MIKE STOBBE
AP Medical Writer

ATLANTA (AP) — Tourette syndrome occurs in 3 out of every 1,000 school-aged children, and is more than twice as common in white kids as in blacks or Hispanics, according to the largest U.S. study to estimate how many have the disorder.

Tourette’s — known for its physical tics and, in some cases, shouted obscenities — has long been considered a rare condition. The new number means it’s more common than some past estimates, but confirms that it’s far less common than other neurological conditions like autism or attention deficit hyperactivity disorder.

The racial gaps are probably the most surprising finding, said study’s authors said.

“Prior to this, we really had very little information about minorities,” said Lawrence Scahill, a Yale University researcher. The study was released Thursday.

It’s not clear why whites have a higher rate or if future studies will find the same disparity, experts said. Some suspect it has less to do with genetics than with a difference in access to medical care or in attitudes about whether repetitive blinking or other tics require medical care.

The study, led by the Centers for Disease Control and Prevention, estimates there are about 150,000 U.S. children with Tourette’s, below the 200,000 threshold for rare diseases.

The researchers also found that:

— Most cases were mild, but one in four were — in the parent’s opinion — moderate or severe.

— Boys had a rate three times higher than girls.

— About 80 percent had been diagnosed with another mental illness or developmental disability.

Tourette’s is characterized by involuntary, sudden and repetitive movements. They may be as mild as blinking, nose twitching or grimacing, or severe as head jerking or other motions that cause chronic pain or injury. Some also have vocal tics that range from grunting and throat-clearing to barking and swearing.

The condition tends to peak in early adolescence and fades afterward. Scientists don’t know the cause, but believe genetics are at least part of the answer.

Earlier studies offered a range of estimates of Tourette’s from less than 1 per 1,000 to close to 30.

The CDC study was the first to employ a national survey. Parents or guardians in more than 91,000 U.S. households were interviewed by phone, and asked if a doctor had ever told them that their child had Tourette syndrome. They also were asked about diagnoses of depression, attention deficit hyperactivity disorder and other behavioral problems or developmental disabilities.

The researchers found that the Tourette’s rate in white children was about 4 per 1,000, while the rate for both blacks and Hispanics was about 1.5.

The researchers looked at parental education and household income, and found those factors did not explain the racial difference.

The report is being published this week in a CDC publication, Morbidity and Mortality Weekly Report.