From my new policy analysis (joint with Angela Dills and Sietse Goffard) on state marijuana legalizations:

In November 2012 voters in the states of Colorado and Washington approved ballot initiatives that legalized marijuana for recreational use. Two years later, Alaska and Oregon followed suit. As many as 11 other states may consider similar measures in November 2016, through either ballot initiative or legislative action. Supporters and opponents of such initiatives make numerous claims about state-level marijuana legalization.

Advocates think legalization reduces crime, raises tax revenue, lowers criminal justice expenditures, improves public health, bolsters traffic safety, and stimulates the economy. Critics argue that legalization spurs marijuana and other drug or alcohol use, increases crime, diminishes traffic safety, harms public health, and lowers teen educational achievement. Systematic evaluation of these claims, however, has been largely absent.

This paper assesses recent marijuana legalizations and related policies in Colorado, Washington, Oregon, and Alaska.

Our conclusion is that state marijuana legalizations have had minimal effect on marijuana use and related outcomes. We cannot rule out small effects of legalization, and insufficient time has elapsed since the four initial legalizations to allow strong inference. On the basis of available data, however, we find little support for the stronger claims made by either opponents or advocates of legalization. The absence of significant adverse consequences is especially striking given the sometimes dire predictions made by legalization opponents.