Many of these studies purporting to show a benefit from drinking included a sizable sample of people in the non-drinking group who had stopped for health reason

Many of these studies purporting to show a benefit from drinking included a sizable sample of people in the non-drinking group who had stopped for health reasons. In other words, they were probably told at some time in the past that they'd destroy their liver if they kept it up, indicating some level of damage to the body. In a society where some level of drinking alcohol is the norm, the non-drinking group has an overrepresentation of these people which skews the results.