As portion sizes have increased, Americans’ waistlines have expanded—and fast-food restaurants haven’t helped the situation.   

A study in last month’s Journal of Consumer Research demonstrates Americans are tricked into drinking more soft drinks when these ubiquitous restaurants eliminate small drink sizes and super-size the menu.   

Regardless of a soft drink’s volume, customers tend to avoid the largest and smallest options, according to researchers Kathryn M. Sharpe, Richard Staelin and Joel Huber, all from Duke University.   

“Our basic premise is that consumer purchases are altered by the portfolio of drink sizes made available,” they explain.   

In an attempt to boost profit margins, fast-food restaurants have eliminated smaller drink sizes and added even larger options. The authors believe these policies have led to a 15% increase in the consumption of these high-calorie drinks.   

“Consumers who purchased a 16-oz. drink when a 12-oz. drink was available later chose a 21-oz. drink when the 12-oz. option was removed, since now the 16-oz. soda is the smallest option,” they write. “This effect also occurred at the large end of the spectrum; people who purchased a 21-oz. drink when the 32-oz. drink was the largest size available moved up to the 32-oz. drink when a 44-oz. drink was added to the range of drink sizes available.”   

By adding the 44-oz. option, the restaurant shifts the demand curve upward, even though the authors believe customers still want 12-oz. drinks.   

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Photo courtesy of McDonald’s