However, recent data suggest that symptoms of ADHD continue into adulthood in up to half of individuals with childhood ADHD. Although research on adult ADHD is reduced compared to childhood ADHD, there is growing evidence that ADHD may present somewhat differently in adulthood.
ADHD is characterized by deficits in neurocognitive processes. Specifically, individuals with ADHD have difficulty with executive function, which includes processes that are important for regulating attention and behavior, such as attentional control, inhibitory control, cognitive flexibility, and planning.
There are many known causes of ADHD, including a strong genetic component. Genetics appears to be the principal cause of ADHD, likely accounting for up to approximately 75% of all cases. In a smaller percentage of cases, ADHD may arise from early brain injuries, low birth weight, exposure to environmental toxins during gestation (e.g., maternal smoking) or youth (such as lead). Research does not support the claim that ADHD is a result of food additives, preservatives, or sugar.
In adults, ADHD frequently occurs with other psychiatric conditions, including mood disorders, anxiety, learning disorders, and substance use disorders. In fact, a large national study found 51% of adults with ADHD suffered from co-morbid anxiety and 32% suffered from co-morbid depression. Because of the amount of overlapping symptoms with other disorders, diagnosing ADHD can be complicated.