You eat what is put in front of you. The more food available, the more you eat.

The sad simple fact is you try to eat as much food as is put in front of you. It’s not just the voice of mom in your head telling you to clean your plate. From a survival point of view this makes perfect sense, from a diet point of view it’s a disaster.

A study by Dr. Brian Wansink at Cornell University found portion sizes are as big a factor in how much you eat as is taste. For example, moviegoers ate 50% more popcorn from a big bucket than when they ate from a smaller bucket. They ate more even when the popcorn was stale and horrible tasting!

Just using a bigger bucket encouraged people to eat more popcorn. And the key is the people didn’t even realize they were eating more. In fact, they thought they would eat the same amount no matter what size container they ate from. People have no idea they unconsciously eat more from large containers and over a whole lifetime this can have a big impact on weight.

Other studies show people don’t even notice when they are given 50% or even 100% more food. This conclusion is backed up by study after study. It doesn’t matter how hungry you are: you eat more food when you are served more food.

Imagine it’s 25,000 years ago and you bagged a deer. You couldn’t store the deer for later, so you had to eat as much as you could. If your body stopped you from eating when food was available, that would be the real problem.

You Eat More from Bigger Packages

Dr. Barbara Rolls, a nutritionist at Penn State College, found in her research that you’ll eat a lot more calories when package sizes are increased.

In a snack study, participants were given a snack of potato chips in an unlabeled opaque package on different days in five different sizes ranging from 1 oz. to 6 oz.

Do you think people will eat more from the larger package of potato chips? If you said yes, why would that be? If you are hungry, shouldn’t you just eat until you stop feeling hungry? If how much you eat depends on the package size, wouldn’t that imply hunger has a lot less with how much you eat than you might think?

It turns out package size does matter. A lot.

Men ate 311 more calories and women ate 184 more calories from the largest bag of chips than they did from the smallest package.

In a similar study using sandwiches instead of potato chips, it was found men ate an extra 355 calories and women ate an extra 159 calories when they ate the largest sandwich compared to the smallest sandwich.

Perhaps it’s not just hunger that determines how much you eat? Perhaps there are many more subtle and deeper mechanisms at work?

Your Stomach Doesn’t Count Calories

Amazing research by Dr. Wansink has shown people use their eyes to count calories, not their stomach. Your stomach doesn’t count calories for you. You need accurate visual cues to prevent unintentional overeating. And because food disappears while you are eating, keeping track of how much you have already eaten can be tricky.

One fascinating study found that people would eat 73% more soup when eating from a self-refilling soup bowl. In this clever study, people ate soup, but the soup level didn’t go down in the bowl. The bowl kept refilling! As the people ate they couldn’t tell how much soup they had eaten, so they just kept on eating and eating.

Without the visual evidence of the quantity of food you are eating, you will eat a tremendous amount more than you would otherwise. Your stomach won’t tell you to stop eating until after you have overeaten.

Eating What You See is Called Unit Bias

Andrew Geier, of the University of Pennsylvania, performed a number of experiments that all confirmed the same basic idea: you eat what you view as a unit, independent of the actual size or number of servings.

“Whatever size a banana is, that’s what you eat, a small banana or a big banana,” says Geier. And “whatever is served on your plate, it just seems locked in our heads: that’s a meal.”

“In terms of food, unit bias applies to what people think is the appropriate amount to consume, and it shows why smaller portion sizes can be just as satisfying,” said Geier. “A 12-ounce can of soda and a 24-ounce bottle are both seen as single units. But be careful, the 24-ounce bottle, though viewed as one unit, is actually more than two and a half servings of soda.”

In one experiment they put a large bowl with a pound of M&Ms® in the lobby of an upscale apartment building. Below the bowl hung the sign: “Eat Your Fill...please use the spoon to serve yourself.” Sometimes the bowl had a small serving spoon and other times it had a much larger serving spoon.

And just like we’ve seen with Dr, Roll’s and Dr. Wansink’s research, people consistently took a lot more M&Ms when using the larger spoon.

The unit bias doesn’t always work. If people are served what is considered less than the normal serving size, they will go back for more.

Are you more satisfied with more food?

Not really. Rolls performed an experiment in a restaurant where on one day they served a standard size portion of pasta and on the next day they served a larger portion with 50% more pasta. They kept the entire meal identical, including the price.

What was the result? People ate nearly all the larger portion and they ate more of the side dishes as well. All-in-all people ate 172 more calories when eating the larger meal than when eating the small portion. You’ll gain 17 extra pounds a year if you eat the larger portion size every day, for just one meal!

Survey responses show that the diners thought both the small and larger portions were appropriate. So they didn’t feel cheated by either meal.

That’s OK, you might say, but in these studies people ate more food yet they didn’t report feeling more satisfied. People didn’t even notice when they were served more food.

Surprisingly, you will feel just as satisfied with smaller amounts of food.

The conclusion: eating more doesn’t make you happier.