Attention-Deficit/Hyperactivity Disorder: Causes of ADHD

What exactly causes ADHD isn't known.

Experts do know that ADHD has a strong genetic component. In addition, they think that genes that control the levels of certain chemicals in the brain called neurotransmitters seem to be different in those with ADHD.

In some cases, though, there is no genetic link to ADHD. Nevertheless, this common behavior disorder is still diagnosed in children whose mothers smoked or drank alcohol during pregnancy. It's also diagnosed in children whose mothers had difficult pregnancies.

Babies with low birth weight may have an increased risk of ADHD. The same is true for children who have had head injuries, particularly an injury to the frontal lobe. Young children who are exposed to lead or other environmental toxins such as PCBs early in life may also have a higher risk of ADHD.

ADHD always begins in childhood. For some people, though, ADHD is not diagnosed until adulthood. That means adults who are newly diagnosed have actually had ADHD for years, and have had to endure symptoms as they've matured. In addition, research shows that between 30% and 70% of children with ADHD continue to have symptoms of the disorder when they become adults.

What is the genetic connection to ADHD?

ADHD tends to run in families. Studies have shown certain genetic characteristics that occur with high frequency in families where one or more family member has ADHD. Also, if one or both parents have ADHD, their children are more likely to develop the condition. And at least one-third of all fathers who had ADHD in their youth have children with ADHD.

What brain changes occur with ADHD?

Studies show that children and adults with ADHD tend to have abnormal functioning, or dysregulation, of certain brain chemicals known as neurotransmitters. There also tends to be abnormal functioning in the nerve pathways that regulate behavior. In addition, children with ADHD may have certain parts of the brain that are smaller or less active than they are in children who don't have ADHD.

Recent studies show that the brain chemical, dopamine, may play a role in ADHD. Dopamine is an important chemical that carries signals between nerves in the brain. It is linked to many functions, including movement, sleep, mood, attention, and learning.

One dopamine study focused on the genetics of ADHD -- specifically, on a particular variation of the DRD4 gene. This gene is associated with a dopamine receptor in the brain. What the study showed is that children with ADHD are more likely to have a certain variation of the DRD4 gene than children without ADHD. Interestingly, not all kids with ADHD in the study had the DRD4 gene variation. But those who did generally had higher IQ scores than other children with ADHD. Plus, the gene variation was most common in children whose ADHD improved over time.

Another dopamine study involving adults with ADHD showed that adults with ADHD had a sluggish dopamine system. The study helped explain why stimulant ADHD medications such as Ritalin and Adderall are beneficial. Stimulant ADHD medications increase dopamine by strengthening the weak dopamine signals in the brain. That counters the decreased brain dopamine activity in adults with ADHD. In addition, drugs of abuse, like nicotine and cocaine, temporarily increase brain dopamine activity. So the study authors hypothesized that the decreased dopamine activity associated with ADHD may help explain why people with ADHD may have a greater risk of drug abuse.