Researchers do not know the exact cause of attention deficit hyperactivity disorder (ADHD). However, they believe it may develop from a combination of genetic and environmental factors.

ADHD can cause inattention, hyperactivity, and impulsivity. Many studies suggest a strong link between genetic factors and ADHD, but this is not the only thing that appears to contribute to it.

Read on to learn more about the causes of ADHD and the research behind them.

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ADHD tends to run in families. Identical twins are more likely to develop ADHD than nonidentical twins. This suggests a genetic component to the condition, as identical twins have the same genes while nonidentical twins do not.

Genes may account for 74% of the cause of ADHD. Research into the specific genes involved in its development is still ongoing, but a large, international 2023 study estimates that around 7,300 genetic variants could increase a person’s risk of ADHD.

However, many are common variants and do not always cause ADHD. This suggests it is not only genetics that influences a person’s chances of having ADHD. The number of these variants a person has, and other factors, may increase the likelihood.

ADHD also appears to cause or originate from changes in the brain. These include:

Brain structure

A 2017 imaging study found that overall brain volume and specific brain regions were slightly smaller in participants with ADHD than in those without ADHD.

The regions included the:

  • caudate nucleus and putamen, which coordinate smooth movement
  • nucleus accumbens, which plays a role in reward processing
  • amygdala, which influences emotional regulation
  • hippocampus, which is involved with emotion and motivation

The differences in brain volume were more noticeable in children with ADHD than in adults. This supports the researchers’ theory that ADHD causes a delay in the development and maturation of several brain regions.

Brain function

According to a 2018 review, functional MRI (fMRI) studies indicate that people with ADHD may have impairments in several brain networks that manage attention, cognitive control, timing, and working memory.

Review authors also state that people with ADHD may have differences in brain networks dealing with reward processing.

For example, the review notes some research into children with ADHD found reduced activation in particular brain regions when performing reward-related decision making tasks.

Brain chemistry

Brain chemistry refers to the balance of chemicals that affect a person’s mood and nervous system. One such brain chemical is dopamine. It plays a vital role in feelings of pleasure, motivation, and reward. Dopamine is also involved in motor function, memory and focus, and executive functioning.

In a 2009 study, study participants with ADHD had lower dopamine levels than participants without ADHD.

Some experts believe that lower dopamine levels may occur in people with ADHD because they have higher concentrations of proteins known as dopamine transporters, which reduce dopamine levels in the brain.

However, researchers do not yet understand the relationship between ADHD and dopamine.

Brain injury

A traumatic brain injury (TBI) is an injury that affects how the brain works.

A 2018 study with children ages 3–7 years old hospitalized overnight for TBI found that TBI correlated to an increased risk of ADHD up to 7 years after the injury.

Specifically, around 62% of the children in the study with severe TBI developed ADHD later. This is known as secondary ADHD.

Some research connects ADHD to complications that affect fetal development during pregnancy, such as:

  • Alcohol consumption: A 2015 study found that children who experienced prenatal alcohol exposure were 1.55 times more likely to have ADHD.
  • Prenatal smoking: Some research suggests a link between smoking during pregnancy and a higher risk of ADHD in children. This includes the 2015 study above, which found a 2.64 times higher chance of ADHD from maternal smoking during pregnancy. However, 2022 research states that, although there is an association, it is unlikely to be causal.
  • Neurotoxins: Some studies suggest that exposure to certain chemicals, such as lead and some pesticides, may have associations with ADHD. For example, a 2016 study found exposure to organophosphate pesticides may impact brain development and contribute to a higher chance of ADHD.
  • Low birth weight: A 2018 review found that, compared to babies weighing at least 5.5 pounds (lb), babies weighing under 3.3 lb were at least twice as likely to develop ADHD. Babies under 2.2 lb were at least four times as likely to develop ADHD.

Some birth complications also have an association with ADHD, such as:

  • Oxygen deprivation: A 2013 study found that children who lack oxygen to the brain before or shortly after birth were significantly more likely to develop ADHD later in life.
  • Cesarean delivery: A 2019 study compared cesarean delivery (C-section) to vaginal delivery. Researchers found a higher rate of ADHD among children born via cesarean delivery. However, the connection between them is currently unclear.
  • Premature birth: A 2022 study found that among children born between 37–41 weeks of gestation, those born before 39 weeks have a higher chance of experiencing ADHD symptoms.

While these studies show an association between ADHD and prenatal or birth complications, they do not prove they are a direct cause. More research is necessary to understand the relationship.

Psychological trauma may play a role in ADHD, but it is unclear how, or to what extent.

Trauma is an emotional and physical response to an intensely distressing event. In studies on child development, scientists refer to these events as adverse childhood experiences (ACEs). ACEs can include:

  • direct experiences of abuse or neglect
  • witnessing violence
  • living in an unsafe or unstable environment

Children with ACEs are more likely to have ADHD. People with ADHD are more likely to have further ACEs.

Scientists do not know why this is, but children with ADHD being more likely to have traumatic experiences could be due to a “cycle of adversity” in which their symptoms make it harder to navigate the world and make them more vulnerable to abuse.

Some people believe ADHD is the result of:

  • eating too much sugar
  • watching TV or playing too many video games
  • poor parenting

However, the Centers for Disease Control and Prevention (CDC) emphasizes that there is not enough evidence that any of these directly causes ADHD.

For example, a 2019 cohort study analyzed children with no ADHD symptoms at the age of 6 years. Sugar consumption did not impact the likelihood of ADHD in the group as they got older.

Researchers suggest that higher sugar consumption in those with ADHD could be an effect of impulsivity or hyperactivity. This would fit with previous research suggesting a link between ADHD and certain food habits, such as eating more often and frequent snacking.

ADHD is also not the product of a lack of discipline from parents. In fact, the opposite may be true. A 2022 review found associations between negative or harsh parenting and ADHD as well as other factors, such as:

  • emotional reactivity
  • intrusiveness
  • maltreatment
  • factors that can put a strain on families, such as divorce or incarceration

However, it can be difficult to tell whether these factors cause ADHD or are present because multiple family members have ADHD. For example, reactivity could result from difficulty regulating emotions or impulses.

ADHD appears to be something a person has from birth, or that develops in early childhood.

According to the National Institute of Mental Health, the symptoms of ADHD can begin between ages 3–6 years.

Researchers are still learning about the causes of ADHD. However, they suspect a combination of genetic and environmental factors increases a person’s chances of developing the condition.

Despite popular views that sugar, excessive TV, or a lack of discipline cause ADHD, there is not enough evidence to conclude they contribute to the condition.