NEW YORK -- Gay and bisexual kids are more likely to be bullied as they're growing up -- even at an early age, according to the first large U.S. study to look at the problem.

Public school students in three cities were asked about bullying in the 5th, 7th and 10th grades. When they reached high school, they were asked if they identified themselves as gay, lesbian or bisexual. The researchers then looked back at what those kids had said through the years about their experiences being hit, threatened, called names, or excluded.

Overall, many of the nearly 4,300 students surveyed said they were bullied, especially at younger ages. But the 630 gay and bisexual children suffered it more.

The researchers found 13 per cent of them were bullied on a weekly basis in 5th grade, compared to 8 per cent of other kids. In both groups, the rates went down as the students got older -- but the disparity persisted.

"In fifth grade, they already were bullied more than other kids" -- even though, at that young age, many gay and bisexual kids haven't discovered their own sexual orientation yet, said the lead author, Dr. Mark Schuster of Boston Children's Hospital.

The data doesn't say why each kid was targeted. But most were likely picked on for being 'different,' he said.

"Some kids may be considered by the bullies to be a more girlish boy, or a more boyish girl," said Schuster.

The pattern reflects what was reported in an earlier study of teens in England. The new study was published in Thursday's New England Journal of Medicine. The research drew from an ongoing study of health behaviours and health risks in Houston, Los Angeles, and Birmingham, Alabama.

Other research has found gay and bisexual high school students are more likely than their heterosexual classmates to attempt suicide or do risky things like smoke and drink alcohol.

In an earlier study, Schuster and his colleagues found that the longer a child is bullied, the more severe and lasting the effect on the kid's health. Bullying is linked to depression and feelings of lower self-worth, Schuster said.

"At one time, bullying was brushed off as 'kids will be kids and that's just part of going through childhood,"' he said. But he said it's more than a little teasing -- the consequences can be "persistent and serious."