What Really Determines What We Eat

The Hidden Truth

Most of us believe that how much we eat is mainly determined by how hungry we are, how much we like the food, or what mood we are in at the time of eating or food selection. While we might acknowledge that others could be influenced by colorful food packages, special lighting, or the size of the plate, we all think we are too smart to be swayed by such gimmicks. But that’s where we’re wrong.

Every single one of us eats what and how much we eat not because of hunger but largely because of what is around us. We eat the way we do because of family and friends, packages and plates, names and numbers, labels and lights, colors and candles, shapes and smells, distractions and distances, cupboards and containers. Dozens of studies confirm this, yet most of us remain unaware of the power of these influences.


That is what makes them so dangerous. We are almost never aware of why we’re eating particular foods or eating them in larger amounts than we may really desire. But by knowing what to look out for, we can head off overeating before we start. This article explores some of the hidden influences that can trip up anyone’s eating habits.

Size matters

The size of a serving bowl can’t possibly influence how much an intelligent, informed person eats, can it? A study conducted at the University of Illinois suggests that it can.

A group of graduate students in a very competitive MBA program at the University of Illinois were brought together and lectured for more than an hour about how bigger bowls could cause them to eat more than they otherwise would. Six weeks later, 40 of them were invited to a Super Bowl party at a sports bar. While they knew they were there as part of a study, they thought the study was related to commercials broadcast during the Super Bowl.

When the students entered the party locale, they were divided equally into two separate rooms. In one room, the group was presented with two four-quart bowls of Chex Mix and offered a snack. Each person was given a plate and told to take as much as he wanted. Each plate was then weighed on a hidden scale, and the student continued on to the Super Bowl party.

In the other room, the same amount of Chex Mix was served in four two-quart bowls. The same thing happened: The students served themselves snacks, and the snack plates were weighed without their knowledge.

The bowls of food were then removed so that no one could take a second helping, and an hour later the researchers collected the plates to see how much had been eaten. They found that the people who had served themselves out of the four-quart bowls took 55% more Chex Mix than those who served themselves out of the two-quart bowls.

However, when the students were informed of this, most did not believe it. A typical response was along the lines of, “Not me. Maybe it happens to other people, but certainly not me.” A write-up of this study was published in The Journal of the American Medical Association in April 2005.

While eating a big portion of Chex Mix may be understandable since it tastes so good, it’s harder to believe that a big container would lead people to eat more of a food they didn’t like, but it does. This was demonstrated in a study conducted in a movie theater in a suburb of Chicago. A group of people who had come to watch the movie Payback, starring Mel Gibson, were offered free popcorn and a free drink if they would agree to answer some questions about the concessions and about the movie after the movie ended. As a pretext for the study, the researchers told the subjects, “It is Illinois History Month and so, because of this fact, we are raising awareness.”

Each person was given either a large bucket of popcorn, weighing about 120 grams, or an extra-large bucket, weighing about twice that much. Half the people were given fresh popcorn that had just been popped that day, and the other half were given popcorn that was 14 days old and, by most standards, tasted terrible.

The researchers’ plan was to ask subjects after the movie how many calories they thought they had eaten and to weigh their remaining popcorn. The thought was that if people only overeat because food tastes good and because they are hungry, the people given the stale popcorn shouldn’t have eaten any of it, both because it didn’t taste good and because the study was done right after dinner, so they shouldn’t have been very hungry.

Here’s what happened: People given fresh popcorn ate 45% more from the extra-large containers than from the large ones. That’s about 190 calories more. But again, if you told them, hey you ate about 45% more than a person with a medium-size container, what would they say? “No I didn’t.”

Even when the popcorn was stale, people ate 34% more from the bigger bucket. When asked what they thought about the popcorn, the typical reply was, “Oh God, I can’t get the taste out of my mouth. Why did you do that to me?” But although they hated the popcorn, they still ate it, and those with bigger buckets ate more.

This experiment demonstrates that when people start eating, they really don’t have an idea of how much they should eat. No one thinks, “I could really use about 112 grams of popcorn today.” Nor do most people notice that they are less hungry than they want to be or more full than they desire. Without those internal bodily cues, people end up using external cues, such as the size of the serving container, to determine how much to eat. And on some level, it seems more normal to take a large serving from a large container than to take a small serving from a large container.

Shape matters, too

The developmental psychologist Jean Piaget described the concept called “conservation of volume.” Essentially, it says that up until a certain developmental stage, young children think that a tall, erect, narrow dish contains more liquid than a flat dish with equal capacity. It is only when children develop cognitively that they can understand that the same amount of water can appear differently in different-shaped vessels.

But understanding that concept and being able to “eyeball” serving sizes in different-size glasses are two different things. And both children and adults can be taken in by optical illusions. When looking at an upside down “T,” for example, most people estimate that the vertical line is longer than the horizontal line by about 20% even when in reality, the two lines are the same length. For whatever reason, people tend to focus on vertical dimensions more than horizontal dimensions and to underestimate horizontal dimensions. This tendency affects people’s perceptions of how much liquid drinking glasses of different shapes hold. Typically, people think that long skinny glasses hold more than short wide glasses.

A study conducted with 133 teens at a nutrition and fitness camp in New Hampshire confirmed the difficulty most people have in correctly estimating how much a short wide glass holds. During breakfast, as campers came through the cafeteria line, each was given either a really tall 22-ounce glass or a very short and wide 22-ounce glass. The campers then got their breakfast and their juice or soda. As they exited the line, the researchers asked them how many ounces of juice or soda they thought they had poured, and their answers were recorded. The researchers then weighed how much they had actually poured.

Because people typically pay more attention to height than width, the researchers expected that campers would pour a lot more liquid into the short wide glasses than the tall skinny glasses, and that is exactly what happened. They poured 88% more into short wide glasses than tall skinny glasses, or about 10 ounces versus about 5.5 ounces. However, they all believed they poured less into the shorter glasses. A typical comment was “I am following my diet well. A little bit more, but not that high.” And these were teens who had been educated about portion sizes, portion control, and calorie estimation.

A similar experiment was done with Philadelphia bartenders as the subjects. Since bartenders pour a tremendous number of drinks every year, they might be expected to be better at estimating amounts of liquids. To test this out, the bartenders were given either tall, slender highball glasses that held about 10.5 ounces or short, wide tumblers that held 10.5 ounces. They were then asked to pour the amount of liquor they’d normally pour into the glasses for a variety of mixed drinks. The result was that even though these people pour all the time, they fell into the same trap as the teenagers in New Hampshire. Bartenders with less than five years of experience poured about 32% more than they should have (2.2 ounces instead of 1.5) into the short tumblers. Those with at least five years of experience were a little more accurate in their estimates.

If even professional pourers are unaware that they pour more into wide glasses, is there any hope for the ordinary person who would like to limit portion sizes of beverages? Yes, there is: Use tall glasses and get rid of your short wide ones. It’s that simple, and the simpler the solution, the more likely it will work.

Your stomach is unreliable

When a group of people were asked the following question, “When you are eating soup, at what point to do you stop eating?” 81% of them said, “I stop eating when the bowl is empty.” Some of them said they stop when the bowl is half empty. Only 19% of them said they stop when they are full.

Like many people, the people who stopped eating when the bowl was half empty were using a visual cue to determine when they were full (or had eaten enough). But what happens if your bowl never empties?

In a study published in the journal Obesity Research in 2005, 60 adults were offered a free bowl of soup. Half of them got normal, 22-ounce bowls, and half got specially designed, bottomless bowls that automatically refilled themselves as soup was removed. However, the refilling was done so slowly it was imperceptible. Theoretically, a person could eat six quarts of soup without realizing that the bowl was being constantly refilled.

The researchers found that bottomless bowls lead to bottomless appetites. All the participants ate for 20 minutes before the researchers stopped them. In those 20 minutes, the people with the bottomless bowls ate 73% more soup than the people with normal bowls, some of whom had them refilled by hand. However, the people with the bottomless bowls did not think they ate more. When asked how many ounces they thought they ate, they said about 9 ounces. When asked how full they were, they didn’t rate themselves as being fuller than their counterparts with normal bowls.

These results suggest that people use their eyes more than their stomachs to monitor how much they eat and how full they feel. Relying on bodily sensations alone, therefore, may not tell us when we have eaten enough.

Avoiding the traps

Homes and workplaces are filled with hidden eating traps that we unknowingly set on our own. To avoid mindless eating, therefore, we must learn to dismantle those traps.

Studies in which secretaries are provided with daily chocolate suggest that simply placing food farther away helps people eat less of it. In one version of the study, researchers placed 30 chocolates on some secretaries’ desks. Every night after they left, the researchers would count how many chocolates they ate and refill the chocolate dish. Other secretaries in the study were also given 30 chocolates, but the candy was placed 6 feet away from their desks. Again, every night after they had gone, the researchers counted how many chocolates had been eaten and refilled the dish.

What happened was that when the chocolate was sitting right on the desk, the typical person ate about 9 chocolates a day. When the chocolate was placed 6 feet away from the desk, the typical person would eat only 4.5 chocolates per day. We expected people to say, when asked why they ate fewer chocolates than the people who have the chocolate right on the desk, that the little extra work to walk to the chocolate was not worth it for them. But hardly anyone said this. What they said was that having 6 feet between them and the chocolate bowl was enough time for them to pause and ask themselves, “Do I really want that?” When the chocolate was right on the desk, there wasn’t even time to pose that question.

It’s important to note that the secretaries who ate less chocolate did not necessarily have more willpower or a stronger desire not to eat chocolate. They had an environment that helped them eat less chocolate.

The best way to avoid mindless eating is to come up with mindless solutions, so that we aren’t tempted by big bowls of tasty, readily available food and don’t have to make hundreds of daily eating decisions. Taking small steps, such as keeping candy dishes out of your office and stocking your kitchen with small bowls, plates, and glasses, is a good place to start. Using smaller serving spoons, and keeping serving bowls in the kitchen and off the dining table can help, too. When serving a meal, place only the amount of food (with a measuring cup, if necessary) that you want to eat on your plate. When you’ve cleaned your plate, you’ll know you’re done.

In general, to alter eating habits permanently, it is best to take small, easy steps and not gargantuan leaps. The key thing is to start. Given time, small steps achieve large success.