In addition to concerns about loss of motivation and reckless driving, many people fear that cannabis intoxication can lead to hostility. Reviews of the cannabis literature invariably reflect writers' prejudices. Summaries of studies on marijuana and aggression may reveal these biases more than any other area of research. Interpretations of this literature are incredibly disparate. One author's evidence for marijuana's connection to violence serves as another author's proof that the drug does not cause aggression.
Interpretations of a study of murderers illustrates this point. In this research, interviews with 268 incarcerated murderers revealed that 72 of them had smoked cannabis within a day of the homicide. Of these 72, 18 claimed that marijuana contributed to the murder in some way. Fifteen of these 18 were intoxicated with other drugs at the time, too (Spunt, Goldstein, Brownstein, & Fendrich, 1994). The researchers reported these facts clearly, but interpretations of their meaning vary dramatically. One review cites this study as an example of cannabis leading to violence (Sussman, Stacy, Dent, Simon, & Johnson, 1996). Another uses it as an illustration of the rarity of marijuana-induced hostility, emphasizing how other drugs likely account for the relationship between cannabis and aggression (Zimmer & Morgan, 1997). Thus, any interpretations of data from this field require a close reading of the original studies.
People have assumed drugs lead to violence at least since the seventeenth century. Intoxication, withdrawal, and chronic use of alcohol and stimulants clearly increase aggressive acts (Kleiman, 1992). Legislators often justify drug prohibition as an effort to decrease violence. Ironically, data suggest that strict enforcement of these laws leads to a hostile underground
market and a climbing murder rate (Miron, 1999). Despite evidence for increased aggression associated with other drugs, the vast majority of work shows that cannabis does not induce hostility. This research includes the standard series of case studies, correlational reports, and laboratory experiments.
Each of these research approaches has strengths and weaknesses, but the general conclusions remain the same. Direct links between cannabis intoxication and violence do not appear in the general population. A few studies show correlations between marijuana consumption and violent acts, but these links frequently stem from personality characteristics or the use of other drugs. People who are violent or who use drugs that lead to violence often also smoke marijuana, but the marijuana does not appear to cause the violence.
Laboratory studies also find no link between cannabis intoxication and violence. Most people who ingest THC before performing a competitive task in the laboratory do not show more aggression than people who receive placebos; occasionally, they show decreased hostility. Numerous scientific panels sponsored by various governments invariably report that marijuana does not lead to violence (Zimmer & Morgan, 1997). Yet two studies reveal small but significant links between cannabis and aggression with very select populations under extremely circumscribed conditions. If these findings replicate, further work may reveal a great deal about aggression in general and subsets of individuals susceptible to provocation during marijuana intoxication.
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