Marijuana Use Increases Violent Behavior

What makes this new study more compelling than previous studies is that the researchers followed the same individuals for over 50 years from a young age to adulthood.  This is precisely what one needs to solve the chicken or egg riddle with respect to cannabis and violence:  just look and see which one happens first.
One fifth of those who were pot smokers (22%) reported violent behavior that began after beginning to use cannabis, whereas only 0.3% reported violence before using weed.  Continued use of cannabis over the life-time of the study was the strongest predictor of violent convictions, even when the other factors that contribute to violent behavior were considered in the statistical analysis.
 In conclusion, the results show that continued cannabis use is associated with a 7-fold greater odds for subsequent commission of violent crimes.  This level of risk is similar to the increased risk of lung cancer from smoking cigarettes over a similar duration (40 years).  The authors suggest that impairments in neurological circuits controlling behavior may underlie impulsive, violent behavior, as a result of cannabis altering the normal neural functioning in the ventrolateral prefrontal cortex.