Approximately two million American children are affected with attention-deficit hyperactivity disorder (ADHD), which is about one child per classroom, with boys being 3-5 times more likely to have ADHD. Symptoms of this disorder begin before the age of seven and often continue into adulthood. ADHD has nothing to do with intelligence, and the precise cause of it is unknown, but it is known that it evolves from an interaction between genetic, biological, and environmental factors. Since the symptoms vary between children, ADHD is commonly over-diagnosed. Additionally, many psychiatric disorders interfere with a child's ability to pay attention and can confuse a diagnosis. Similarly, anxiety, depression, drug abuse, and stress can interfere with attention and cause hyperactivity which can be misdiagnosed as ADHD. Adults who have extreme expectations about behavior can also mislabel a child as having ADHD. Children who actually have ADHD and are not appropriately treated may experience academic and social failure, low self-esteem, drug addiction, and trouble with the law.