What causes ADHD? Genes, birth and other links

There's no single answer to what causes ADHD. It's a highly heritable condition, so genes play a large part, and family and school setting shape how it shows up. Some pregnancy and birth factors are linked with a higher chance of ADHD, but a link isn't proof of cause, and a risk factor isn't a fault.

What causes ADHD, according to the research?

If you're asking what causes ADHD, a good place to start is a broad expert review by Hinshaw (2018). It sums up research on ADHD across the whole life span, from preschoolers to adults, including the author's own long study of girls with ADHD followed from elementary school into their twenties. It's an expert review, which means it's one researcher's informed judgment of the field rather than a single new study.

Hinshaw describes ADHD as real, impairing and neurodevelopmental, meaning it's tied to how the brain develops. He also describes it as highly heritable. In plain terms, genes explain much of why some people have ADHD and others don't. Most of the estimates he reports sit high.

Genes aren't the whole picture, though. Hinshaw argues that ADHD is expressed through a back and forth between genetic vulnerability and the world a child grows up in: family, school, friends, neighborhood and even public policy. That's a point about how ADHD shows itself over time. It isn't a claim that a family or school creates ADHD.

Some cautions come with this review. First, Hinshaw notes that family studies without a genetic design can't rule out shared genes as the hidden reason two things travel together in a family. Second, the author says his coverage is highly selective, because one review can't cover everything. He also says the true rate of ADHD is impossible to know without biological markers for it. Those limits matter whenever someone offers a confident, simple answer.

Pregnancy and birth factors linked with ADHD

Some pregnancy and birth factors are linked with a higher chance of ADHD. They're risks, not faults.

Being born very early or very small is one clear link. Franz (2017) pooled 34 studies of people born very preterm or at very low birth weight and compared them with people born at or near full term or a normal weight. Most were assessed as children, some as teens and a few as adults, and nearly all lived in high-income countries. Those born early or small were more likely to have an ADHD diagnosis and higher symptom scores, and the link was stronger for those born extremely early or extremely small. The studies covered births across a 30-year period, while newborn care changed, so the size of the link today isn't certain.

A wider umbrella review by Kim (2020) gathered earlier pooled analyses of observational studies, where researchers watch and record rather than assign anything. For factors like eczema or asthma, it kept to studies of children. It rated the evidence of a link with ADHD as convincing for high blood pressure during pregnancy, for pre-eclampsia and for eczema in childhood. It rated the evidence as highly suggestive for childhood asthma and for smoking during pregnancy. When the review looked only at studies that followed families forward over time, smoking during pregnancy kept its rating, but the links for blood pressure, pre-eclampsia, eczema and asthma no longer reached those top ratings.

The authors are clear that these links don't necessarily mean one thing causes the other. For smoking during pregnancy, the link didn't hold up in studies comparing brothers and sisters within the same family. That suggests shared family or genetic factors may explain it. The review also couldn't look at boys and girls separately, so it can't say whether these links differ by sex. For more on girls, see ADHD in women and girls.

Try this today: Write down the one question about pregnancy or birth that's been on your mind, and bring it to your next appointment with a pediatrician or other clinician.

Why a link isn't the same as a cause

Most of what's known about the causes of ADHD comes as links. A link means two things tend to show up together. It doesn't show that one brings about the other, because something else can sit behind both.

A large study from South Korea shows why this matters (Oh, 2024). Researchers used national health records for children born there in the 2010s and followed them from birth into childhood. They looked at antibiotics prescribed to mothers during pregnancy and to babies in their first 6 months. Children exposed to antibiotics had a slightly higher chance of an ADHD diagnosis later on. Across childhood mental health conditions as a group, the link showed up for exposure in either window and grew stronger with more and longer prescriptions.

The authors couldn't separate any effect of the antibiotics from the infection they were prescribed for, or from the inflammation that comes with it. They say their results show a link, not a cause, and that the absolute risk is low. It's a finding about young children in one country, and it isn't advice about antibiotics either way.

Most risk factors are studied by watching, not testing, so a link can reflect genes, family circumstances or an illness rather than the factor itself. And an unanswered question isn't a settled negative.

These findings can't explain any one child. A list of risk factors isn't a checklist, and it can't tell you whether ADHD is present. The day to day side is covered in what ADHD feels like. Only a qualified clinician can assess an individual child, and they're the right person to talk through questions about family history, pregnancy or birth.