The exact causes of ADHD are not known with certainty.

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, but other common behaviors, such as smoking or drinking during pregnancy, as well as other obstetrical complications have been linked to ADHD in children.

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 or pesticides 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.

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 or mothers who had ADHD in their youth have children 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.

The presence of certain toxins in the environment, such as lead, PCBs, or pesticides, may also have a role in causing ADHD. Researchers believe that some toxins may interfere with the brain development of children. That, they say, could possibly lead to ADHD.

Past studies indicate that too much TV may be associated with ADHD. But some newer studies show that time spent watching TV may not affect young kids' risk of developing ADHD.

In one study, experts reviewed data from a national survey of parents, teachers, kindergarteners, and first-grade students. They concluded that kindergartener's TV time was not linked to the chance of having attention problems in first grade.

Quick GuideADHD Symptoms in Children

Studies show pregnant women who smoke or drink alcohol may have an increased risk of having a child with ADHD. Some studies also show that women who smoke during pregnancy are more likely to have ADHD themselves. Other studies conclude that alcohol or drug abuse during pregnancy can reduce activity of the neurons that produce neurotransmitters.

In addition, ADHD is higher in children who had a low birth weight or whose mothers had difficult pregnancies.

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