Childhood bullying may seem like a normal part of growing up, but a new study shows that it can cause severe long-term mental health problems, even worse than maltreatment by adults.

There is already an established link between maltreatment by adults and the mental health consequences for children. But researchers at the University of Warwick wanted to determine whether long-term mental health issues among victims of bullying were related to having been maltreated by adults as well.

They looked at data from 4,026 participants in the UK ALSPAC study (Avon Longtitudinal Study of Parents and Children) and 1,273 participants from the US Great Smoky Mountain Study.

For the ALSPAC study, they looked at reports of maltreatment between the ages of 8 weeks and 8.6 years; bullying at ages 8, 10 and 13; and mental health outcomes at age 18. Data from the Great Smoky Mountain Study had reports of maltreatment and bullying between the ages of 9 and 16, and mental health outcomes from 19-25 years old.

"The mental health outcomes we were looking for included anxiety, depression or suicidal tendencies. Our results showed those who were bullied were more likely to suffer from mental health problems than those who were maltreated. Being both bullied and maltreated also increased the risk of overall mental health problems, anxiety and depression in both groups," lead study author Professor Dieter Wolke said in a statement.

In the ALSPAC study, 8.5 percent of children reported maltreatment only and 29.7 percent reported bullying only, while 7 percent reported both maltreatment and bullying. In the Great Smoky Mountain Study, 15 percent reported maltreatment, 16.3 percent reported bullying and 9.8 percent reported both.

"Being bullied is not a harmless rite of passage or an inevitable part of growing up," Wolke said, "it has serious long-term consequences. It is important for schools, health services and other agencies to work together to reduce bullying and the adverse effects related to it."

The findings were published in the journal Lancet Psychiatry and are being presented at the Pediatric Academic Societies (PAS) annual meeting in San Diego on April 28.

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