Marijuana Decriminalization and Prevalence of Use

"Proponents of criminalization attribute to their preferred drug-control regime a special power to affect user behavior. Our findings cast doubt on such attributions. Despite widespread lawful availability of cannabis in Amsterdam, there were no differences between the 2 cities [Amsterdam and San Francisco] in age at onset of use, age at first regular use, or age at the start of maximum use."
"Our findings do not support claims that criminalization reduces cannabis use and that decriminalization increases cannabis use."

Source: 

Reinarman, Craig; Cohen, Peter D.A.; Kaal, Hendrien L., "The Limited Relevance of Drug Policy: Cannabis in Amsterdam and in San Francisco," American Journal of Public Health (Washington, DC: American Public Health Association, May 2004) Vol 94, No. 5, pp. 840 and 841.
http://ajph.aphapublications.org...

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