Mother Jones, July 2009 by Kevin Drum
http://www.motherjones.com/politics/2009/07/patriots-guide-legalization
The most likely conclusion from the overall data is that if you fully legalized cannabis, use would almost certainly go up, but probably not enormously. MacCoun guesses that it might rise by half—say, from around 15 percent of the population to a little more than 20 percent. “It’s not going to triple,” he says. “Most people who want to use marijuana are already finding a way to use marijuana.”