Many drinkers seriously underestimate the amount of alcohol they consume:
[R]esearchers surveyed over 40,000 people with standard alcohol survey questions about their quantity and frequency of alcohol consumption — “How many drinks have you had in the past month?” and so on. But in a smart twist, they then asked a more immediate question: “How many drinks did you have yesterday?” This method is useful for detecting under-reporting because of the improbabilities it reveals. For example, if 50 percent of people who say they drink once a month acknowledge drinking yesterday, one can infer that this group is severely under-reporting their consumption: If they were truly once-a-month drinkers, only about 3 percent should acknowledge drinking on a particular randomly selected day of the month.
Men and women were comparably good (or bad, depending on your perspective) at accurately reporting their drinking. But as the chart above shows, a large difference emerged when types of drinkers were compared. Putatively low-risk drinkers grossly under-reported, acknowledging only about one in four of their actual drinks consumed. The heaviest drinkers actually recalled their consumption most accurately, but in absolute terms they still only reported about half of it.