Does Dieting Affect How You Think?

You are served a slice of pizza and you notice that it’s quite a bit larger than the slice that someone else is getting. Then cookies are served. Does the perceived size of the pizza slice have any effect on how many cookies you go on to eat?

Yes — but exactly what the effect is depends on whether or not you’re a dieter.

This is the latest finding in a huge body of work by University of Toronto psychologist Janet Polivy, who’s been studying human eating behaviour and the dieter personality since 1974. In this latest experiment, she and her colleagues invited 106 young women into the lab on the pretext of rating cookies that someone was hoping to market on campus. The women were told not to eat for three hours before the experiment, and that they would be given a light lunch before the cookie tasting, so that ratings would not be biased by hunger.

The light lunch consisted of a slice of pizza. In the control scenario, the volunteer saw only her own slice of pizza, which is a standard Pizza Pizza slice, exactly like those sold on campus. In the experimental scenario, the women got exactly the same sized piece, but they also saw the piece that another participant would be receiving — and it was either one-third larger or one-third smaller than the one they were given.

After eating the pizza, the volunteers were given three plates of cookies — oatmeal raisin, chocolate chip and double chocolate chip — and asked to rate them. They could eat as many as they wanted. Unbeknownst to them, the plates and remaining cookies were then weighed to see how much they’d consumed.

The volunteers were also asked to fill out questionnaires, including one assessing whether they are dieters (“restrained eaters”) and another on how they were feeling.

Interestingly, dieters and non-dieters behaved very differently after receiving the apparently larger slice of pizza. Non-dieters ended up eating fewer cookies than controls did. But dieters ate significantly more. Polivy thinks it has to do with what she calls the “what the hell effect”. That is, the diet has been broken by the big slice anyway, so why be good?

What’s more, the questionnaires revealed that dieters were delighted to get the “large” piece. “They’re secretly actually happy when they get the larger portion and get to break their diet,” says Polivy. “The diet is wrecked, but it wasn’t their fault.” The study is published in the journal Appetite.