An innovative study involving more than three thousand adult twins from Australia investigated the contributions of genetic and environmental factors to blacking and passing out after drinking. Twins are a vital resource for health research, including alcohol studies. By comparing the frequency of an outcome ─such as blackout ─among identical twins (who share a commonality of all genes) and fraternal twins (who share only half of their genes but similar environmental aspects), researchers can better understand the roles of genetic and environmental factors in influencing the outcome.

The participants in this study were between 27 and 40 years old. All 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 essential part of the statistical evaluation was to control for how often participants became intoxicated – while the frequency of heavy drinking would affect the likelihood of blacking and passing out, the researchers wanted to look beyond individual differences in drinking habits to understand susceptibility.

Fifty-three percent of the participants reported having had at least one alcohol-induced blackout in their life; a similar proportion (56%) related passing out from alcohol. Although blacking out and passing out are strongly associated with each other. People who experience blackouts are 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 influential regarding the tendency to pass out. Here, environmental influences ─particularly ‘nonshared’ environmental factors that can differ between twins ─have a more significant role, especially in women. These influences might include social and cultural norms around alcohol use, the drinking context, and the type of alcohol consumed.

The Australian study is one of very few to investigate the contribution of genetic and environmental factors to people’s liability for alcohol-induced blacking and passing out. The results suggest that interventions should target possible harmful environmental influences. Women, particularly, are advised to avoid drinking game participation, mixing drinks and taking shots. The research results will aid future investigations and may help to reduce the negative consequences of heavy drinking