ADHD is caused primarily by genetic and neurological factors. It is one of the most heritable conditions in psychiatry, with genetics accounting for roughly 70 to 80 percent of cases. Differences in brain development, particularly in the prefrontal cortex and the dopamine and norepinephrine systems, are consistently observed in people with ADHD. Certain biological risk factors such as premature birth or prenatal tobacco exposure can also elevate risk, but these are secondary to genetic influences.