Selection bias, that is chosing who is to be studied, has a lot to do with research outcomes.
An analysis from 2007 by an international group of alcohol epidemiologists and addiction researchers, published in Annals of Epidemiology, notes that “as people progress into late middle and old age, their consumption of alcohol declines in tandem with ill health, frailty, dementia, and/or use of medications.” That decline means that, as people become less well—even if they’re not elderly—they will also tend to stop drinking. So when they enroll in a study on drinking and get lumped into the group of non-drinkers, they’ll artificially inflate the mortality risk—even though their deaths have nothing to do with alcohol abstention. It’s not that teetotaling made them more likely to die during the course of the study; it’s that being closer to death made them more likely to quit alcohol. And people who drink, but are able to keep their drinking at a reasonable level, are likely to have the kind of health and social advantages associated with living longer.
“People who in their teens or twenties begin to drink, don’t die or become alcoholics, and are able to maintain drinking at low levels—that’s a select group of drinkers,” says Naimi. And that means when epidemiologists go looking at people who are still moderate drinkers in middle age, they’re missing some key people. “If you look at people in midlife who are stable, moderate drinkers, that means they didn’t die from their drinking and they didn’t quit drinking.” That sounds obvious, but let’s take a second to think about it: Alcohol is an addictive substance. People who have had a healthy relationship with it their entire lives are probably people who also have a bunch of other advantageous qualities.
There is still a chance that one or two drinks a day has some benefit for your heart. If it does exist, researchers like Naimi think it’s probably pretty small, and that matters because there are a lot of other diseases that we know alcohol increases your risk of contracting. These include liver cancer, breast cancer, colon cancer, oral cancer, tuberculosis, pancreatitis, cirrhosis and related liver diseases, and a whole host of others.