Attention-deficit/hyperactivity disorder (ADHD) is the most common behavioral disorder among children in the United States. It is a behavioral and neurocognitive condition characterized by inattention, hyperactivity, or both lasting for at least 6 months. The onset is typically before age 7 years. Although ADHD begins in childhood, fewer than 40 percent of such children continue to meet diagnostic criteria in their teenage years. In studies, adults with childhood histories of ADHD were found to have higher rates of accidents, injuries, health problems, pregnancies, and job and marital problems. Although prevalence estimates for ADHD vary by country and region, as well as by age, a pooled estimate of worldwide prevalence is 5.29 percent. This figure showed the most variation geographically between Europe and North America.

The disorder has been identified in the literature for many years under a variety of terms. In the early 1900s, impulsive, disinhibited, and hyperactive children—many of whom had neurological damage caused by encephalitis—were grouped under the label hyperactive syndrome. In the 1960s, a heterogeneous group of children with poor coordination, learning disabilities, and emotional lability but without specific neurological damage were described as having minimal brain damage. Since then, other hypotheses have been put forth to explain the origin of the disorder, such as genetically based condition involving abnormal arousal and poor ability to modulate emotions. This theory was initially supported by the observation that stimulant medications help produce sustained attention and improve these children’s ability to focus on a given task. Currently, no single factor is believed to cause the disorder, although many environmental variables may contribute to it, and many predictable clinical features are associated with it.

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