Is ADHD Hereditary? What Twin Studies Show

After I got the diagnosis, something odd happened while I was reading symptom lists: in sentences where I thought I was reading myself, I began to see other people.

My father looks for his keys every day. Sometimes he looks for them while holding them. My aunt starts a new hobby every few months, buys the materials, and then the hobby disappears completely.

None of these people has been diagnosed and I am not about to make a diagnosis. But I cannot deny that these scenes opened a question in me. The answer to the question surprised me too: it is no coincidence.

What the number says

The review by Faraone and Larsson (2019), based on twin studies, gives the heritability of ADHD as about 74 percent.

What this figure means needs to be made clear, because it is very often misunderstood.

What it does not mean: It does not mean "it passes to my child with a 74 percent chance." It does not give a personal probability.

What it means: When you look at how much ADHD symptoms differ from person to person in a population, about three quarters of that difference can be explained by genetic differences.

This rate is one of the highest among psychiatric conditions. For comparison: the heritability of height is reported in a similar range.

Heritability does not measure a fate. It measures where the difference in a population comes from.

It took me time to understand this distinction. When I first read it, I thought "so 74 percent is fate" and my morale dropped. Yet the sentence does not say that.

There is no single ADHD gene

For a long time an "ADHD gene" was searched for and it could not be found. The reason it could not be found is that there is no such gene.

The genome-wide study by Demontis and colleagues (2019) identified the first significant regions associated with ADHD. The picture that emerged is this: many genetic variants, each with a very small effect. This is called a polygenic structure.

The practical consequence is important: there is no genetic test for ADHD. A diagnosis cannot be made with a saliva test, and it is not worth paying for any product that promises such a test. Diagnosis is still made by clinical assessment; I explained how the process works in the article on how adult ADHD is diagnosed.

How it looks in the family

The frequency of ADHD in first-degree relatives is markedly higher than in the general population (Faraone et al., 2021).

In everyday terms it looks like this:

• The number of parents who get their own diagnosis after their child is diagnosed is not small at all. Many people recognize their own childhood while the questions are being asked in the assessment room.
• Different presentation types can be seen among siblings: one active, the other quiet and absent-minded. Both can be inside the same picture, and usually only the active one is noticed.
• Traits in a family that are brushed off with "we have always been like this" can actually be a described condition.

The third item is like this in my family too. The sentence "everyone in our family is absent-minded" was used as an explanation and nobody asked what lay behind it. If a family shares a common difficulty for long enough, that difficulty becomes invisible, because nothing is left to compare it with. After I got the diagnosis I heard that sentence as a question for the first time.

There is another meaning to this: nobody in the family is to blame. A hereditary trait is not the result of parenting. I am writing this sentence on purpose because the comment "so they did not raise them well" is still going around a lot.

I gathered the factors other than heredity, the difference between causation and association, and the things shown not to cause ADHD in the article on what causes ADHD.

So what is the environment's share

74 percent heritability leaves 26 percent, and that is not counted as a small share.

Also, a distinction is essential here: the appearance of the symptoms and how life goes are not the same thing.

Genetic predisposition largely explains the presence of the symptoms. But what determines whether someone living with those symptoms will finish school, keep their job and what they will think about themselves is the environment:

• Whether it was noticed early
• Whether accommodations were made at school
• Whether they could reach treatment
• Whether the people around them read the picture as laziness

The consensus statement (Faraone et al., 2021) shows that untreated ADHD is associated with measurable negative outcomes in education, work and health. The difference between the course with intervention and without it is real.

What is meant by environment

What the remaining share covers is also often misunderstood. The "environment" here does not mean parenting style or screen time, first of all.

The factors emphasized in research are more on the biological side: factors belonging to pregnancy and birth, premature birth, very low birth weight, some environmental exposures. The consensus statement reports that these factors have small but measurable contributions to risk (Faraone et al., 2021).

Popular accusations such as sugar, screens and bad parenting do not appear on this list with the same weight. I dealt with them one by one in the article on ADHD myths and misconceptions.

Put the family history in writing. If you are going for an assessment, think beforehand about whether there are similar patterns in your family and write a few lines: who struggled with what, how school life went. This information does not make a diagnosis, but it strengthens the weakest link of the developmental history, that is, the remembering part.

Three common mistakes

Taking heritability for certainty. High heritability does not mean it will necessarily be seen in a child. In some families one sibling has it and the other does not.

Erasing the environment completely. The sentence "it is genetic anyway" makes everything that can be done look pointless, and above all it makes people put off getting support.

Using genetics as an excuse. I saw this in myself. For a period I explained every behavior of mine with "it is like this in my family too." Explaining does not make anything easier; it only takes the possibility of changing it off the table.

Will it pass to my child

This is the most frequently asked question and the honest answer is this: the risk goes up, there is no certainty.

Be careful with any answer that tries to give an exact probability. What is known is that the frequency increases in first-degree relatives.

The benefit of knowing is not calculating the risk; it is noticing early. A parent who has got their own diagnosis recognizes the signs in their child much earlier, and support starts much earlier. And that child has the chance to grow up without hearing the sentence "you are lazy."

I described how the picture looks in children in the article on ADHD symptoms in children, and the particular difficulties of being a parent with ADHD in the article on being a parent with ADHD.

What I am left with

ADHD is largely hereditary and this explains its repeating in the family. There is no single gene, there is no genetic test, and being hereditary does not mean the course is determined.

This knowledge changed something in me. My way of looking changed when my father was looking for his keys. I am not attaching a diagnosis to him; I only know now that something described for years as "absent-minded" may have another name. Knowing this does not bring a solution, but it brings a softening.