What Causes ADHD? Causes, and Things That Do Not

After a diagnosis there is a question everyone asks, and it usually comes second: why?

Behind this question there is often something else. A parent asking for their child is really asking "is it my fault?" An adult asking for themselves is asking "could it have been prevented?"

I will answer both in this article. But first I have to say how clear, and how blurred, the answer is.

First, an honest framework

ADHD does not have a single cause, and in saying so I am not filling in a blank.

In medicine some conditions are explained by a single cause: a germ, a gene, a deficiency. ADHD is not of that kind. The present understanding is that a large number of factors, each individually small in effect, play a part cumulatively.

So the answer to the question "why" is not a name but a distribution of weights. Most of the weight lies in heredity.

The share of heredity

The figure is surprisingly high.

Reviews based on twin and family studies report the heritability of ADHD as about 74 percent. This is one of the highest figures in psychiatry, comparable to strongly heritable traits such as height and intelligence.

What this number means needs spelling out, because it is often misread.

Seventy-four percent does not mean "74 percent of your ADHD is genetic." It shows how much of the difference between individuals in a population can be explained by genetic differences. It is not a declaration of individual fate; it is a measure at the population level.

I went into this distinction in more detail in the article on whether ADHD is hereditary.

There is no single gene

If heredity is this strong, the thought arises: then let's find the gene and run a test.

It cannot be found, because there is no such gene.

Genome-wide studies have identified genetic regions associated with ADHD, and that was an important step. But what was found is not a single key gene; it is a large number of variants, each with a very small effect. Risk comes together through the adding up of these small effects.

The practical result is this: no genetic test is done for an ADHD diagnosis, and there is no sign that one will be in the near future. Diagnosis is made from history and observation; I described this in the article on diagnosis.

What is known on the brain side

Imaging studies report differences on average between groups with and without ADHD: a delay in the developmental course of certain regions, changes in the connection patterns of some networks.

Two warnings are needed here.

First, these differences are seen in group averages and the distributions largely overlap. So it is not possible to make a diagnosis by looking at one person's brain scan.

Second, seeing a difference does not show that the difference is the cause. The brain changes as it is used; part of the observed difference may be a result, not a cause.

This is also a point at the center of the debate on whether ADHD is a real disorder.

Prenatal and environmental factors

Care is needed here, because this is the heading that produces the most guilt.

Some factors have been found to be associated with ADHD in research: preterm birth, low birth weight, some exposures in pregnancy, lead exposure in early childhood.

All of these are reported as associations. Finding an association does not prove causation, and this distinction here is not a theoretical detail; it changes the outcome completely.

The best example is smoking in pregnancy. For many years it was found to be associated with ADHD and was reported as if it were a causal factor. Then sibling-comparison studies were done: children of the same mother, one exposed and the other not, were compared. The association largely disappeared.

The explanation that emerged is this: ADHD symptoms may be more common among mothers who smoke, and what passes to the child is not the smoking but the familial predisposition. The association was real; the cause had been misread.

When writing this, one thing must not be confused. Smoking in pregnancy is not recommended anyway for many other reasons. The only thing said here is that causation for ADHD has not been shown.

Things that do not cause it

This part is perhaps the most important section of the article, because these wrong beliefs both produce guilt and delay treatment.

Parenting style. It does not cause ADHD. Parenting markedly affects the course of symptoms and how much the child struggles; it does not produce the picture. This distinction matters because the second sentence explains why support helps, while the first only produces guilt.

Sugar. A meta-analysis combining twenty-three double-blind controlled studies concluded that sugar does not affect children's behavior or cognitive performance. The observation is real but the explanation is different; I covered this in detail in the article on sugar.

Screen time. There is no evidence of causation. ADHD was described long before smartphones became widespread. There are studies that find small associations, but the reverse direction is at least as plausible: people with ADHD may be drawn more to media that give fast rewards. I opened this up in the article on screens.

Vaccines, food dyes, modern life. None of these headings has been shown to be a cause of ADHD. For food dyes a small behavioral effect is reported in some children; that is not the same thing as causing the condition.

So why are more people being diagnosed

This is a separate question that gets confused with the question of causes.

Studies using standard diagnostic methods show that the proportion of people in the population meeting the ADHD criteria has not clearly increased over decades. What has increased is the rate of diagnosis.

The reason is largely awareness: groups that used to be overlooked, especially girls, women and adults, now go for assessment. I took up this debate in the article is ADHD diagnosis exaggerated.

So the answer to the question "why has it increased so much" has to do not with causes but with visibility.

What the question is really asking

I come back to what I said at the beginning.

When a parent asks "why," what they are often asking is "is it my fault?" The answer to this question is no, and this answer is not a consolation; it is what the research says.

When an adult asks "why," what they are generally asking is "could it have been prevented?" The answer to this too is largely no. What could have been prevented is not the picture but the picture going unseen for years.

The two are different things, and the second is a real loss. I wrote about that loss separately in the article on grief after diagnosis.

What I am left with

In writing this article I did not want to build the answer to the question of causes as a consolation; it should not be read that way either. Still, I think it changes something.

A person who explains their own story through a series of wrong choices puts themselves at the center of every explanation. If only I had worked harder, if only I had not given that up, if only I had built that habit. Each explanation turns the past a little more into a criminal file.

Knowing the share of heredity does not close that file. What I take from this article is that the file is very often opened on the wrong person.

The information here is a general framework. Decisions about diagnosis and treatment are made with your doctor.