We don't know the exact causes of ADHD. However, research has demontrated that genetics play an important role in the transmission of this disorder which affects between 4 and 5% of the population. Symptoms of ADHD are present as of childhood and we know now that they persist through adulthood in the majority of cases. We, as physicians, thus treat children but also parents and adults who struggle to reduce the impact that the symptoms of ADHD entail in their daily lives.
ADHD has a genetic component in most cases and can, more rarely, also be tied to the consequences of a neurological condition at an early age (e.g., premature birth, neonatal problem, early neurological disease of an infectious nature, such as meningitis).
ADHD is a neurological condition; it is not caused by a poor upbringing or by psychological stressors. However, one's environment can modulate its expression and course. For example, the presence or absence of help and support to remedy this disorder can generate anxiety, undermine self-esteem, and give rise to behaviour problems (e.g., opposition and delinquency).
To explain ADHD, scientists have advanced the hypothesis of a dysfunction at the level of certain information-transmission mechanisms involving neurotransmitters such as dopamine and noradrenaline. To gain a better understanding of the effects of ADHD, we might compare how information circulates in the brain to a road network. Studies of brain function in persons with ADHD have revealed an impairment of the region responsible for controlling or inhibiting certain behaviours (in medical jargon, these are referred to as "executive functions"), they are what allow, among other things, turning on the engine, breaking, changing directions and prioritizing on the road. In ADHD, the information-transmission network appears to be defective, as though it was lacking traffic lights and road signs and as though the automobiles were equipped with a faulty ignition or brake system.