Hochelaga was the original Iroquoian name for the village that ultimately became Montreal, but it is also the name of a rough-hewn French–Canadian neighbourhood located east of — and a world away from — the cosmopolitan city centre. The district's tidy two- and three-storey brick duplexes, adorned with Montreal's characteristic wrought-iron staircases, predominantly house families that have, because of poverty and lack of education, never quite attained thriving middle-class status.

During the 1980s, public-school officials identified Hochelaga and many other impoverished neighbourhoods in the eastern part of Montreal as places where kindergarten children disproportionately displayed severe behavioural problems, such as physical aggression. The school system asked a young University of Montreal psychologist named Richard Tremblay for help.

“Their parents didn't have a high-school diploma, and many of the mothers had their first child before the age of 20,” Tremblay says of the families he began to study, as he walks along Rue Ontario in Hochelaga on a sunny afternoon in September. Those were the women, he adds, “most at risk of having children who have problems”.

Over the past three decades, Hochelaga and similar neighbourhoods have served as living laboratories in the study of the roots of aggression. Since 1984, Tremblay and his collaborators have followed more than 1,000 children from 53 schools in the city from childhood into adulthood. And in 1985, he initiated a ground-breaking experiment in which some families of at-risk children were given support and counselling to help curb bad behaviour. His research overturned ideas about when aggressive behaviour first emerges, and showed that early intervention can deflect children away from adult criminality.