New data suggest that we probably shouldn't discount the growing use of crack cocaine and methamphetamines.
"The major story was a major split in the use of crack," said Mr. Johnson.
He was among the first to document the growing use of crack in 1984.
A decline in the use of crack cocaine may also have helped.
Experts say few statistics are available on the use of crack.
It spread as the use of crack cocaine increased and sex in exchange for drugs became common, health officials said.
That, combined with a decline in the use of crack cocaine, an increase in imprisonments and tough policing strategies, helped push violent crime "down hugely," he said.
He was divorced from the children's 36-year-old mother and had cited her use of crack cocaine in court papers about custody of their children.
Toronto mayor Rob Ford faced renewed questions on Tuesday over his alleged use of crack cocaine and reports that a video shows him smoking the drug.
Experts debate why: better policing, less use of crack cocaine, fewer crime-prone youths, more criminals in jail, or some or all of the above.
That is less likely than in the past, the professor said, because tougher sentences have kept criminals off the streets and the use of crack cocaine has declined.
