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April 12, 2014

How eating several smaller sized chocolates makes you look greedy and impulsive: the unit size effect of indulgent food

Imagine you are offered a package full of delicious chocolates. Would the size of the pieces of chocolate influences how much you eat?

Yes, the size of your piece of chocolate matters. This so-called unit size of food refers to the number of units in which a portion of food is divided. Earlier studies have shown that smaller units typically lead people to eat less. For example, Wansink and colleagues found that people being given four 100-calorie packs of crackers ate about 25% less than when given one 400-calorie pack. But why does this happen? Is it because everytime you start with a new piece, you realise you are still eating and wonder whether that is a good thing? A kind of pause moment? Or do other psychological processes play a role?

In a series of studies together with Hans van Trijp and Christos Kavvouris (published in Psychology and Health), we show that with a small unit size, people eat less because they have the impression (that others think) they eat more and are impulsive. Feeling or looking impulsive is something that many people want to prevent in our society.

Judging someone else eating chocolate (experiment 1)
In our first experiment, participants watched a movie featuring Michelle enjoying chocolate during a break. Half of the participants saw Michelle eating five small chocolates (about 50 grams in total). The other half of the participants saw Michelle eating exactly the same amount of chocolate, but now she ate one big chocolate bar. Interestingly, even though she ate the same amount of chocolate, participants considered the amount to be more excessive, impulsive and inappropriate in the case of the smaller pieces. 
 
Michelle eating 50 grams of chocolate in 5 small pieces (left) or 1 large piece (right)
 
Eating a fixed amount of chocolate (experiment 2)
In a second experiment, we asked one group of participants to eat all 5 small chocolates (50 grams) and another group to eat one entire large chocolate bar (50 grams). We asked them to imagine that they choose themselves to eat the amount they ate. Again, despite consuming the same amount of chocolate, eating smaller sized chocolates felt more excessive and inappropriate than eating one large piece.
 
Eating as much as you like (experiment 3)
Now participants got either 15 small-sized chocolates (150 grams) or 5 large-sized chocolates (also 150 grams) in a supposed taste test in which they were free to decide how much to eat. We also varied the wrapping of the chocolates (see picture below). Unwrapping a chocolate may draw attention to the decision to continue eating, which makes it less automatic.

About 23% less chocolate was eaten when it is presented in smaller units. Participants ate most in the groups being presented with unwrapped large chocolates. This unit size effect could be explained by people feeling more impulsive eating several smaller sizes chocolates than eating a large size chocolate.

Equal amounts of chocolate in the 4 conditions of study 3
Enjoy more, eat less
Across studies, the unit in which the chocolate was offered changed people's perceptions. Eating smaller units looks and feels more excessive and impulsive. Feeling more or less full had nothing to do with it. This brings us back to research on the numerosity effect in the 1990s. Essentially, we think that more pieces of something usually turn out to be more of something. In other words, bigger numbers equal bigger quantities. For example, a seven seater care is usually larger than a five seater car. A quick decision strategy that leads to good decisions most of the time, but it may also lead to wrong estimations.

Clearly, unit size is a cue that helps people to assess what an acceptable portion is. Smaller sized portions may help consumers to control themselves and eat less. Food companies could make item sizes smaller (indulgent snacks such as ice cream scoops or candies) or bigger (fruit, vegetables, whole wheat bread slices), depending on whether you want to make consumers effortlessly eat less or more.

van Kleef E., Kavvouris C. & van Trijp H.C.M. (2014). The unit size effect of indulgent food: How eating smaller-sized items signals impulsivity and makes consumers eat less, Psychology & Health, 1-41. DOI:

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