Attention Deficit Hyperactivity Disorder (ADHD) is a neuro-developmental mental disorder. It's characterized by problems in paying attention, excessive activity, or difficulty in controlling behavior (typically not appropriate for a person's age). Symptoms usually begin at between six to twelve years of age, and, to be classified as ADHD must be present for more than six months and cause problems in at least two settings (e.g. school, home, or in recreational activities).
The causes are as yet unknown, but genetic, environmental and societal factors have been found to affect its prevalence.
See: Wikipedia
Scientists are studying cause(s) and risk factors in an effort to find better ways to manage and reduce the chances of a person having ADHD. The cause(s) and risk factors for ADHD are unknown, but current research shows that genetics plays an important role."
Source : US Centers for Disease Control and Prevention
A 2023 study performed by University College London found that the prevalence of ADHD in the UK appears to be increasing year by year, and that "ADHD diagnoses and prescriptions are associated with socioeconomic deprivation." (ref)