Friday, 23 June 2006

The 'power of one' - why larger portions cause us to eat more

The effect of portion size on how much people eat is something of a mystery – why don’t they simply leave what they don’t want, or alternatively, where possible, why not help themselves to more? Andrew Geier and colleagues at the University of Pennsylvania think it has to do with ‘Unit bias’ – “…the sense that a single entity (within a reasonable range of sizes) is the appropriate amount to engage, consume or consider”.

To test this, the researchers left a bowl of M&M sweets in the hallway of an apartment building with a sign that read “Eat Your Fill: please use the spoon to serve yourself”. Some days they left a tablespoon-sized scoop, other days they left a quartercup scoop that was four times as big. Passers-by could obviously help themselves to as little or as much as they wanted regardless of which spoon was provided, but on average, 1.67 times more M&M’s were taken on the days the big scoop was left compared with the tablespoon-sized scoop.

In another experiment, the researchers found that, measured by weight, significantly more pretzels were taken by passers-by when a complimentary bowl of 60 whole pretzels was left in an apartment building, compared with when a bowl of 120 half-pretzels was left. And it was a similar story when either a bowl of 80 small Tootsie rolls (an American snack bar) or a bowl of 20 large Tootsie rolls was left in an office building.

In other words, throughout the study, people took more food when the unit on offer was larger. “Consumption norms promote both the tendency to complete eating a unit and the idea that a single unit is the proper portion”, the researchers said. However, they also acknowledged that other factors must have been at play because the amount of food taken did not vary in direct proportion with the increase in unit size on offer.

The researchers concluded that this ‘unit bias’ applies in other walks of life too – they cited the example of films: “double features are rare, but very long movies are not”, and amusement-park rides: “one ride on a particular attraction is usually enough, whether it takes one or five minutes”.

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, 521-525.

Post written by Christian Jarrett (@psych_writer) for the BPS Research Digest.