We now also know more about other factors that influence blackouts, such as drinking on an empty stomach or when sleep deprived. Another major risk has to do with how fast alcohol is consumed, as the quicker we gulp the faster our blood alcohol level spikes. A blood alcohol level of between 0.20 to 0.30 percent seems to be able to induce a total blackout, where nothing is remembered. That’s about the equivalent of 10-15 units of alcohol in four hours, depending on body weight.
But blood alcohol levels do not explain why only some people lose whole chunks of their memory while others who drink similar amounts don’t. A 2016 study led by Ralph Hingson, also of the National Institute on Alcohol Abuse and Alcoholism, provided some answers.
“The frequency with which people reported bingeing and being drunk in the past month played a role, as did whether they smoke and took more than one psychoactive drug,” he says.
Blackouts are more common in people with lower body weights. They’re also more common among university students, who are known to ‘pre-game’ to get “a buzz on before they start to socialise, and that makes your blood alcohol level rise fast”, says Hingson.
Across the board there seems to be inherent brain vulnerabilities, and genetic vulnerabilities, that put a person at risk
Women also experience blackouts more often. They are smaller on average than men and have a higher percentage of body fat, which means their bodies have less water to dilute the alcohol they drink – so their blood alcohol level rises faster. In 2017, Amie Haas of Palo Alto University in California found that women would routinely black out with three fewer drinks than men. A 2015 study showed that women who consumed only one more drink than their usual amount had a 13% higher chance of blacking out than men.
Aside from the sex differences, there could be a genetic component to who is more likely to black out. Individuals whose mothers had a history of alcohol problems were found to be more at risk. Another study, this time on more than 1,000 pairs of twins, found that a genetic link accounted for half the blackouts experienced.
The genetic difference seems to play out in the brain, too. One longitudinal study of adolescents aged 12-21, led by Reagan Wetherill of the University of Pennsylvania, showed that certain individuals who later went on to abuse alcohol and experience blackouts, were less able to suppress their actions. This could be seen on brain scans, even before they were drinking alcohol.
“Across the board there seems to be inherent brain vulnerabilities, and genetic vulnerabilities, that put a person at risk,” she says.
Worse, studies on mice suggest that heavy drinking may even lead to additional changes in the brain. Equally worrying is that the same people who are more prone to blackouts – teenagers and university students – are at a physically more vulnerable age. “There’s growing evidence that particularly if you are younger, it’s really quite unsafe for a developing brain,” says Haas. That’s because adolescents are more sensitive to the effects of alcohol compared to adults. One reason for this is that the frontal lobe of the brain is the last to develop, at around 25.
Like the risk factors, the consequences of blacking out are not only worse for adolescents, but also for women.
Haas and colleagues showed that women who experienced blackouts were more likely to engage in risky sexual behaviour while in a blackout state, compared to men and to drinkers who didn’t black out at all. These women also showed more regret the following day.
You have to rely on a preponderance of evidence to determine whether or not consent was given
Evidence also shows that women with a history of sexual assault are more likely to be re-victimised if they are in an alcohol-induced blackout – compared to binge-drinkers who didn’t blackout. This is because they are risk while they under the influence due to impaired decision making, especially when it comes to assessing potentially dangerous situations, but they are also at risk afterwards because they cannot rely on their memory of what happened.
This means there is a catch-22. Those experiencing blackouts may be more vulnerable to potential perpetrators in the moment. But if they try to press charges after, they also are vulnerable to having their cases dismissed.
That is true even in places with an “affirmative consent” standard, where unless someone has indicated their willingness, it’s assault. “If it’s ‘he said/she said’, you have to rely on a preponderance of evidence to determine whether or not consent was given,” says Wetherill.
One party being blackout complicates that evidence. Take Canada, where affirmative consent is necessary. As a Globe & Mail investigation recently found, the courts tend to want a complainant who is so drunk she probably has blacked out at least partially – but at the same time, a complainant who has blacked out isn’t seen as a reliable source of information about what happened.
It’s tricky because people can black out and look quite sober
In the US, meanwhile, laws vary by state. Most say that someone who is “mentally incapacitated” cannot give consent. But New York, for example, says mental incapacitation can legally result only from involuntarily being given a drink or drug, not from having chosen to drink.
States that include voluntary drinking, on the other hand, usually include the caveat that the accused must ‘reasonably’ have realised the person was incapacitated. But since people who have blacked out can seem highly functioning, the accused can argue that they didn’t realise.
“It’s tricky because people can black out and look quite sober,” says White. “You don’t always have to [appear] severely intoxicated to blackout.”