Do 'snack packs' make you eat more?

Behind the Headlines

Thursday July 3 2008

It is not certain that smaller bags of crisps make people eat more

A study has found that “diet-sized snack portions encourage people to eat more," The Daily Telegraph reported. The newspaper said that researchers had warned that smaller sized packets could make people believe they had already limited their food intake, and therefore did not need to “exercise further self-control”. It added that it is not only diet-sized or “fun packs” that make dieters drop their guard - the effect could extend to apparently healthy food, such as nuts and berries.

This research resulted in some contradictory findings, and there is no certainty that it actually suggests that pack size is a major determinant of how much a person consumes. The study’s results showed that pack size alone did not make any difference to how many crisps were eaten, and any difference in consumption levels was only clear in those people who had been made to think about their diet. This study does not indicate an optimum packet size for limiting binging, nor a size that is linked to people eating more.

Where did the story come from?

Rita Coelho Do Vale, an assistant professor of marketing at the ISEG Economics and Business School in Lisbon, Portugal, carried out the research with two colleagues from Tilburg University in the Netherlands. The study was supported by a grant from the Portuguese Foundation for Science and Technology. The study was published in the peer-reviewed Journal of Consumer Research.

What kind of scientific study was this?

The research paper described three studies that were all part of the first author’s academic dissertation on the effects of package size on the self-regulation of consumption. The first two studies were based on qualitative research, and served to generate ideas for the third main quantitative study, which was a randomised controlled trial. In this third study, the researchers randomly allocated 140 undergraduate students to four groups and compared eating behaviours across the groups.

The first two studies looked at the beliefs of about 120 Dutch students regarding package size and its influence on self-control. The theory was that people with different levels of confidence in their self-control when it came to eating snacks would have different perceptions about how package size affected how much they ate. For these parts of the study, the researchers concluded that consumers believe that providing snacks in small packages could theoretically control their consumption, but would not work with non-tempting “utilitarian” products. This belief was apparently more prevalent in consumers who had difficulties with self-control.

The third part had a “two-by-two design” and set out to prove this theory with 140 student volunteers who were randomly allocated to four groups. Half of the 140 volunteers had their 'self-regulatory concerns activated', by being made to think about calories and diet through being questioned about any concerns they had about their size, and then being weighed. The other half were not pre-conditioned to think about diet in this way. Both these groups were then split in half again, and each half either given crisps packed in small bags, or the same weight of crisps in large bags. All four groups were then shown TV programmes and commercials while seated next to the allocated bags of crisps.

Complex statistical methods were used to assess the significance, or lack of it, between groups in terms of the number of volunteers who opened a bag, and the weight of crisps eaten.

What were the results of the study?

[The study] suggests that there is some aspect of the pre-conditioning that was an important determinant of eating behaviour – rather than pack size itself.

The researchers reported that those given the large bags were less likely to open them, but that overall, the quantity of crisps eaten did not differ between groups offered the large and small bags. When the researchers compared those who were pre-conditioned to think about diet (in both the large and small bag groups) with those who were not conditioned, they found that consumption was lowest when “self-regulatory concern was activated”.

What interpretations did the researchers draw from these results?

The researchers say that activating the self-regulatory concerns had no effects on deliberation and consumption when snacks came in small packs. However, when self-regulatory concerns were activated (i.e. by thinking about diet), consumers were almost twice as likely to eat tempting products from small bags compared to large packs. The activated people who ate from the small packs consumed nearly twice as much as the activated volunteers who were offered large packs.

What does the NHS Knowledge Service make of this study?

There are several results reported from this complex series of studies and these results have been selectively reported by the newspapers. The main research finding, that pre-conditioned volunteers ate less, is not unexpected. However, there are contradictory findings when the results are looked at across the four groups by regression analysis, a technique which attempts to tease out how much snack consumption can be attributed to pack size, pre-conditioning or a combination of both. There are some limitations to this analysis:

  • The small numbers in the sub groups - about 35 in each group - means that some of the differences between groups may have arisen by chance.
  • It is not clear how aware the volunteers were of the aims of the research when they sat in front of the TV screen. For example, those taking part in the first study, or knowing the results of this, would be likely to consider their snacking more carefully. The researchers do not report how many volunteers were used in both parts.
  • Overall, the researchers found that the amount of crisps consumed (grams) did not vary between package sizes, but only in the groups who were pre-conditioned. This suggests that there is some aspect of the pre-conditioning that was an important determinant of eating behaviour – rather than pack size itself.
  • The researchers say that their findings do not imply that the best way to regulate consumption levels is by using smaller packs such as “multiple-serve and family-sized packs”. However, telling consumers to not purchase large packages or to “never eat out of a big bag or package” is also not the answer. Their suggestion is that “consumption that proceeds mindlessly may stop at the bottom of the bag, which is more problematic if it is deep”.

This study does not indicate a size that would be best at limiting binging, nor a size that is linked to people eating more. Future studies would need to investigate this further to answer this question.


Sir Muir Gray adds...

An important study; it’s not just the advertising or hunger that makes us act in the way we do, it’s lots and lots of little nudges.


Analysis by Bazian

Edited by NHS Choices

Links to the headlines

Diet sized snacks make you eat more. The Daily Telegraph, July 03 2008

How 'snack-size' makes you bigger. BBC News, July 03 2008

Small portions make you eat more, researchers claim. Daily Mail, July 03 2008

Diet-sized snack packs turn off willpower. New Scientist, July 03 2008

Links to the science

Coelho do Vale R, Pieters R, Zeelenberg M. Flying under the Radar: Perverse Package Size Effects on Consumption Self-Regulation. JCR 2008 [Published online June 19]


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