Self-injury makes some people feel part of a group. Teenagers who self-injure often report that there is no adult they could talk to who accepts them for who they are.

“A 13-year-old can go on the Internet and instantly find community and get hitched to this behavior,” Dr. Whitlock said. “When they don’t want to self-injure anymore, it means they have to leave a community.”

Self-injury can be manipulative, an effort to make others care or feel guilty or to drive them away. More often, though, it is secretive. Self-injurers may try to hide wounds under long pants and long sleeves even in hot weather, and may avoid activities like swimming.

Who Is Vulnerable?

Self-injury often starts in the emotional turmoil of the preteen and early teenage years and may persist well into adulthood.

Although female self-injurers are more likely to be seen by a professional, in-depth studies indicate that the behavior is practiced equally by young men and women. No racial or socioeconomic group has been found to be more vulnerable, although self-injury is slightly less common among Asians and Asian-Americans, Dr. Whitlock said.

Interviews with self-injurers have found background factors that may prompt and perpetuate the behavior. A history of childhood sexual, and especially emotional, abuse has been reported by half or more of self-injurers. Some seek relief from the resulting emotional pain. Others self-inflict pain to punish themselves for what they perceive as their role in inviting the abuse.

Low self-esteem is common among self-injurers. Childhood neglect, social isolation and unstable living conditions have also been cited as risk factors. In about 25 percent of self-injurers, there is a history of eating disorders, as well as an overlap with risky drinking and unsafe sex.