The 18-year-old question as to whether or not legalizing medical 
marijuana causes an increase in crime seems to be answered in a recent study by a team of researchers from The University of Texas at Dallas this week. The results did not indicate a “crime exacerbating effect” of medical marijuana on any of the Part I offenses, which (according to the FBI) include homicide, rape, robbery, aggravated assault, burglary, larceny, and auto theft.
Alternatively, states with medical marijuana laws showed a reduction in homicide and assault rates. This is congruent to other studies by The National Academy of Sciences that found THC -- the active ingredient in pot -- actually causes a decrease in "aggressive and violent behavior" in chronic marijuana users.
"The findings on the relationship between violence and marijuana use are mixed and much of the evidence points toward reductions in violent behavior for those who smoke marijuana," Robert Morris, the study’s lead author, said in a recent interview with the Huffington Post. "In fact, researchers have suggested that any increase in criminality resulting from marijuana use may be explained by its illegality, rather than from the substance itself."
These findings run counter to arguments that suggest making marijuana legal for medical purposes poses a danger to public health, in terms of exposure to violent crime and property crimes.