Effects of portion size on eating and obesity

5 04 2007

Paul Rozin and Andrew Geier write an interesting article in the Chronicle of Higher Education on the powerful effect of smaller or larger portion sizes on our consumption of food. It is so often that when I go to a movie and order the small soda or the small popcorn, the cashier seems surprised I’d not want a larger size for pennies more. Why not? Cause I’ll CONSUME IT!

The Chronicle: 4/6/2007: Want Fewer Fries With That?: (requires subscription or password — talk to a PCC librarian for the password)

In a 2006 article with Gheorghe Doros in Psychological Science, we named that phenomenon “unit bias.” We demonstrated it in a number of ways. For instance, we left pretzels in the lobby of an apartment house as free snacks for the residents. When we offered full pretzels (containing 300 calories each), people consumed 71 percent more calories than when we offered pretzels cut in half. Of course, people could take a second half-pretzel, but most didn’t.

It is not only the food portion that is subject to unit bias, but also the serving instrument or receptacle: the size of the glass, plate, or serving spoon. At other times in the same lobby, we left out a big bowl of peanut M&M’s with either a tablespoon or a quarter-cup scoop tethered to the bowl. A sign urged individuals to serve themselves as much as they wanted. With the scoop, four times as large as the tablespoon, people took 75 percent more candy. Of course, they could have dipped the tablespoon in repeatedly, but they tended not to do so.

Citation for the original research:

Geier, A. B., Rozin, P., & Doros, G. (2006). Unit bias. A new heuristic that helps explain the effect of portion size on food intake. Psychological Science 17 (6), 521–525.

Link to the original article at Psychological Science (requires subscription).

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