Many of us think that making healthy choices when dining out is a case of mind over matter. If you tell yourself you’re only going to eat half of that personal pizza, you’ll only eat half, right?

Not necessarily, says Julie Downs, Ph.D., an associate research professor of social and decision sciences at Carnegie Mellon University: “It turns out that people are spectacularly bad at knowing when they’ve had enough.”

You might be surprised at how much external forces—such as the size of your meal, what’s on the menu, and even the people you’re with—can affect what you order and the amount you end up eating. Here are five things to be aware of that might inadvertently cause you to overindulge when you’re dining out.

1. Going Out With Friends

Sharing a meal with your buddies can be a great way to blow off steam, but according to research done in the past three decades, dining out in groups could cause you to consume more food than you would otherwise, a phenomenon researchers have dubbed the “social facilitation” effect.

more on dining out

Scientists aren’t exactly sure why people eat more in the presence of others. It typically happens when you’re dining with friends or relatives. Eating with people you don’t know well appears to make some people self-conscious, so they eat less. Some researchers believe that socializing distracts you from realizing that you’re satisfied.

Another theory is that people view meals they eat with others differently from how they view meals they eat alone. Dining out feels more celebratory, and that can lead you to order more food than you’d normally eat—cocktails, appetizers, and dessert. And in a restaurant, you’re more likely to treat yourself because there are so many tempting options, while at home, you’re limited to what you have in your fridge.

The fix: When the food arrives at your table, take a minute to focus on it before digging in. Notice what’s on your plate as well as the portion size, and consider what you really want to eat (vs. what you might reach for just because it’s there) and whether it’s the amount you’d normally eat. It’s okay to indulge occasionally, but if you frequently go out for group lunches or dinners, try to plan ahead by perusing a menu beforehand to come up with a meal game plan.

2. Falling for Misleading Ads

Remember when Subway pitched itself as a sandwich shop that could help you lose weight? Although the chain does have healthier options, those ads were very effective at giving all the dishes on the chain’s menu a healthy reputation, Downs says.

“People tend to think of Subway as healthy relative to other fast food chains, so they end up ordering incredibly high-calorie and high-sodium meals, all while thinking they’re making a healthy choice,” she says. But it’s just as easy to overdo or make less healthy choices at a restaurant that has a “health halo” as it is at any other establishment.

Indeed, a study led by the Palo Alto Medical Foundation Research Institute in California found that teens took in an average of about 1,000 calories and 2,000 mg of sodium at both Subway and McDonald’s. Another study, from Harvard Medical School, found that customers more often underestimated the number of calories they consumed at Subway than they did at other restaurants.

The fix: Ask questions about how your meal is prepared and pay attention to the nutritional information, if available. A study published earlier this year found that diners choose healthier options when calorie counts are printed on menus. And soon, it may be easier to find that information while dining out: Chain restaurants will be required to post calorie counts on menus starting in May.

Until then, take advantage of the information many restaurants already supply. You can often find calories as well as other nutrition information on the menus, on their websites, or in pamphlets you can ask for at the restaruant. That’s good, because “healthy sounding” dishes may not be your best pick. For example, the Frontega Chicken Panini at Panera Bread has 110 more calories (750 vs. 640) than the chain’s Roasted Turkey & Avocado BLT. The chicken option also has 2,050 mg of sodium—the daily recommended maximum is 2,300 mg—compared with 1,060 mg in the BLT. 

3. Cleaning Your Plate

In a recent analysis, researchers in the U.K. evaluated 64 studies on consumption and portion size—and found that when presented with larger amounts of food in packages or on plates, people tend to eat more than when the portions are smaller.

Why? Probably because it’s there, Downs says. When food is in front of them, people tend to eat everything on their plates, even if they don’t plan to.

One research group experimented with using this tendency to people’s nutritional advantage: Scientists in the Netherlands tried replacing some of the meat on restaurantgoers’ plates with more vegetables instead. They found that diners ate less meat and more veggies—a healthier meal overall—and felt just as satisfied.

The fix: Order smaller meals to begin with when dining out. Instead of the unlimited buffet or the entrée-sized fettuccine, try an appetizer and a salad, or a small plate and a soup. Some restaurants offer half orders or appetizer sizes of main courses. When you do get an entrée, ask your waiter to box half your meal before he brings it out to you. Or better yet, share it with a friend.

4. Adding to Your ‘Light’ Meal

Many restaurants have a separate section on the menu, or an entire menu, that offers “lighter” choices to appeal to diners who want a healthier meal. But a study from Babson College in Massachusetts found that the mere presence of healthy dishes on a menu actually caused people to select a less nutritious meal.

Here’s why: “When there are healthy dishes available, people do think about ordering them. And just the fact that they’ve considered those options leads them to feel kind of virtuous—but then they get the french fries,” Downs says. Or, she notes, people may order the salad, but then they’ll overcompensate for their healthier choice by getting dessert.

The fix: Order ahead. In one study, Downs and her colleagues found that choosing a meal beforehand—placing your lunch order just after you’ve eaten breakfast, say—will help you eat fewer calories at that next meal. According to Downs, people may be less swayed by high-calorie options if they’re not hungry when they decide what to have.

If it’s not practical to order ahead, plan ahead. Many establishments provide detailed nutritional information on their websites and apps. If you go into a restaurant armed with some healthy options, you’re less likely to be swayed by the junk.

5. Eating Too Much for Your Size

When you sit down at a restaurant and see that a dinner entrée is labeled as having 800 calories, it’s hard to know if that’s high or low for you. Most people are familiar with the information on the Nutrition Facts label on packaged foods, which is based on a 2,000-calorie diet. So they may assume that’s how many calories they need. But, Downs says, that’s not the right level for everyone.

For example, a somewhat active 120-pound, 5-foot-4-inch, 30-year-old woman needs 1,750 calories per day to maintain her weight. A somewhat active, 175-pound, 6-foot, 55-year-old man should aim for 2,250 calories per day.

The fix: Calculate it out. Figure out how many calories you should eat per day, divide that number by three, and that’s roughly how many calories you should eat per meal. (Of course, some people eat less in the morning and more at night, so adjust that calculation as needed. Don’t forget to allocate some calories to snacks.)

There are online calculators that use your activity level, age, gender, height, and weight to estimate how many calories you need. Know this information before you dine, and if possible, check the menu’s nutritional information online beforehand to find out which meals fall within your range.