What causes ADHD

Researchers have been looking into what causes ADHD for decades. While they haven’t found an exact cause of ADHD, they’ve identified two key factors that play major roles.

The first is differences in how the brain develops and functions. The second is genetics — ADHD very often runs in families.¹ Some researchers have also looked into potential outside risk factors, like exposure to chemicals.

In the end, the cause of ADHD may be a combination of factors and not just one.

ADHD and the brain

ADHD is a neurodevelopmental disorder, meaning it’s a difference in how the brain develops and functions. Brain imaging studies have shown that the brain develops similarly in kids who do and don’t have ADHD — but at different rates. 

With ADHD, there’s a delay in the development in certain areas of the brain that are involved in what’s known as executive function.¹ This is a set of mental skills that allow us to focus, plan, manage time, and organize. Kids and young adults with ADHD tend to be three to five years behind in developing those areas of the brain.

A large-scale study using brain imaging scans also showed that some areas of the brain are smaller in people with ADHD.²

These areas include:

• Accumben: Controls mood, motivation, and experiencing pleasure

• Amygdala: Plays a role in emotional control and prioritizing actions

• Caudate: Key in decision-making and purposeful behavior

• Hippocampus: Responsible for long-term and working memory

• Putamen: Helps with learning, memory, and regulation of movement

Researchers have also found differences in how the brain functions. These differences involve:

• The level of activity in certain areas of the brain

• The chemical signals that travel through the brain

• Interaction between certain areas of the brain³

Genes and heredity

If you have ADHD, chances are that others in your family do, too. Over the years, twin, family, and adoption studies have shown a strong genetic connection.⁴ If you have ADHD, there’s a one-in-four chance that one of your parents also has it. And it’s likely that another close family member, like a sibling, will have ADHD. (ADHD isn’t something parents “give their kids.” It’s just something in our genetic code.)

Often, undiagnosed parents will recognize signs of ADHD in themselves when their child is diagnosed with ADHD. More and more adults, especially women, are now getting evaluated for ADHD.

Connections to other disorders

There are a number of disorders that often co-occur with ADHD. They don’t cause ADHD, but researchers are looking into possible links — including genetic links.

Conditions that often co-occur with ADHD include: 

Learning disorders: Certain learning disorders often co-occur with ADHD, although the exact amount of overlap isn’t clear.⁵ The learning differences that most often co-occur are written expression disorder, dyslexia, and dyscalculia.

Autism spectrum disorder (ASD): Kids who have ADHD sometimes also have ASD.⁵ When someone has both ADHD and ASD, you may hear it being called AuDHD.

Mental health conditions: A number of mental health disorders often co-occur with ADHD. In adults, the ones that overlap most are mood disorders like depression and anxiety.⁶

In kids, the conditions that co-occur most include learning disorders, mood disorders like depression and anxiety, disruptive mood dysregulation disorder (DMDD), and oppositional defiant disorder (ODD).⁵

Environmental factors 

Genetics aren’t a part of all ADHD cases. Researchers are also looking into outside or “environmental” factors that might play a role. None of these factors are shown to cause ADHD, but they may increase the risk of having it. 

Here are some of the factors that researchers are looking at: 

• Exposure to certain chemicals⁷

• Exposure to heavy metals and lead⁸

• Prenatal exposure to alcohol or tobacco⁹

• Trauma¹⁰

• Pre-pregnancy obesity¹¹

• Parental stress, particularly in mothers¹²

It’s important to know that experiencing any of those factors doesn’t mean you or your children will have ADHD.

Watch: An expert’s take on genes and ADHD

Understood Expert Dr. Ari Tuckman sorts fact from fiction about what causes ADHD.

Summary

There’s no single cause of ADHD, but researchers have found two factors that play a key role.

The first is genetics. ADHD has a high tendency to run in families. People with this common condition have a one-in-four chance of having a parent with it. It’s also likely that another close family member, like a sibling, has it, too. (ADHD isn’t something parents “give their kids.” It’s just something in our genetic code.)

The second is differences in brain development. With ADHD, certain areas of the brain take a few years longer to develop. These areas are involved in executive function, a set of skills that include attention, self-control, and working memory. 

There are also differences in how the brain functions in people with ADHD, from the chemical signals that go throughout the brain to the connections between different areas of the brain.

Researchers are looking into environmental factors that may raise the risk of ADHD. These include exposure to chemicals, tobacco, and alcohol, among other things.