Being bullied may increase a child's risk of developing psychotic symptoms, researchers have found.

Children who were teased had nearly twice the risk of developing psychotic symptoms compared with those who weren't picked on, Dieter Wolke, Ph.D., of the University of Warwick in Coventry, England, and colleagues reported in the May Archives of General Psychiatry.

The association was even greater -- about four and a half times stronger -- when bullying was chronic or severe.

"We found that bullying victimization is a moderate to strong predictor of psychotic symptoms," the researchers said. "These results support emerging evidence about a relationship between peer victimization and psychotic experiences from cross-sectional or retrospective studies."

There's evidence that traumatic events such as abuse in childhood are linked to psychosis in adulthood, but there's only emerging evidence that peer victimization may be associated with psychotic symptoms.

So the researchers conducted a prospective cohort study of 6,437 children who were followed from before birth until about age 13.

About 46% reported being victimized and 54% had not been victimized.

The researchers found that children who reported being repeated victims had a greater likelihood of developing psychotic symptoms (OR 1.94, 95% CI 1.54 to 2.44), and parent and teacher reports showed significant associations with psychotic symptoms as well.

Also, the relationships were even stronger for chronic and severe victimization (OR 4.60, 95% CI 3.24 to 6.50), and there appeared to be a dose-response relationship between severity of victimization and development of psychotic symptoms.

Even when controlling for prior psychopathology, family adversity, or childhood IQ, the results remained significant.

"When we additionally considered family adversity, prior common psychopathology, and IQ as possible confounders, we found that the strength of the associations hardly changed," the researchers said. "Altogether, this is suggestive of a possible causal relationship between victimization and psychotic symptoms."

The researchers noted, however, that victims of bullying are often more withdrawn, unassertive, physically weaker, easily emotionally upset, or have poor social understanding compared with kids who aren't bullied.

Therefore, victimization may be a marker of a risk factor for psychosis rather than a cause.

"It is likely that the children may have already differed in their social skills," Dr. Wolker said. "Thus, victims of bullying are more likely to have social deficits but they get worsened by the bullying experiences."

For example, Dr. Wolker said that even in monozygotic twins, which share the same genes but may have a different victimization experience, the bullied twin will show more mental health problems and lower coping skills.

Potential mechanisms by which bullying can lead to psychotic symptoms include a heightened sensitivity to stress, as well as altered cognitive, affective, and biological processing.

The researchers concluded that a "reduction of peer victimization and of the resulting stress caused to victims could be a worthwhile target for prevention and early intervention efforts for common mental health problems and psychosis."

One limitation of the study was the fact that the initial assessment covered only psychotic symptoms occurring during the six months before the interview, the researchers noted. The age at which the symptoms actually started was not assessed, and there was no assessment of psychotic symptoms before the bullying assessment.

"Thus, it cannot be ruled out that psychotic symptoms might have been present before the peer victimization," they wrote.