What causes ADHD? What we know, don’t know and suspect

Neurodevelopmental disorders are a diverse group of conditions that affect the brain from early development. They include attention-deficit hyperactivity disorder (ADHD), autism and learning disabilities, such as dyslexia.

These conditions usually become more evident over time. This is because delays in the skills a child is expected to have developed at each age become more apparent.

ADHD is the most common neurodevelopmental disorder. It affects around 8-10% of children and 2-5% of adults.

ADHD affects a person’s efficiency at completing tasks (for example, because they get distracted) and their behaviour (such as losing things or struggling to pay attention).

ADHD can affect all aspects of functioning including problems learning and maintaining friendships. If undiagnosed, the challenges are likely to persist and may lead to anxiety, depression and low self-esteem.

How is it diagnosed?

There is no specific genetic or brain abnormality that causes ADHD and no single reliable test to diagnose it.

A formal diagnosis depends on whether a child shows at least six of the diagnostic criteria for inattention (at least five for adults) and/or at least six of the criteria for hyperactivity-impulsivity (at least five for adults). These have to persist for at least six months.

The diagnostic criteria include:

• difficulty concentrating (for example, trouble listening, poor attention to detail, not getting tasks finished)

• hyperactivity (including fidgeting, feeling restless and running around, constantly chatting)

• impulsivity (for example, interrupting conversations and games, difficulty waiting their turn).

Not everyone with ADHD is hyperactive. For people with inattentive-type ADHD, their main difficulty is inattention, for example, concentrating consistently on everyday tasks that are not particularly interesting.

If someone meets the criteria for hyperactivity-impulsivity and for inattention, they have combined-type ADHD.

How reliable is diagnosis?

One problem with these criteria is they’re not specific to ADHD. For example, difficulties concentrating can also be a symptom of depression.

This is why it’s not enough to simply tick a symptom checklist. The formal diagnostic criteria emphasise these symptoms must interfere with daily functioning.

The key question is: are ADHD symptoms causing day-to-day problems or holding this person back?

What this means will vary from person to person, depending on what their everyday activities involve.

For example, someone may struggle to concentrate at school but excel later on in a creative career such as photography, or in a high-intensity job with hard deadlines, such as journalism.

It also means a person may only meet the full diagnostic criteria at certain stages of their life. Subthreshold ADHD – when someone meets some criteria but not enough for a diagnosis – can still cause significant difficulties.

Gender differences

Boys aged between four and 11 are up to four times more likely to be diagnosed with ADHD than girls.

This may partly be because the diagnostic criteria are especially good at identifying hyperactive young boys. But they are not as effective for girls, particularly those who are not hyperactive or disruptive, or who try to hide their difficulties concentrating.

Girls and women are likely to be diagnosed later and show more “internalising symptoms”, such as depression. However the rate of underdiagnosis in girls has been improving over the last four decades.

The gender disparity also evens out with age. The female proportion of young adults diagnosed with ADHD is closer to half (38%).

What about genetics?

There is also a strong genetic component. Heritability for ADHD is around 70–80%. This describes how much of the person-to-person differences in ADHD are due to genetics, rather than environmental influences.

The more closely someone is related to a person with ADHD – in other words, the more genes they have in common – the more likely they are to have ADHD.

However the genetics are complex. It’s not as simple as finding a gene or selection of genes “responsible” for ADHD.

For example, early research linked ADHD to six genes that target neurotransmission (how the brain sends chemical signals). But the effect of each gene was small.

ADHD is now understood to be a polygenic disorder, with thousands of common genetic variants involved.

Each of these genes is capable of making a discrete but minuscule contribution to the overall expression of ADHD. Because these genes are common, the traits of ADHD are distributed throughout the population, with no clearly defined cut-off between those who do and do not have the condition.

Within a family, the interaction between shared genetics and a shared environment (their household) make it difficult to study these separately.

Does environment play a role?

A supportive family can help a child with ADHD cope better with everyday tasks, as parents often adapt their parenting style to their child’s behaviour. This may mask the ADHD and delay diagnosis.

But if one or both parents also has ADHD, this may affect their parenting style. It can be difficult to determine how much of that child’s behaviour is due to their inherited ADHD, and how much to the family environment and parenting.

Studies have also shown children who are relatively young for their year when they start school have higher rates of treatment for ADHD. This points to their environment playing a role in when their ADHD is diagnosed, but not necessarily its cause.

For more information about ADHD, as well as information about support groups, visit the ADHD foundation or ADHD Australia websites.