Nearly a quarter of young people who have been bullied go on to bully others, according to a major British survey.
The findings also suggest that more than twice as many boys as girls bully - 66 per cent of males compared to 31 per cent of females.
And 50 per cent of young people have been bullied at some point in the past year, according to the anti-bullying charity Ditch the Label.
It surveyed 8850 people aged between 12 and 20.
Students were allocated time periods to fill in the compulsory survey.
This year's annual bullying survey by Brighton-based Ditch the Label looked at the various reasons why young people bully others.
Around a third of those who bully said they rarely or very rarely spent time with their parents, and almost the same number said they had daily rows at home.
The findings suggest people who bully are more likely than average to have suffered a traumatic event, such as their parents splitting up or a major family fall-out.
And 44 per cent of young people who have been bullied suffer depression, 33 per cent have suicidal thoughts and 31 per cent go on to self-harm, the survey found.
Consultant psychiatrist Dr Rick Fraser of Sussex Partnership NHS Foundation Trust said there needed to be a "unified approach" to combat bullying.
"Whether perpetrator or recipient, bullying has negative consequences for young people that can have lasting effects into adulthood," he said.
Ditch the Label founder and CEO Liam Hackett said bullying frequently has a huge impact on the health, welfare and future prospects of millions of young people.
Psychologist Professor Ian Rivers, of Brunel University, said bullying remains a significant concern in UK schools.
"It is very important that we understand the context in which bullying takes place, and how and why young people are bullied by their peers," Rivers said.
Ditch the Label surveyed online 8850 people aged from 12 to 20 in partnership with schools and colleges across the UK between November 1, 2015 and February 28, 2016.
