If cannabis were legalized would the crime rates in the US increase?
* It has not been conclusively proven either way whether the legalization of cannabis would increase or decrease crime rates in the US; the debate goes on, with different people putting forward different theories. There is a possibility that crimes related to fund the habit would increase, but it is not likely; people who use the drug are likely to do so regardless of the law, as we see in society today.
If cannabis were legalized, those who would use it more regularly would probably be the type who would use it more socially and less often, paying for it with money earned from work, rather than from crime. The logic behind this is that the current users, the ones breaking the law, are the more regular users who can only afford to fund the habit by committing crimes. This would mean that crime rates would remain more or less the same.
However, you also need to consider that everyone smoking marijuana would no longer be breaking the law when they used it, so crime rates here would go down. The overall effect of this would probably be a reduced crime rate.
Further research would be required by experts before a government decision on the subject, but the outlook does look positive in terms of crime rates. Another factor to consider is that there may be knock-on effects to the legalization of cannabis: Some people claim it can lead to the use of stronger substances, but the evidence goes against this. If those people who started using cannabis went on to using stronger drugs, then the crime rate could be affected adversely, with people commiting crimes to fund addictions to heroin, cocaine or other drugs. It very much depends on the scale of the knock-on effects. Although unlikely (research HAS proven that cannabis usually doesn't lead on to harder drugs) they could possibly occur.
My conclusion, therefore, is that the result of legalizing cannabis would most likely lead to a reduction in US crime rates, but not definitely.
* Adding to what the previous respondent said in the last paragraph, this is recognized by many political parties across the world, usually those that are left of center, which have the legalization of cannabis and other substances written into their manifesto. In some countries, there are parties solely aimed at achieving this; the best example is the Legalise Cannabis Alliance in the U.K. In some countries whose government stance is more liberal in this area, it is already legal, e.g., Belgium and Russia.
As far as violent crime goes, the statistics are inconclusive as to whether legalization would result in a reduction. In Belgium, where the drug is legal, the most recent study of homicides per 100,000 people showed a low rate, just 1.50. Then again, in Russia, where the drug is also legal, the most recent study revealed a 16.5/100,000 homicide rate -- a definite correlation can not be seen between legalized cannabis and reduced murder rates. In places where the drug is illegal the same thing is true; no pattern seems to exist.