Adults with ADHD have a high incidence of coexisting or comor-bid conditions, according to the 2004 National Comorbidity Survey, which studied the entire U.S. population between the ages of 15 and 45. This research showed that 45 percent of adults with ADHD had mood disorders, including depression. Those rates were three times the instance of depression in the general population.

In addition, 59 percent suffered from anxiety disorders (3.2 times higher than the general population), 35 percent had issues with substance or alcohol abuse or dependency (2.8 times the general population), and 69 percent suffered from impulse disorders (5.9 times the general population). Other research showed that adults with varying types of ADHD had an even higher rate of coexisting conditions than did children with ADHD.

Among adults with the combined type of ADHD, 69 percent had some history of substance abuse and dependence. In addition, at some point in their lives 63 percent had been treated for depression, 35 percent for anxiety, and 30 percent for conduct disorders. Studies show that more than a third of adults with ADHD also suffer from oppositional disorder at some time in their life, and nearly a quarter of them suffer from social phobia.

Executive Function Impairments

Adults with ADHD are also highly likely to suffer from a wide variety of executive function impairments that can impact every area of their lives. Put another way, the brain of an adult with ADHD can be compared to a computer with an erratic operating system that interferes with running the essential software they need to succeed at work, at home, and in their relationships.

Cluster Theory of Adult ADHD

Why 88 percent of adults with ADHD — six times the rate of the general population — suffer from some type of psychiatric condition is a factor that continues to puzzle scientists. Some experts attribute it to genetics and the fact that people simply inherit a particular form of psychiatric problems. Others believe the answer may stem from a theory holding that ADHD is not one disorder, but a complicated syndrome made up of a cluster of impairments that affects many different parts of the brain, and which causes or contributes to many different types of psychiatric illnesses.