A study of 54 people with bipolar disorder found that the illness, long considered an adult affliction, also affects children.
The research, published in Archives of General Psychology this week, said that 44% of those who had manic episodes as children continued having them as adults.
"Children with mania grow into adults who have mania," said Dr. Barbara Geller of Washington University School of Medicine in St. Louis, who led the National Institutes of Health-funded study.
People with bipolar disorder experience severe mood swings between depression and mania, a state marked by excess energy and restlessness. Between mood swings, many patients are symptom-free. However, severe cases carry a risk of suicide.
Some experts have been skeptical that bipolar disorder exists in children. Yet increasing numbers of children are diagnosed as bipolar, a phenomenon that has been attributed in part to diagnostic confusion. Certain characteristics of bipolar disorder, such as aggression or irritability, also are symptoms of attention-deficit hyperactivity disorder and other conditions.
To avoid that pitfall, researchers selected children who displayed not only irritability or aggression but also some form of grandiose behavior -- dashing into traffic because of a sense of invincibility, for example -- a classic symptom of bipolar mania in adults. Children entered the study between ages 7 and 16 and were tracked into adulthood.
At the end of the study, 24 of the 54 participants had at least one manic episode after turning 18. That rate was 13% to 44% higher than in the general population, researchers said.
Geller argued that recognizing grandiose behavior in children was a key to making correct diagnoses. She said that children were not properly diagnosed in the past because physicians are trained to identify grandiose behavior in adults but not in children.
"Children don't get married four times, or max out their credit cards, or buy real estate they can't afford," she said. "We have to find the childhood equivalent of these behaviors."