Violence inflicted on male children and adolescents by each other begins with parenting, may be evident in junior infants and is most common among children living in socially isolated housing estates. Those are the conclusions of extensive research at the University of Colorado into children and violence. For younger children, healthy "`rough and tumble" play has a value in the development of friendship, fighting skills and the establishment of one's position in a dominance hierarchy. When rough and tumble play persists in pursuit of domination, it becomes bullying, which is a precursor to antisocial and aggressive behaviour and is evident in four- and five-year-olds.

Rough and tumble play is likely to continue beyond childhood into early adolescence, when children have few alternative means of establishing their social position, such as success in school. Boys who consistently harm other boys in school (and it usually is boys against boys) are more likely than others to end up as juvenile offenders. Usually, such anti-social behaviour has developed by the age of eight or nine, when bullying can turn into serious, violent crime. Most at risk of becoming nine-year-old fighters are children in disrupted families who have been abused, who have anti-social parents and aggressive siblings. To prevent such boys victimising others, the most effective interventions are at home, rather than school.