Cyber-bullying may be even harder on the victims than physical beatings or name-calling, U.S. researchers reported on Tuesday.

And unlike traditional bullies, cyber-bullies seem to be less depressed than their prey, the team at the National Institutes of Health found.

Jing Wang, Tonja Nansel and Ronald Iannotti of the NIH's National Institute of Child Health and Human Development analyzed data from an international survey from 2005/2006 that included 4,500 U.S. preteens and teens.

They were asked specifically about feelings of depression, irritability, grouchiness and ability to concentrate, and also asked specifically if they had been hit, called names, shunned or sent negative messages via computer or cell phone -- or done any of these things to other people.

Unlike traditional bullying which usually involves a face-to-face confrontation, cyber victims may not see or identify their harasser, Iannotti's team wrote in the Journal of Adolescent Health.

As such, cyber victims may be more likely to feel isolated, dehumanized or helpless at the time of the attack.

Physical and verbal bullies are often depressed themselves. But while there was little difference in depression between physical bullies and their targets, the NIH team found that cyber-bully victims reported significantly higher levels of depression than frequent bullies.

Bullying can be a policy issue -- it harms learning and can lower a school's test scores. U.S. schools are increasingly under pressure to bring up scores and to show regular improvements.

Last year the same team found that more than 20 percent of all U.S. adolescents in school had been bullied physically at least once in the last two months, 53 percent were bullied verbally, 51 percent bullied socially by being excluded or ostracized and 13.6 percent were bullied electronically.

Depression