The legalisation of recreational cannabis is weeding out teenaged users

WHEN JUSTIN TRUDEAU promised to legalise cannabis use across the border in Canada, his main reason for doing so was to protect the young. Cannabis is bad for the developing brain and a worrying number of minors were taking the drug. The counterintuitive proposal was based on the idea that regulated sales would drive out illegal sellers, who do not care how old their customers are. Legal sellers, however, will generally abide by age restrictions in sales to keep their licence.

It is too early to tell whether Canada’s change, at the end of last year, will have the desired effect. Yet there is a wealth of historical data in America, which has been tinkering with various forms of liberalisation since the 1990s. Today 33 states permit medical cannabis, and 11 have legalised recreational use. The most recent legalisation bill, for recreational use, was signed in Illinois on June 25th.