In most if not all known cultures, many people have engaged in activities that alter their state of consciousness, whether through meditation, trances, or the use of mind-altering substances such as alcohol, marijuana, cocaine, opiates, MDMA (ecstasy), methamphetamine, and hallucinogens (McKenna, 1993; Rosen & Weil, 2004; Weil, 1972). Although these drugs can be appealing for a variety of reasons, both the resulting altered states and the desire for these drugs can contribute to aggression in a variety of ways.

Some theorists have argued that the criminalization of recreational drugs and the extremely ineffective “war on drugs” play a substantial role in crime and violence. These drugs generate huge profits that legitimate authorities are unable to regulate and that bankroll other illegal activities (Goldstein, 1986). Both small-time drug dealers and large-scale drug cartels must therefore devise their own means of protection. As a result, the illegal drug trade contributes to a large percentage of violent crimes, ranging from assault to homicide, in countries such as the United States (Roth, 1994) and Mexico (Miroff & Booth, 2010). For example, Mexican drug cartels hire sicarios, assassins who protect their turf and business interests. The book El Sicario: The Autobiography of a Mexican Assassin (Molloy & Bowden, 2011) tells the story of one such hit man who killed hundreds of people and was very well paid for doing so.

The effects of drugs on users also can contribute to violence. Although a common belief has it that addicts turn to crimes such as muggings and burglary to support their addiction, at least for heroin, evidence for this link is not clear (Kretschmar & Flannery, 2007). However, drugs can increase physiological arousal, heightening emotional reactions to provocations and reducing higher-order cognition and impulse control. Some drugs can create a sense of paranoia, which also intensifies feelings of being threatened. Unfortunately, experimental research on the effects of illegal drugs on aggression is rare because of the ethical and practical difficulty of giving such drugs to participants. In one of those rare experiments, participants given a high dose of cocaine gave higher-intensity shocks to an opponent in a competitive game than did those given a placebo (Licata et al., 1993). However, complementary correlational evidence on this topic has not supported a clear link between cocaine and amphetamine use and aggression. More research is needed before strong conclusions can be drawn about the direct effects of these and other stimulants on aggression (Kretschmar & Flannery, 2007; Kruesi, 2007).

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The vast majority of studies on the effects of drugs on aggression have focused on alcohol (Kretschmar & Flannery, 2007; Kruesi, 2007). This is partly because alcohol is a legal recreational drug and partly because it is the most commonly used (and abused) recreational drug in the United States and most other large cultures. Imagine that you are at a sporting event, a party, or a concert, and a fight breaks out. How likely do you think it would be that at least one of the combatants is drunk? If you’re like most people, you’d probably think it would be pretty likely. Correlational research strongly supports this intuition. In fact, when researchers review the literature and various crime reports, they find that alcohol is involved in about half of all violent crimes and sexual assaults worldwide (Beck & Heinz, 2013; Chermack & Giancola, 1997). Some studies show even higher rates. In one study of 882 persons arrested in Ohio for felonies, over 75% of offenders who were arrested for violent crimes, ranging from assault to murder, were legally intoxicated (Shupe, 1954). Clearly, alcohol use is positively associated with aggression. This is especially true for people with aggressive dispositions, tendencies to hostility, and low impulse control (Geen, 2001).

Experimental research further supports a causal role of alcohol use in aggression. When given an opportunity to deliver electric shocks or aversive noise to another person, participants who have consumed alcohol under conditions of threat or competition engage in more aggression than sober participants. The higher the dose of alcohol, the greater the aggression (Bushman & Cooper, 1990; Taylor & Leonard, 1983). In contrast, threatened participants who have consumed the psychoactive ingredients in marijuana generally have been found to be less rather than more aggressive than sober participants (e.g., Taylor & Leonard, 1983). Why is alcohol intoxication such a significant contributor to aggression?

One reason is that alcohol impairs higher-order thinking such as self-awareness, and therefore reduces inhibitions and impulse control (e.g., Hull et al., 1983; Ito et al., 1996). A minor slight or insult that a sober person would likely ignore or deflect is more likely to provoke aggression in a drunk person. In addition, this cognitive impairment makes it less likely that drunk people will consider the consequences of their actions (Steele & Josephs, 1990; Taylor & Leonard, 1983). Neuroscience research further shows that patterns of electrical activity in the brains of intoxicated people indicate that they are less distressed than sober participants by the mistakes they make (Bartholow et al., 2012). All these factors highlight alcohol’s tendency to reduce the self-regulatory controls with which we ordinarily inhibit aggressive impulses.

A second reason that alcohol increases aggression is that we expect alcohol use to lead to aggression. Participants who drank a placebo beverage that they believed contained alcohol (but actually did not) showed increased behavioral signs of aggression compared with those who knew they were not drinking alcohol (Rohsenow & Bachorowski, 1984). In fact, simply exposing participants to alcohol-related pictures (e.g., a beer bottle or martini glass), or even flashing alcohol-related words on a computer screen so quickly they can’t be consciously seen, increases the accessibility of aggressive cognitions, leads participants to interpret a person’s behavior as more aggressive, and leads to more hostile evaluations of another person when frustrated (Bartholow & Heinz, 2006; Friedman et al., 2007). These effects tend to be stronger the more strongly people believe that alcohol use causes aggression.

Although the bulk of the experimental research has focused on the effects of aggression on perpetrators, some research suggests that victims of violent crimes are also more likely to be drunk (e.g., Chermack & Giancola, 1997). One explanation for the latter finding is suggested by research showing that when people have consumed alcohol, they become less able to read the emotions on the face of another person and other social cues (Steele & Josephs, 1990; Taylor & Leonard, 1983), a phenomenon Steele and Josephs refer to as alcohol myopia. So in a bar or at a party, a drunk person may, because of alcohol’s disinhibiting effects, become increasingly annoying to another person, but because of their insensitivity to facial and social cues, they are insufficiently aware of how much they are angering that person. Consequently, alcohol consumption not only makes a person a more likely perpetrator of violence, but also a more likely victim of it. Worth keeping in mind!

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