Another menu, with "bundled" options, offered a 16-ounce soda for $1.59, a pair of 12-ounce drinks for $1.79, and a pair of 16-ounce drinks for $1.99. And a third "no bundle" menu only offered a 16-ounce drink for $1.59.
The researchers found that the participants wanted to buy more drinks when they had the choice of "bundling" them. When the participants only had a choice of just one size, only 62 percent chose to buy a soda, compared to 84 percent of those who had a choice of "bundled" options and 79 percent of those who faced the "unregulated" options.
Wilson said the study suggests that restaurants could make 70 percent more money from drinks if they offered the bundled options instead of just 16-ounce drinks. "The bundled options just felt like a better deal" to study participants, Wilson suggested.
It's not clear how much actual extra profit restaurants could make, however.
Wilson dismissed the prospect that his study was giving restaurants ideas about how to bypass the super-size ban. "It's good for policymakers to know about any unintended consequences and think about them now," he said.
Barbara Jean Rolls, chair of nutritional studies at Penn State University, said the study findings are "provocative" but lack a real-life component because they didn't involve an actual restaurant or actual drinks. (Wilson said that's the next step for future research.)
"Human eating and drinking behavior is very complex," Rolls said. "A lot of studies indicate that what you say on paper isn't what you're going to do [in real life]. I don't think just asking people on paper is going to tell us how this will play out."
The study was published online April 10 in the journal PLoS One.