Marijuana Use Causes 7 Fold Increased Risk of Violent Behavior

Marijuana use and violence New research reported in the journal Psychological Medicine, concludes that continued use of cannabis causes violent behavior as a direct result of changes in brain function that are caused by smoking marijuana over many years.

Marijuana use and violence

Researchers have long debated a possible link between use of marijuana and violent crime.  Many previous studies have linked marijuana use to increased delinquency, aggression, and other psychological disorders.  From a pharmacological perspective this makes sense, because cannabis inhibits neural circuits in the prefrontal cortex that control impulsive behaviors and self-control.  In addition, long-term use of cannabis, especially in adolescent years, can cause long-term changes in brain development, including an increased risk of schizophrenia. The sticky problem in such studies are the many confounding factors involved in interpreting this correlation.  It is very difficult to determine whether any statistical correlation between marijuana use and violent behavior are causally linked, or instead the two are associated through some other factor, such as socioeconomic status, personality traits, or many other variables that are related to the propensity to use marijuana. 

Moreover, the causal relation between smoking pot and violent behavior could be in exactly the opposite direction.  That is, individuals who are involved in violence or who commit criminal offenses may also be people who are more open to using marijuana.  After all, marijuana is an illegal substance in most places, so people with antisocial personality traits and those with tendencies toward lawlessness may be the type of individuals inclined to be more open to obtaining and using the illegal substance. 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 do to solve the chicken or egg riddle with respect to cannabis and violence:  just look and see which one happens first.

These subjects were in the Cambridge Study in Delinquent Development, comprised of 411 boys who were born around 1953 and living in working-class urban neighborhoods of London.  97% of them were Caucasian and all of them were raised in two-parent households.  The researchers took into consideration other factors, including antisocial traits, alcohol use, other drug use, cigarette smoking, mental illnesses, and family history. The statistics showed that most of the participants never used cannabis and they were never reported to have violent behavior.  38% of the participants did try cannabis at least once in their life.  Most of them experimented with cannabis in their teens, but then stopped using it. However, 20% of the boys who started using pot by age 18 continued to use it through middle age (32-48 years). 

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. "Together, the results of the present study provide support for a causal relationship between exposure to cannabis and subsequent violent outcomes across a major part of the lifespan," write Tabea Schoeler and her colleagues at Kings College London in their paper.  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.

In fact, their 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). This one study may not settle the debate in this complex issue.  One problem is that it is not logically valid to conclude causation from the fact that one event precedes another.  For example, you may be correct in concluding that the red light caused the car to stop at the intersection, because the red light came first, but you would be incorrect to conclude that the light turned red because the car approached the intersection before it turned red.  Still, the long-term analysis of individuals over 3/4 of their life-span is valuable evidence in this important debate.    

About the Author

Douglas Fields

R. Douglas Fields is Chief of the Nervous System Development and Plasticity Section at the National Institutes of Health, NICHD, in Bethesda, Maryland, and author of the new book about sudden anger and aggression “Why We Snap,” published by Dutton, and a popular book about glia “The Other Brain” published by Simon and Schuster.