The primary features of attention-deficit/hyperactivity disorder include inattention and hyperactive-impulsive behavior. ADHD symptoms start before age 12, and in some children, they're noticeable as early as 3 years of age. ADHD symptoms can be mild, moderate or severe, and they may continue into adulthood.

ADHD occurs more often in males than in females, and behaviors can be different in boys and girls. For example, boys may be more hyperactive and girls may tend to be quietly inattentive.

There are three subtypes of ADHD:

• Predominantly inattentive. The majority of symptoms fall under inattention.
• Predominantly hyperactive-impulsive. The majority of symptoms are hyperactive and impulsive.
• Combined. The most common type in the U.S., this is a mix of inattentive symptoms and hyperactive-impulsive symptoms.

Inattention

A child who shows a pattern of inattention may often:

• Fail to pay close attention to details or make careless mistakes in schoolwork
• Have trouble staying focused in tasks or play
• Appear not to listen, even when spoken to directly
• Have difficulty following through on instructions and fail to finish schoolwork or chores
• Have trouble organizing tasks and activities
• Avoid or dislike tasks that require focused mental effort, such as homework
• Lose items needed for tasks or activities, for example, toys, school assignments, pencils
• Be easily distracted
• Forget to do some daily activities, such as forgetting to do chores

Hyperactivity and impulsivity

A child who shows a pattern of hyperactive and impulsive symptoms may often:

• Fidget with or tap his or her hands or feet, or squirm in the seat
• Have difficulty staying seated in the classroom or in other situations
• Be on the go, in constant motion
• Run around or climb in situations when it's not appropriate
• Have trouble playing or doing an activity quietly
• Talk too much
• Blurt out answers, interrupting the questioner
• Have difficulty waiting for his or her turn
• Interrupt or intrude on others' conversations, games or activities

Additional issues

In addition, a child with ADHD has:

• Symptoms for at least six months
• Several symptoms that negatively affect school, home life or relationships in more than one setting, such as at home and at school
• Behaviors that aren't normal for children the same age who don't have ADHD

Normal behavior vs. ADHD

Most healthy children are inattentive, hyperactive or impulsive at one time or another. It's normal for preschoolers to have short attention spans and be unable to stick with one activity for long. Even in older children and teenagers, attention span often depends on the level of interest.

The same is true of hyperactivity. Young children are naturally energetic — they often are still full of energy long after they've worn their parents out. In addition, some children just naturally have a higher activity level than others do. Children should never be classified as having ADHD just because they're different from their friends or siblings.

Children who have problems in school but get along well at home or with friends are likely struggling with something other than ADHD. The same is true of children who are hyperactive or inattentive at home, but whose schoolwork and friendships remain unaffected.

While the exact cause of attention-deficit/hyperactivity disorder is not clear, research efforts continue. Factors that may be involved in the development of ADHD include:

• Genetics. ADHD can run in families, and studies indicate that genes may play a role.
• Environment. Certain environmental factors, such as lead exposure, may increase risk.
• Development. Problems with the central nervous system at key moments in development may play a role.

Risk factors for attention-deficit/hyperactivity disorder may include:

• Blood relatives, such as a parent or sibling, with ADHD or another mental health disorder
• Exposure to environmental toxins — such as lead, found mainly in paint and pipes in older buildings
• Maternal drug use, alcohol use or smoking during pregnancy
• Premature birth

Although sugar is a popular suspect in causing hyperactivity, there's no reliable proof of this. Many issues in childhood can lead to difficulty sustaining attention, but that's not the same as ADHD.

Attention-deficit/hyperactivity disorder can make life difficult for children. Children with ADHD:

• Often struggle in the classroom, which can lead to academic failure and judgment by other children and adults
• Tend to have more accidents and injuries of all kinds than do children who don't have ADHD
• Tend to have poor self-esteem
• Are more likely to have trouble interacting with and being accepted by peers and adults
• Are at increased risk of alcohol and drug abuse and other delinquent behavior