An array of sources on the subject of marijuana decriminalization. As always, feel free to add to this list in the comment thread.

Marijuana Decriminalization & Its Impact on Use (an assortment of links to studies from the National Organization to Reform Marijuana Laws)

Poll results on pot: Legalization doesn't appear to have majority support (though the latest Gallup poll showed, for the first time, a narrow edge -- 50%-46% -- for legalization) , but medical marijana enjoys significant support (77 percent; 70 percent; 78 percent; 60 percent; 73 percent; 81 percent).

Emanuel explains support of decriminalizing pot possession

Under Emanuel’s proposal, police officers would have the discretion to issue tickets with fines ranging from $100 to $500 for people carrying 15 grams or less of marijuana. The plan could be considered by City Council at its next meeting on June 27.,,,Currently people caught in possession face a misdemeanor charge punishable by up to six months in jail and a $1,500 fine

The Impact ot Marijuana Decriminalization, Journal of Public Health Policy

Abstract: The available evidence indicates that the "decriminalization" of marijuana possession had little or no impact on rates of use. Although rates of marijuana use increased in those U.S. states which reduced maximum penalties for possession to a fine, the prevalence of use increased at similar or higher rates in those states which retained more severe penalties. There were also no discernable impacts on the health care systems. On the other hand, the so-called "decriminalization" measures did result in substantial savings in the criminal justice system.

Roger Roffman,a professor of social work at the University of Washington, quoted in Room for Debate (New York Times):

Will more people use marijuana and become dependent if marijuana is decriminalized? Probably not. A number of U.S. studies tell us decriminalization would not likely have an effect on the rates of marijuana use by adults or adolescents.

The Effect of Marijuana Decriminalization on the Budgets of Massachusetts Governments, With a Discussion of Decriminalization’s Effect on Marijuana Use by Jeffrey A. Miron Professor of Economics Boston University:

Evaluations of decriminalization experiences in other states and countries provides little indication that use increases to any substantial degree as the result of decriminalization....MacCoun and Reuter (1997, 2001) discuss the evidence on marijuana decriminalization in the United States, Australia, and the Netherlands. They conclude there is no evidence that decriminalization increases marijuana use. Single, Christie, and Ali (2000) reach the same conclusion regarding evidence on the experience with decriminalization in several Australian states. The result that decriminalization has little impact on marijuana use might seem surprising, since standard economic principles suggest that lowering the penalties for use should increase demand and therefore the quantity consumed. The explanation for the counter-intuitive result of little impact on use is that decriminalization frequently ratifies what has already taken place in the form of reduced enforcement of marijuana laws. Furthermore, the fact that some states decriminalize while others do not might reflect incidental factors rather than any real difference in policy. Thus, actual enforcement of marijuana laws probably did not change much in decriminalizing states or changed across states in ways that bear little relation to the criminal status of marijuana. Nevertheless, existing evidence provides no indication that marijuana decriminalization causes increased marijuana use. In particular, there is no reason to expect a substantial increase in marijuana use in Massachusetts since enforcement of marijuana laws already provides little disincentive to use.

Removal of penalties/decriminalization (Marijuana Policy Project information page) and Regulation, Not Prohibition is Key to Reducing Teen Marijuana Use (MPP, includes reference to 2010 Pew survey in the Netherlands)

Drug Decriminalization in Portugal: Lessons for Creating Fair and Successful Drug Policies (CATO Institute White Paper by Glenn Greenwald):

None of the nightmare scenarios touted by preenactment decriminalization opponents — from rampant increases in drug usage among the young to the transformation of Lisbon into a haven for "drug tourists" — has occurred.....decriminalization has had no adverse effect on drug usage rates in Portugal, which, in numerous categories, are now among the lowest in the EU, particularly when compared with states with stringent criminalization regimes..by virtually every metric, the Portuguese decriminalization framework has been a resounding success. Within this success lie self-evident lessons that should guide drug policy debates around the world.

The Office of National Drug Control Policy: Drug Decriminalization in Portugal: Challenges and Limitations:

The Cato Institute report does not present sufficient evidence to support claims regarding causal effects of Portugal’s drug policy on usage rates. More data are required before drawing any firm conclusions, and ultimately these conclusions may only apply to Portugal and its unique circumstances, such as its history of disproportionately high rates of heroin use. However, it is safe to say that claims by drug legalization advocates regarding the impact of Portugal’s drug policy exceed the existing scientific basis.

""I saw that Rhode Island just became the 15th state to decriminalize small amounts of marijuana. It's true. Which explains why today state lawmakers are like, 'Wait, how are we both a road and an island?'"...Jimmy Fallon

An Analysis of Marijuana Policy--National Research Council of the National Academy of Science,

Reports from California, Oregon, and Maine indicate no appreciable increase in use following decriminalization of use, at least in the short term. Oregon, the first state to repeal prohibition of use (in October 1973) has been studied in a series of Drug Abuse Council surveys (National Governors' Conference, 1977). Surveys in 1974 and 1975 showed no major increase following decriminalization. While the percentage of adults who were current users had increased by January 1977 (from 20 to 24 percent), use had increased similarly nationwide in the same period, suggesting that the causes for the adult increase in Oregon were the same as those for increases in the rest of the country rather than the result of changes in the law....The state of Maine, which repealed criminal penalties for marijuana use in May 1976, surveyed the effects of legislation in July and August 1978 (State of Maine Department of Human Services, 1979). Its study concluded that the change from criminal to civil penalties has not caused a large increase in marijuana use:

Interpreting Dutch Cannabis Policy: Reasoning by Analogy in the Legalization Debate (Science)

While the Dutch case and other analogies have flaws, they appear to converge in suggesting that reductions in criminal penalties have limited effects on drug use—at least for marijuana.