Does ADHD Affect Intelligence? Capacity vs. Performance

When I reread my past after the diagnosis, the sentence I got stuck on most was this: "She's smart, if only she'd work a bit harder."

I heard this sentence for years and it never occurred to me to object to it. My grades were not bad; I did my homework the last night, but I did it. From the outside there seemed to be no problem. From the inside, every term I got stuck in the same place: I could not produce what I knew, on time and in full.

Later I saw that some version of the same sentence stops at nearly everyone who has been diagnosed. "They knew I could do it, and I knew it too, and still it didn't happen." That is why I do not want to brush this question aside; the answer has a few more layers than you might expect.

The short version first

ADHD is not counted as an intellectual disorder. The diagnosis is made at every level of intelligence: among people who score very high, among those in the average range and among those below average. Intelligence is neither one of the diagnostic criteria nor a reason for exclusion.

The long answer goes through this: intelligence is an estimate of capacity, and what you meet in life is performance. ADHD does not touch capacity itself; it touches the mechanism that turns capacity into results.

What lies between knowing and doing

Knowing something is one ability. Applying that knowledge at the right moment, in the right order and all the way to the end is another ability.

ADHD struggles with the second. The meta-analysis by Willcutt and colleagues (2005) shows that differences in executive function are consistently found in ADHD: planning, initiating, sustaining, sequencing, inhibition. The same study has one more finding that should not be skipped: these differences are consistent at the group level but are not found in everyone with ADHD. So scoring well on an executive function test does not remove the diagnosis.

The everyday form of this distinction looks like this. In the exam you know the subject, but you read the question halfway and jump to the answer. The content of the report is finished in your head, and opening the file and writing the first sentence takes two weeks. In the meeting what you want to say is clear, but by the time it is your turn the sentence has scattered. In all three the knowledge stays in place; the carrying mechanism catches.

I explained the details of the mechanism in the article on executive functions.

Intelligence is the power of the engine. Executive functions are the gearbox and the steering wheel. A powerful engine does not get you down the road if the gears do not shift.

The gap between capacity and result

This gap is one of the most visible consequences of ADHD:

• Understanding in class and losing it in the exam
• Producing ideas and being unable to finish the project
• Knowing the job and still missing the deadline
• Being unable to send the text you have written
• Finishing the hard part and stopping at the last five percent

A person looking from outside sees only the result and chooses between two explanations: either she is not smart enough or she is not trying hard enough. Both read the picture wrongly.

This misreading has a long-term cost. A person who hears "you're not using your potential" for years ends up saying that sentence to herself in her own voice. I covered how the label settles in in the article on the "not living up to potential" label, and the mark it leaves in the article on shame and self-compassion in ADHD.

What intelligence tests show

This is the place where the sentence "ADHD does not affect intelligence" stays too flat, so I am writing it as it is.

The meta-analysis by Frazier and colleagues (2004) reports that the average total IQ score of the ADHD group came out lower than that of the comparison group; the effect size is moderate (d = 0.61). So there is a real average difference in the measured scores.

What that difference means is a separate question. A few things need to be held together:

• The groups overlap to a large degree. A difference between averages says nothing about individual people. There are many people with ADHD who score very high.
• Some parts of the test are sensitive to attention. Subtests that demand working memory and processing speed are directly affected by fluctuations in attention. The review by Barkley and colleagues (2008) shows that performance differences of this kind are common in ADHD.
• What is measured is performance on that day. Someone who is sleepless, anxious or sitting down to a task that does not interest her at all can score below her capacity.

The practical equivalent is this: in an ADHD assessment, the spread across subtests can be more informative than the total score. It also makes a difference whether the person interpreting the test knows ADHD. A single low subtest score neither makes the diagnosis nor removes it.

How high capacity delays the diagnosis

This combination is one of the most frequent reasons for a late diagnosis, and it stands right in the middle of my own story.

A child with high capacity can get adequate results for a long time despite ADHD. She studies the last night and passes, gets by on what she heard in class, closes the gap quickly. Nobody notices anything, because what triggers being noticed is usually a low grade.

Then the demands go up: high school, university, the first job, her own business. While the structure from outside decreases, the workload grows and compensation begins to fall short. The first real blockage usually shows up in one of these transitions.

I learned the name of this only after the diagnosis: a compensation strategy. The systems I built over the years worked, but each of them stood on energy, and when the energy ran out the system ran out too.

Does ADHD bring creativity

This claim is repeated often. The honest answer is this: the evidence in this area is weak and inconsistent.

What is reported is mostly qualitative. Adults with ADHD describe being able to make unusual connections, to work very deeply in the area that interests them and to move fast under pressure. These are real experiences. But they cannot be presented as a proven advantage for the average of a group.

The whole discussion is in the article on the question of strengths in ADHD.

What closes the gap

What closes the distance between capacity and result is not trying harder; you are already trying. What works is building the missing structure from outside:

• Write the first step. "Write the report" does not count as a task description. "Open the document and type the title" does. I explained the problem of not being able to start in the article on task paralysis.
• Make the time visible. A clock that counts down, a timer standing on the desk. If you struggle to feel time, you need to see it with your eyes.
• Break up the deadline. A single deadline in ADHD often ends up being the name of the last night. Three intermediate dates means three separate starts.
• Count finishing as a separate job. The last five percent asks for a different skill than the first ninety-five. Write it in the calendar separately.
• Move the checking outside. Having someone read the text you cannot send often speeds up finishing.

The sum of these is called external scaffolding, and it is no different from wearing glasses: it does not increase capacity, it makes the use of capacity possible.

The most common mistake is right here: when the scaffolding works, people drop it. "I'm used to it now, I don't need the list." For me this ends up in the same place every time.

What we do not know

The contested side of this question has to be said too.

It is not clear how the diagnosis should be made when high capacity and ADHD exist together. The criteria are written for an average level of functioning; someone with high capacity can look from outside as if she is "getting by" while being far below her own line. There is no common rule on how to reflect this in the assessment.

How much space an intelligence test should take up in an ADHD diagnosis is also debated. The test does not make the diagnosis alone. The diagnosis is made by evaluating the developmental history, information from more than one setting and loss of function together, and this evaluation is the work of a clinician.

Keep one for a month. The gap between capacity and result is hard to describe, because the examples get forgotten. For a month, write the jobs "that I knew how to do but could not produce" in a single place: what was expected, where you stopped, what happened at that moment. When you go to an assessment you will have much better material than your memory.

What stays with me

The damage done by the sentence "you're smart but you don't work" comes not from its being wrong but from its being incomplete. The smart part may be true. The work part may be true too; someone who struggles a great deal can hear the same sentence.

The missing third piece is the mechanism needed for work to start, to continue and to finish. What I took from this article is that the solution is not to push yourself harder. It is to put a structure where you stopped pushing.

There is a separate entry in the diary on being counted as smart: Those who think they are smart: the ordinariness of evil and stupidity.