The effect of portion size on consumption: the unit bias

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In Italy, but in general in most western cultures, children are raised since their infancy to eat all that is presented on a plate, because “food must not be wasted”. In addition, it can be considered as a lack of respect towards the person who prepared the dish. One does not eat until they are satiated, but the plate must be finished, because that is the norm for consumption, it is a “fair” portion size. This happens, however, at the expense of calorie assumption or one’s own feeling of hunger.

This cognitive error is called unit bias, in other words the tendency to consider a single entity as the appropriate amount to consume, consider or engage. This phenomenon is applicable for both physical objects and activities: if you are reading a volume, for example, you try to finish the chapter before putting down the book.

In the nutrition field, many scientists have researched then the correlation between this effect and food consumption. The main hypothesis was that the portion size that was presented was directly correlated to the quantity of food consumed by a person. Therefore, if the portion is bigger, the consumption of food will increase. In 2006, three researchers from the University of Pennsylvania conducted three experiments, aimed at confirming this hypothesis.

Candies, pretzels, and chocolate

The researchers have tested food consumptions placing big bowls of sweets in the common areas of three different buildings. According to their hypothesis, the smaller the product presented was, the smaller the consumption would have been. In two of the three cases, the size of the candies and the pretzels was varying, while in the third they changed the size of the spoon with which one could have served themselves some small chocolates. They filled the bowls at the start of the day, then controlling the quantity consumed in the afternoon.

The candy bowl was filled with 80 small candies, weighing 3 grams, or 20 big candies, weighing 12 grams. The pretzels were instead presented whole or cut in halves, therefore they offered either 60 whole pretzels or 120 halves. It was a different matter for chocolates: there was a full bowl, on which there was written: “Eat your fill (please use the spoon to serve yourself)”. The container was in fact attached, through a small chain, to a tablespoon (15 grams) or a quarter-cup spoon (60 grams).

The unit bias is a cognitive in error that consists in considering a single entity as the appropriate amount to consume, consider or engage.

After weeks of testing, the results seemed to confirm the hypothesis. When the big candies were served, the consumption increased by 127%. The same thing occurred for pretzels and chocolates, respectively increasing by 69% and 67% when compared to smaller portions.

The researchers proposed two additional hypotheses to explain why one would stop after eating a single unit. The first hypothesis is social: if one person took more than one portion, they could seem to be eating too much, or could appear greedy, since the food offered was free. The second is cultural: since childhood, everybody learns that a portion is the right quantity, therefore you cannot exceed.

Many companies that work in the food industry know this effect, so much so that they use this strategy to increase sales, even at the expense of their customers’ health.

The negative effects of supersizing

Supersizing is a marketing tool consisting in offering bigger quantities of food for a discounted price. This technique encourages the purchase, but can lead to excessive consumption. In fact, portions that are bigger than recommended are considered as a potentially relevant factor in the increase of obesity rate by several government agencies. In the United States, where the obesity rate has reached 40% in the last years, an individual portion of yogurt weighs 227 grams on average, while in France, where obesity is at 15%, weighs about 125, which is the classic single cup that everyone thinks of.

Some researchers from the Bond University have demonstrated how doubling a portion increases consumption by 35% on average. In addition, the smaller the initial portion is, the bigger the effect will be.

The effect seems to be stronger in men (+52%) than in women (+27%), while kids are less influenced. The consumption is higher for snacks (+37%), compared to other kinds of food (+27%).

Obviously, the unit bias cannot be the only explanation for the increase or decrease of consumption, otherwise the change rate would have been 100%. Some factors that diminish the effect of this bias are a greater attention for food when someone is eating, following a precise nutritional plan, such as a diet, or different cultural influences. In some countries of the world, leaving some food on the plate is considered as an act of politeness, that sends the message to whom has prepared the dish that the quantity of food was more than enough.

Carlo Sordini


  1. Geier AB, Rozin P, Doros G. Unit Bias: A New Heuristic That Helps Explain the Effect of Portion Size on Food Intake. Psychological Science. 2006;17(6):521-525. doi:10.1111/j.1467-9280.2006.01738.x
  2. Zlatevska N, Dubelaar C, Holden SS. Sizing up the Effect of Portion Size on Consumption: A Meta-Analytic Review. Journal of Marketing. 2014;78(3):140-154. doi:10.1509/jm.12.0303