Portion distortion: We tend to think of everything on our plates as a single portion.

Could you tell how many calories were in the breakfast you had today? Probably not, but that's normal, research presented at a previous American Psychological Association. But not knowing how many calories we eat may be a big reason that obesity rates are risk, experts say.

In the study, college students were shown three portions—small, medium (two times the size of "small"), and large (three times the size of "smal"”)—of six junk foods (a Snickers bar, a cookie, Skittles candies, Red Vines licorice, a brownie, and peanuts) and asked to estimate the number of calories in each portion. The study authors write: "As hypothesized, the participants were insensitive to sample size. On average, as the portion size tripled, participants’ calorie estimates increased by only about 50 percent." When it came to the peanuts, the students were off even more: They gave roughly the same calorie estimate for all three portions.

Even if we know overeating is unhealthy, the human brain doesn't seem wired to calculate calories or portion sizes. "Research has shown unequivocally that people eat the entirety of what they’re served and consider what they’re served a single portion, no matter how large what they’re served happens to be," says Yale University psychologist Andrew Geier, PhD, an expert on portion control. What's more, he says, the abundance of food that most of us have access to leads us to focus more on value—how much we’re getting for our money—than simply how much we’re getting, period. "We’re so focused on value in America that we really have to make a point of reteaching ourselves that personal health, as a priority, trumps value," says portion-control expert Andrew Geier, PhD. "Besides, start to lose your health, and you’ll quickly find out how expensive that is."

But you can change all that: The trick is not only to control what you’re served, but to know exactly how much you should be served in the first place.

Here are five ways to identify a healthy portion and avoid consuming too many calories:

•Check labels. The portions-per-package info on a food label is the most important number on it, insists Geier, who is currently lobbying to have that value emphasized. Until that happens, however, emphasize it in your own mind by checking portion information on the food you buy.

• Learn familiar portion sizes. You have to know what a single serving looks like before you can be expected to recognize it—or estimate the number of calories you’re taking in—on your plate. A mini primer: According to the National Heart, Lung, and Blood Institute, a single serving of meat or fish is 3 ounces—or, the size of a deck of cards. A single serving of cheese is 1.5 ounces—or the size of four playing dice. And a single serving of ice cream or frozen yogurt is ½ cup—or, the size of half a tennis ball.

• Buy in portion-sized bags. Don’t trust yourself to eat just one portion? Let food companies define your limits for you. “Some companies have started packaging high-calorie foods like chips and cookies in 100-calorie bags,” says Geier. “It is more expensive to buy that way, but a behavioral economist will show that it’s really cheaper in the long run because of the epidemiological public health outcome of eating less and maintaining our health.” Just remember to check the label to make sure it really is 100 calories per bag. And when it comes to time for a snack, pull out a serving, and put the rest out of reach so you won't keep snacking.

• Create your own portion-sized bags. When you buy an econo-size bag of chips, for instance, the first thing you should do, says Geier, is read how many portions the bag contains then separate the chips into that many sandwich-size bags. “Research shows that even this relatively minor environmental cue will have a huge effect on your caloric consumption,” says Geier.

• Set your own portions in restaurants. “I used to advise people, ‘When you’re served your dinner, ask the waiter for a to-go container and immediately put half of your meal into the bag,’” says Geier. “But I’ve found that people felt cheated when they do that. So now I tell them to ask the waiter to put half of it in a to-go container immediately, in the kitchen, and just bring half the meal to the table. You never see the entire portion, and that makes a big difference.”