Not so long ago, attention deficit disorder (ADD), which is now called attention deficit hyperactivity disorder (ADHD), was a disorder experts believed was limited to children. The conventional wisdom was that kids grew out of the condition sometime during puberty and that symptoms did not continue into adulthood. Today, however, experts know that while ADHD begins in childhood, many people continue to have the disorder as adults. In fact, research now shows that 4 percent of the adult population suffers from ADHD — a total of more than 8 million people.