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Children as young as 7 may intentionally hurt themselves by cutting, burning or hitting themselves, according to a new study of non-suicidal self-injury. It's a dangerous practice, say researchers, that can have significant impact on school, relationships and social interactions.

That's according to a study published Monday in the journal Pediatrics by researchers at the University of Denver and Rutgers University. The study included 665 youths in the third, sixth and ninth grades at area schools who were invited to be part of a laboratory study. They were interviewed about non-suicidal self-injury.

Researchers found that 8 percent of the children had injured themselves deliberately — 9 percent of the girls and 6.7 percent of the boys. The self-injury was highest among ninth-graders at 12.7 percent. In the third grade, the figure was 7.6 percent, while the number for sixth-graders was 4 percent. Ninth-grade girls reported much higher rates of non-suicidal self-injury, at 19 percent, compared to 5 percent of the boys at that age.

There were also differences in how the children hurt themselves, based on gender. Girls said they cut themselves and carved their skin more often, while boys reported hitting themselves more often.

The researchers said in a written statement about the study that children engaging in what they call NSSI "tend to be depressed, angry and consumed with negative thoughts. And the injuries can have a significant effect on academics, relationships and social functioning."

"It's unfortunately probably more common than we want to think," lead researcher Benjamin Hankin, an associate psychology professor at the University of Denver, told the Denver Post. He said many kids find that causing physical pain helps them cope with emotional stress. Some researchers believe that when they hurt themselves, their bodies release feel-good hormones called endorphins.

The Denver Post said that "family strife, troubles in school and bullying are among reasons some kids hurt themselves; details on what caused the behavior among the study kids wasn't included in the published report."

Steve Pastyrnak, division chief of pediatric psychology at Helen DeVos Children's Hospital in Grand Rapids, Mich., told Web MD Health News that "self-injury is typically done for stimulation or soothing. It is, in a sense, treating emotional pain with physical pain. Some children say they need the pain because they feel emotionally numb."

He suggested that children who hurt themselves may suffer from depression or anxiety and said that in his practice he teaches children and teens strategies for coping with anxiety and stress.