According to statistics recently released by the government in Denver, the amount of robberies and violent crimes significantly decreased since marijuana legalization went into effect. It is important to mention that this strong correlation is not definitive proof that legalization is the cause of this drop in crime, but it does strongly suggest that this is the case.
These statistics are especially convincing considering the short amount of time that this drastic reduction in crime has taken place. In just one short year the number of homicides dropped by 52.9%. Sexual assaults were reduced by 13.6%. Robberies were down by 4.8% and assaults were down by 3.7%.
The statistics measured the first few months of the year for both 2013 and 2014, and then compared those numbers with one another to determine whether they were higher or lower after legalization went into effect.
There are many different factors contributing to this drop in crime, and it is likely that marijuana legalization is a very big piece of the puzzle. Legalization has had a profound impact on local economies, and has created a large boom in new residents who have moved to the area to flee persecution. This increase in prosperity surely has some effect on the amount of robberies and burglaries that have taken place.
Additionally, marijuana is traditionally known to mellow people out and calm them down, making them far less likely to act out in anger or plan a murder.
One final possibility that comes to mind is the fact that possibly, police resources are being diverted towards serious crimes instead of nonviolent offenses. Unfortunately, they are still writing plenty of fines and locking up plenty of people for nonviolent offenses, but marijuana smokers and traders have been one of the largest group of persecuted nonviolent offenders for a very long time.
See the UCR Citywide Report