Blackout or pass-out? What twins tell us about sensitivity to alcohol

Newswise — A new study involving more than three thousand adult twins from Australia has investigated the contributions of genetic and environmental factors to blacking and passing out after drinking. Twins are an important resource for health research, including alcohol studies. By comparing the frequency of an outcome ─such as blackout ─among pairs of identical twins (who share all their genes) and non-identical twins (who share only half of their genes but many aspects of their environment), researchers can better understand the roles of genetic and enviromental factors in influencing the outcome.

The participants in this study, all aged between 27 and 40 years, had completed a computer-assisted telephone interview, answering questions on a variety of drinking behaviors and consequences – including their experience of blackouts, passing out, and intoxication. An important part of the statistical analysis was to control for how often participants became intoxicated – while the frequency of heavy drinking would clearly affect the likelihood of blacking and passing out, the researchers wanted to look beyond individual differences in drinking habits in order to understand susceptibility.

Just over half the participants (53%) reported having had at least one alcohol-induced blackout in their life, and a similar proportion (56%) had passed out from alcohol. Although blacking out and passing out were strongly associated with each other ─i.e., people who had experienced blackout were more likely than others to have passed out after drinking, and vice versa ─their causes were different. The findings showed that after accounting for frequency of intoxication, susceptibility to blackout among men and women depends in part on the individual’s genetic make-up. However, genetic factors are less important in susceptibility to passing out. Here, environmental influences ─particularly ‘nonshared’ environmental factors that can differ between twins ─have a greater role, especially in women. These influences might include social and cultural norms around alcohol use, the drinking context, and the type of alcohol consumed.

This is one of very few studies to have investigated the contribution of genetic and environmental factors to people’s liability for alcohol-induced blacking and passing out. The findings will help guide future research, and suggest that interventions to target potentially harmful environmental influences for women ─such as avoiding drinking game participation, not mixing drinks, and not taking shots ─may help to reduce the negative consequences of heavy drinking.