The Montreal Longitudinal Experimental Study was set up in 1984 to follow the development of over 1,000 boys from poor and deprived areas who were deemed at risk of growing up to be aggressive.

The children were split into three groups: one that would receive a preventive parenting intervention (called Preventive Treatment Program), another who were observed for their interactions and a third who received neither intervention nor observation. A large amount of contextual data was also collected.

The study continues to follow the lives of the participants who are now nearing their thirtieth birthdays