There is a standard serving size for a glass of wine in the U.S.: 5 ounces. Not that anyone is counting. One person’s glass of wine is rarely the same as another’s, be it a waiter’s stingy pour at a fancy restaurant or the super generous portions of a hostess trying to get a dinner party kicked into high gear.

But even when tipplers are trying to be consistent, they rarely are. Researchers have shed some light on why people tend to pour — let’s be honest — more wine than they think they are, and under what circumstances. “If you ask someone how much they drink and they report it in a number of servings, for a self-pour that’s just not telling the whole story,” says lead author Doug Walker, an assistant professor of marketing at Ohio State University. “One person’s two is totally different than another person’s two.”

The researchers looked at several different factors, but the three biggest ended up being wine color, glass size, and position of the glass. The most intuitive finding is glass size: A wide wineglass receives 11.9 percent more wine than a normal-size goblet. “People have trouble assessing volumes,” says Iowa State’s Laura Smarandescu. “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.”

The fact that people pour more generous portions of white wines than reds makes some sense, too, when you think about it. And no, it’s not just red wines tend to be valued more than whites — though dinner hosts probably are more generous with that California chardonnay than with the Brunello di Montalcino. It’s that people can’t really see how much white wine they’re pouring into a clear glass, so they pour 9.2 percent more, the researchers found.

The third criteria is the biggest, and it’s a puzzler: People tended to pour 12.2 percent more into a chalice when the recipient is holding the glass, versus when the glass is on a table.