Marijuana Use May Increase Violent Behavior

50-year study finds possible causal link between cannabis and later violence

Posted Mar 20, 2016

Wikipedia Commons
Source: Wikipedia Commons

New research published online in advance of print in the journal Psychological Medicine concludes that persistent use of cannabis may cause violent behavior as a result of changes in brain function due to smoking weed over many years.

Researchers have long debated a possible link between use of marijuana and violent crime.  In contrast to alcohol, meth, and many other illegal drugs, the mellowing effects of cannabis seem unsuited to promoting violent behavior.  However, ample previous research has linked marijuana use to increased violent behavior. 

The sticky problem in such studies is the existence of many confounding factors involved in interpreting the correlation.  It is very difficult to determine whether any statistical correlation between marijuana use and violent behavior represents a causal link or whether, instead, the two are associated through some other factor,s 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. 

Not so, conclude neuroscientist Tabea Schoeler at Kings College London and her colleagues, "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."  Let's examine the evidence provided by this new study.

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.

The 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.  Ninety-seven percent of them were Caucasian, and all of them were raised in two-parent households.  The researchers took into consideration factors including antisocial traits as assessed by the Antisocial Personality Scale, alcohol use, other drug use, cigarette smoking, mental illnesses, and family history. 

Hers's what they found:  Most of the participants never used cannabis and they were never reported to have violent behavior.  Thirty-eight percent 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 lifetime 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.  The level of risk is equivalent 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.  

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Fields, R.D.  Inhale or Don't?:  Marijuana Hurts Some, Helps Others, Scientific American MIND, Sept. 1, 2009.  

Fields, R.D. The Absurdity of "Medical Marijuana,"  BrainFacts.org  Dec. 20, 2014.  

Fields, R.D.  Creativity, Madness, and Drugs.  Scientific American, November 22, 2013.