Children as young as four are routinely bullying their nursery or pre-school classmates, according to the first study of violent behaviour in infants.
The findings revealed school bullies are already set in their ways by the time they enter school although their targets change on a weekly basis.
The report, presented yesterday at the British Psychological Society's annual conference, has prompted calls for anti-bullying strategies to be targeted at the youngest school children.
Although all schools must by law have an anti-bullying policy little emphasis, claim the researchers, is placed on eradicating it at an early age.
"Bullying needs to be stamped out at the age of four, when children first enter school. We need to break the cycle before it takes hold," said psychologist Claire Monks, who conducted the study.
Her findings showed up to one-quarter of the sample, aged four to six, were identified as bullies by their peers over a four-month period. One-in-seven children's names cropped up on a regular basis, indicating a hard core of bullies even at the age of four.
Their victims, however, changed frequently as aggressive infants picked out the "soft" targets in the classroom.
Victims only become regular targets as children get older, said Ms Monks, a researcher at Goldsmiths College, London.
"What we are suggesting is that these aggressive children are trying out a variety of targets and they learn from the responses and then limit their bullying to fewer children as they get older," said Ms Monks.
She studied 123 children in four primary schools across London, showing them basic cartoons to represent different types of bullying and noting the names of children they identified as bullies and victims.
She also discovered 16 per cent of infants acted as "white knights", willing to step in to prevent victimisation. Almost half the boys were identified by classmates as bullies, compared to just seven per cent of girls.
Bullying consisted of physical and verbal assaults but the most common was a form of social exclusion, where children refused to let victims play in their group.Reuse content