Commentary on McCarthy et al. (2012): Understanding how alcohol affects decisions to drive while intoxicated—the need to deconstruct impulsivity


McCarthy et al. [1] conducted a laboratory study to determine if individuals who drink and drive might also become more impulsive in response to alcohol compared with those with no history of drinking and driving. College students were surveyed about their history of driving after consuming alcohol, and the authors classified those who reported driving within 2 hours of having three or more drinks as ‘drink drivers’. The authors reported that these drink drivers became more impulsive in response to a moderate dose of alcohol compared with a control group with no history of drinking and driving.

McCarthy et al. address a significant social issue. The public health costs associated with alcohol-related traffic accidents have prompted considerable research aimed at identifying characteristics of driving under the influence (DUI) offenders in efforts to improve treatment and prevention. The vast majority of this research has relied on findings from surveys and personality inventories. Indeed, personality inventories reliably show high levels of impulsivity and sensation-seeking among DUI offenders [2-4]. However, self-reported traits of impulsivity provide limited insight into the specific behavioral and cognitive mechanisms that contribute to decisions to drive after drinking. McCarthy et al. take advantage of advancements in the assessment of specific behavioral mechanisms involved in impulsive behavior by employing behavioral tasks that measure two specific components of impulsivity: (i) poor inhibitory control over pre-potent actions and (ii) inability to delay reward. McCarthy et al. demonstrate a potentially important dissociation between these two aspects of impulsivity. Compared to control drivers, drink drivers were more sensitive to the impairing effects of alcohol on the ability to delay reward, but not on the ability to inhibit pre-potent action.

The authors offer a reasonable interpretation for the finding by suggesting that impaired ability to delay immediate reward could contribute to the drinker's impulsive decisions to drive after drinking. Deciding to drive after drinking yields the immediate reward of convenience for the drinker, to travel home, to the next bar or elsewhere, and the decision to drive after drinking could reflect a failure to delay or forego that immediate reward. As such, McCarthy et al. have taken an important initial step towards deconstructing the specific components of impulsivity towards the goal of understanding more clearly its influence on decisions to drive after drinking.

Another important contribution of the study is that McCarthy et al. recognize the importance of examining impulsivity of the driver in the intoxicated state. Factors affecting decisions to drive after drinking have traditionally been examined by self-reports of sober subjects on the likelihood of driving in hypothetical situations (e.g. ‘if I had three drinks and my drive home was less than 1 mile, I would drive’). Such studies highlight the importance of situational factors on a sober decision-making process. However, they provide no indication of how such decisions might be altered by acute alcohol intoxication. McCarthy et al. show that only under alcohol do drink drivers display more impulsive behavior than control drivers. This suggests that certain groups of drivers might have a heightened sensitivity to the disruptive effects of alcohol. Indeed, we have shown that certain groups [e.g. reckless drivers and drivers with attention deficit hyperactive disorder (ADHD)] show greater sensitivity to the disruptive effects of alcohol on their driving performance and report greater willingness to drive while intoxicated [5-7]. Taken together, these findings highlight the importance of assessing how at-risk drivers respond to alcohol, as such groups might demonstrate heightened sensitivity to the behaviorally disruptive effects of the drug at a time when these individuals are confronted with decisions about whether or not to drive.

Finally, given that McCarthy et al. focus on impulsive behaviors in the intoxicated state, it was surprising that the authors did not report the blood alcohol concentrations (BACs) of the subjects. BACs were measured and the authors reported no group differences in BACs. However, information on the actual BACs of the subjects is critically important for understanding how impulsivity changes in response to alcohol. Although impairing effects of alcohol are typically BAC-dependent, the point of the authors' study is that certain groups of drivers might have a heightened sensitivity to the disruptive effects of alcohol. A heightened sensitivity implies a lower ‘threshold’ BAC for the onset of impairment, a BAC that actually might be below the legal limit for driving. This might be especially relevant in countries with higher legal limits for driving, such as Canada and the United States, where the legal limit is 0.08%. Indeed, laboratory studies of the general population find that many behavioral functions are reliably impaired by alcohol at BACs below 0.08% [8]. Thus, it seems likely that at-risk populations, such as those studied by McCarthy et al., might have heightened sensitivities to alcohol that result in pronounced impairment, even at BACs below 0.08%. As such, identifying the BAC-dependence of alcohol effects on impulsive decision-making in these drivers is important in understanding when they are at increased risk for driving after drinking.

Declarations of interest