Before ordering lunch, diners at McDonald’s restaurants in Brooklyn and Manhattan were given slips of paper containing either their daily recommended calorie counts or per-meal recommended calorie counts, while a control group received no recommended calorie information. After analyzing the meal choices of over 1,000 people, researchers found that individuals who received information about daily or per-meal calories tended to make the same lunch choices as those who did not get said calorie info.  In fact, about a third of the people purchased fare clocking in at 1,000 calories or more, well beyond what anyone (except maybe Michael Phelps) needs for one meal. “It could be that people don’t want to change their behavior,” says lead study author, Julie Downs, Ph.D., associate research professor of social and decision sciences at Carnegie Mellon University. When you hit a fast food joint, you might’ve already decided that you’re splurging, and seeing the hard numbers of how a cheeseburger and fries will impact your diet for the day doesn’t make a difference.  Plus, we’re really bad at estimating how much we’re actually eating. Experts already know that we tend to underestimate our actual calorie intake—so even if you order the high-cal combo meal, you might start noshing with the intention to only eat half. Then there’s the problem of so-called extras: At around 500 calories, even a cheeseburger by itself doesn’t seem awful. But then we forget to factor in the soda or handful of fries we picked off our friend’s tray, Downs says, and that leads to trouble.  To encourage healthier choices, Downs and her colleagues suggest that restaurants incentivize healthier menu options, like offering a discount for ordering diet soda or water. For now, though, we’re on our own—and mindfulness might be the best defense against that we’ve got. “Always be wary of little things adding up,” says Downs. “Those little extras—like adding a drink and fries to a sandwich that seemed like it had a reasonable number of calories—may be what pushes people up to high.”  Cheeseburger and a side of nothing, it is.  More from Prevention: How to Prevent Overeating at Restaurants