts the gap in measured IQ between ADHD and comparison groups at roughly half a standard deviation, on the order of seven to ten points on average. That figure is real and it is also three things people routinely forget:

• It is a group average with enormous spread. The ADHD distribution overlaps the comparison distribution almost entirely. It tells you nothing about any individual.
• It is a gap in measured IQ. Much of it comes from the two suppressed indexes described above, which means it partly measures the measurement problem rather than a difference in reasoning.
• Samples are drawn from people who got diagnosed. Clinic samples over-represent those whose difficulties were severe enough, or visible enough, to reach assessment. People who compensated their way through school are systematically less likely to appear in them.

High ability and ADHD together

The combination has a name, twice-exceptional, and it is probably the most commonly missed profile in the whole area, because the two exceptionalities hide each other.

Strong reasoning lets someone compensate for attention difficulties for years, often until the work gets hard enough that compensation stops working, which is frequently at university or in a first demanding job. From the outside they look fine, so nobody investigates. Meanwhile the attention difficulties hold their output below what their ability would predict, so they also do not look exceptional. Both signals get cancelled.

On a cognitive profile this often shows up as exactly the pattern described here: high verbal and reasoning indexes, markedly lower working memory and processing speed, and a Full Scale IQ sitting unremarkably in the middle. It is one of the clearer illustrations of why the shape of a profile carries more information than its average.

What to ask for in an assessment

If you are having, or have had, a formal cognitive assessment, four things are worth asking about:

• The index scores, not just the Full Scale figure. All four, individually. This is the informative part of the report.
• Whether a General Ability Index was calculated. If your profile is uneven in the way described here, it should have been.
• The confidence intervals. Every index carries error, and a good report states the range rather than just the point estimate.
• Whether you mentioned ADHD beforehand. Tell the assessor. It changes how the profile is interpreted and makes the assessment more accurate, not less.

And the thing not to do: treat an uneven profile as self-diagnosis. Plenty of people have low processing speed relative to their reasoning and do not have ADHD. The pattern is consistent with it, which is a much weaker claim than being evidence of it.

Frequently asked questions

The bottom line

ADHD does not straightforwardly lower intelligence. It lowers two of the four indexes a Full Scale IQ is built from, and the composite then understates the reasoning ability the other two indexes actually measured. The General Ability Index exists to correct for exactly this, and close to six in ten adults with ADHD in one WAIS-IV study scored significantly higher on it than on their Full Scale figure.

If your profile is uneven, the single number is the least informative thing in your report. The index breakdown is where the useful information is, and a qualified assessor is the person to interpret it.