Are some people more likely to blackout than others?
       Based on anecdotal evidence, it seems that some people are more prone to experience blackouts than others.  I frequently hear stories about individuals that either a) never experience blackouts, regardless of how much they drink, or b) frequently experience blackouts, even at relatively low levels of intoxication.  Unfortunately, no matter how compelling anecdotes can be, they do not make good evidence.  Is there any scientific evidence, beyond the factors we have already discussed (e.g., body weight, gender, etc), to suggest that some people are more prone to blackouts than others?  Yes, but not much.  In Goodwin's classic studies of hospitalized alcoholics (Goodwin et al., 1969a;b), 36 out of the 100 patients he interviewed indicated that they had never experienced a blackout.  In some ways, these individuals are every bit as interesting as the patients that did experience blackouts.  What was it about these 36 patients that kept them from blacking out, despite the fact that their alcoholism was so severe that it required hospitalization?  One very real possibility is that they actually had experienced blackouts, but were simply unaware of them.  However, another possibility is that there was something fundamentally different about these individuals that diminished their likelihood of experiencing memory impairments while drinking.  
       In support of this possibility, a recent study by Hartzler and Fromme (2003) suggests that individuals with a history of blackouts are more vulnerable to the effects of alcohol on memory than those without a history of blackouts.  The authors recruited 108 college students, half of whom had experienced at least one fragmentary blackout in the previous year and half of whom had not.  While they were sober, the two groups performed comparably in memory tasks.  However, when they were mildly intoxicated (0.08% BAC), those with a history of fragmentary blackouts performed worse than those without such a history.  There are two possible interpretations for these data, both of which support the hypothesis that some individuals are more susceptible to blackouts than others.  One plausible interpretation is that subjects in the fragmentary blackout group have always been more vulnerable to alcohol-induced memory impairments, which is why they performed more poorly during testing under alcohol, and why they are members of the blackout group in the first place!  A second interpretation is that subjects in the blackout group performed more poorly during testing as a result of drinking enough to experience alcohol-induced memory impairments in the past.  This latter possibility seems unlikely, given that previous research has failed to find differences in cognitive performance of hospitalized alcoholics with and without a history of blackouts (Tarter and Schneider, 1976).  Thus, in my opinion, the most logical explanation is that subjects in the blackout group were always more susceptible to alcohol-induced memory impairments, which is at least part of the reason they were members of the blackout group to begin with.
      The potential for an inherent vulnerability to alcohol-induced memory impairments, including blackouts, is strengthened by two other recent studies.  In a very impressive longitudinal study, Baer et al. (2003) examined the drinking habits of pregnant women in 1974-1975, and then studied alcohol use and related problems in the offspring at seven different time points during the following 21 years.  One observation was that prenatal alcohol exposure was associated with increased rates of experiencing alcohol-related consequences, including blackouts, even after controlling for general drinking habits.  In addition, a report by Nelson and colleagues (2004) suggests that there might actually be a genetic contribution to susceptibility to blackouts, strongly suggesting that some individuals are simply built in a way that makes them more vulnerable to alcohol-induced amnesia (click here for the abstract to the manuscript).  
       A subsequent section reviews the brain mechanisms underlying memory formation.  It is easy to imagine that variability could exist in the impact of alcohol on this circuitry, rendering an individual more or less sensitive to the memory-impairing effects of the drug.