Science has already reassured us that throwing back a glass of cabernet or shiraz can actually help boost our health. Various studies have shown that people who drink one glass of red wine per day tend to live longer and have fewer heart problems than those who drink more or less.

But a September 2013 study by researchers at Iowa State and Cornell universities found that limiting your consumption to that magical one glass is even harder than you think. The study, published in the journal Substance Use and Misuse, found that specific environmental factors such as glass size and color cause wine drinkers to unintentionally overpour, causing us to drink much more than we intended to.

One serving of wine is 5 ounces according to the National Institute on Alcohol Abuse and Alcoholism, but unlike other beverages that come in single-serving bottles or shot glasses, it’s almost impossible to pour an exact amount. And the study found that each person’s perception of “one normal glass” varies, and is even further skewed by glass size, color and location.

Study subjects were asked to pour what they would consider a normal-sized drink in a few different glasses and in different settings. The participants ended up pouring 12 percent more when the glass was wider and when they held the glass instead of setting it on the table. They also over poured (9 percent more) when there was less of a contrast in color between the glass and the wine (i.e. when pouring white wine into a clear glass).

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“People have trouble assessing volumes,” said Laura Smarandescu, an assistant professor of marketing at Iowa State and co-author of the study. “They tend to focus more on the vertical than the horizontal measures. That’s why people tend to drink less when they drink from a narrow glass, because they think they’re drinking more.” It also explains why it’s easier to over pour wine that sharply contrasts in color from the glass.

So how can we trick our eyes into seeing the full depth of our pours? “If you want to pour and drink less wine, stick to the white wine glasses and only pour if your glass is on the table or counter and not in your hand—in either case you’ll pour about 9-12 percent less,” says Brian Wansink, director of the Food and Brand Lab at Cornell and co-author of this study.

It’s not realistic to try to eliminate this disconnect in our perception, so the only way to get a 100 percent accurate reading on how much your imbibing might be whipping out a measuring cup. But if you’d rather save yourself the social faux pas when you’ve got company, being conscious of the factors that trip up your judgment can help you adjust a little more next time.

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