Cause
The exact cause of attention deficit hyperactivity disorder (ADHD) is not known. However, inherited genetic factors are likely responsible. Ongoing research is focused on identifying genes that cause a person to be susceptible to ADHD.

Using various imaging tests, researchers have been able to observe the brain at work. They have found a possible link between ADHD and:

Brain structure.
The function of chemicals in the brain that help regulate attention and activity (dopamine and norepinephrine).
Differences in function of some of the areas of the brain that affect attention and impulse control.

Also, research shows that a mother's use of cigarettes, alcohol, or other drugs during pregnancy may increase the risk for ADHD. Some studies suggest that exposure to lead may cause symptoms associated with ADHD.

Although many parents believe that foods with sugar and food additives make their children more hyperactive, these foods have not been shown to cause ADHD. In a comprehensive review of research, the National Institutes of Health (NIH) found that restricted diets seemed to help only about 5% of children with ADHD, mostly either young children or children with food allergies.Other research has shown that sugar is not related to ADHD.

Some people have misconceptions about ADHD, such as that children with ADHD are lazy or dumb. Discuss any questions or concerns you have about these issues with a health professional.

Symptoms
The symptoms of attention deficit hyperactivity disorder (ADHD) include:

Inattention, which is having a short attention span and being easily distracted.

Impulsivity, which can cause a person to do dangerous or unwise things without thinking about the consequences.

Hyperactivity, which is inappropriate or excessive activity.
These symptoms affect people in all age groups who have ADHD. However, typical behavior varies by age.

In preschool-aged children, symptoms may be hard to identify. Normal behavior in young children periodically includes all of the major symptoms. ADHD is distinguished from normal behavior by the severity and consistency of symptoms.

Children between the ages of 6 and 12 usually show more obvious signs of ADHD than other age groups. School expectations can make symptoms more noticeable. For some children, school is the first setting where academic performance and socialization abilities are assessed. However, it often is more difficult to detect ADHD when inattention is the primary symptom and the child is otherwise well-behaved. The presence of ADHD may be indicated by:
Low grades or test scores, including achievement tests.
Poor organization and study skills.
Socialization problems and feeling rejected by peers.
Problems with self-esteem.

Teens between the ages of 13 and 18 may be in better control of disruptive behavior related to hyperactivity. Other problems that began in earlier years may continue or become worse when ADHD is not treated. Teens with inattention problems who previously managed to cope may start to fall behind in schoolwork. This is especially true when major changes occur, such as starting at a new school or going to college.

Symptoms of ADHD in adults may not be as noticeable. Many adults with ADHD have not been diagnosed and treated. These people can develop problems such as depression and difficulty maintaining a job.

There are several other conditions with symptoms similar to ADHD. For example, sometimes bipolar disorder and ADHD can be confused. It can be difficult to determine whether symptoms are caused by ADHD, another condition, or both.