- "Probationers who had perpetrated violence in the past were significantly more likely to have used a host of drugs — marijuana, hallucinogens, sedatives, and heroin — than probationers who had never been involved in a violent episode."
- "Almost 50% of all state and federal prisoners who had committed violent felonies were drug abusers or addicts in the year before their arrest, as compared to only 2% of the general population."
- "Individuals who used marijuana or marijuana and cocaine, in addition to alcohol, were significantly more likely to engage in violent crime than individuals who only used alcohol."
- Among adolescent males, "marijuana use in one year frequently predicted violence in the subsequent year."
"Compared with nonsmokers, cigarette smokers have a higher rate of criminality," they wrote in their 2011 book Drugs and Drug Policy: What Everyone Needs to Know. "Smoking in and of itself does not lead to crime, but within the population of smokers we are more likely to find individuals engaged in illicit behavior."
"There is little direct association between marijuana or opiate use and violent crime. ... it is also possible that for some would-be offenders, the pharmacological effect of certain drugs (marijuana and heroin are often given as examples) may actually reduce violent tendencies."