Just what size is a "small" drink-8 ounces, 12 ounces, 16 ounces? The truth is, those are all "small" sizes depending on what restaurants and fast food joints you go to. As customers, we are used to ordering food based on relative size, but according to a new study from Cornell University, these seemingly standard labels impact our entire eating experience.

Dr. David R. Just and Dr. Brian Wansink of the Cornell Food and Brand Lab designed a study to understand how portion labels impact what you're willing to pay for your food as well as how much you actually eat. The researchers served two different portion sizes of lunch items, including spaghetti: either 1 cup (small) or 2 cups (large).

The twist was in the labeling: for some participants, the small and large portions were labeled "Half-Size" and "Regular" respectively, giving the impression that the large 2-cup portion was the norm. For the others, however, the same portions were labeled "Regular" and "Double-Size"-indicating that the smaller 1-cup portion was the norm. These varying concepts of "Regular" portions made all the difference in how much people would spend and subsequently eat.

The researchers examined how people's eating habits differed depending on these food labels. When served identical large portions of spaghetti, individuals ate much more when it was labeled "Regular" than when it was labeled "Double-Size;" in fact, those who thought it was "Double-size" left 10 times as much food on their plates!

To explore how portion labels impact how much patrons will pay, the researchers had people bid on each portion in an auction-like set up. When the portion was labeled "Half-Size," participants were only willing to pay half as much as when the same portion had was labeled "Regular." The labels themselves, rather than the visual appearance of each serving, acted as indicators of the amount of food on each plate compared to a hypothetical normal serving.

The study indicated that people primarily use labels alone to dictate how much food is a 'normal' portion and that they adjust their intake accordingly. These studies together show that people are not only willing to pay more for a portion that sounds larger but also that they will eat more of an enormous portion if they believe it is "Regular" to do so.

The huge impact of size labels suggests that both consumers and producers could benefit from standardization of food size-labeling; clearly defining the actual amount of food in a "small" or a "large" would inform customers of just how much food they are ordering every time they ask for a certain size. Until then, take the time to think about what portion you're really getting when you order your standard "Medium" meal!

Just, D. R., and Wansink, B. (2013). One man's tall is another man's small: How the framing of portion-size influences food choice. Health Economics, in press.