In a series of experiments, participants expressed a preference to purchasing a drink offered in a 24 oz. (682 ml) cup over two 12 oz. (340 ml) cups.
When the choice was expanded to either a 24 oz. drink or a 16 oz. (455 ml) drink with free refills, participants chose the drink with refills consuming 44% more calories compared to purchasers of the 24 oz. drink.
When participants were required to get their own refill, calorie consumption was reduced by 40% when compared to participants, who received waiter service.
"Our research provides insight into the effectiveness of a portion limit policy," explains lead study author Dr Leslie John, behavioural scientist at the Harvard Business School.
"We identify one circumstance - bundling - where the reduction in purchasing of sugar-sweetened beverages is likely to be realised, and another - refills - where the policy can in certain cases have an unintended consequence of increasing consumption."
The findings add more insight into how size perception and social image concerns affect consumer purchase consumption choices.
Adopting these cues may provide a cost effective approach for promoting healthy behaviour that provides consumer choice and minimises impact on businesses.
In January of this year, France made selling unlimited soft drinks either at a fixed price or for free illegal.
Prior to this move, the country already had a soft drinks tax, which targets soft drinks and sports drinks with added sugar or sweeteners. In addition, vending machines have been banned from in schools.
In New York regulations that restricted the serving containers of sugary drinks to a maximum size of 16 oz. were overturned in 2013.
In four closely related experiments, the collaborative efforts from Harvard Business School and the University of Pennsylvania began by enrolling 623 participants, (45.1% male) and offering them the choice of buying a medium drink for $0.20 (€0.18) or a large drink for $0.30 (€0.28).
The large was served in either a single 24-oz cup or two 12-oz cups. The medium drink was served in a single 16-oz cup. Participants were also assigned to waiter-service or self-service to obtain refills.
In experiment two, 470 participants (49.9% male) were offered a 16 oz. medium drink or a 24 oz. large drink was 24 oz. They were also offered a large 16 oz. drink was 16 oz. that came with unlimited refills provided by a waiter.
Experiment three involved 557 participants (48.5% male), who were asked to choose either no drink, a 16-oz medium drink with no refills for $0.20 or a large drink that was either (a) 20 oz, (b) 16 oz with waiter-service refills for $0.30 (0.28) or (c) 16 oz with self-service refills also for $0.30 (0.28).
The final experiment was made up of 285 participants (48.4%) were given drinks with smaller serving sizes. The participants were given either a 10 oz. drink with no opportunity for refills or a smaller, 8 oz. drink with a refill option.
“Taken together, these results suggest that this method of complying with a sugary-drink portion limit could have the perverse effect of increasing consumption," the researchers commented.
"However, requiring the participants to stand up and walk a tiny distance to obtain their refills helped to curb it."
In discussing the results, the researcher’s thought that in light of the potential for a sugary-drink portion-limit policy to curb consumption, there was also a risk of inciting business and consumer backlash.
“In contrast to a similar study the results of experiment one suggest that bundling would not backfire and may curb purchases of sugary drinks,” they said.
“The findings of the other experiments suggest that such a policy could be undermined if businesses offered free refills with a 16-oz beverage.
The participants found this smaller drink with free refills appealing, but it ultimately led them to consume more calories from sugary drinks, particularly when waiters served refills.”
The portion size minefield
Reducing portion sizes is now one of the approaches the food industry claim they are using to reduce sugar, fat and salt content in its products.
However, critics have suggested this is one way of increasing profits by giving the consumer slightly less product without reducing the retail price.
In 2012 Nestle 1 kg tin of Quality Street went down to 820 g. In a similar vein, the Mars Bar's 58 g serving decreased to 51 g. Snickers' 58 g bar shrank to 48 g in 2013.
Cadbury's Creme Eggs also reduced its offerings from six in a box to only five.
In November last year Mondelēz’s Toblerone chocolaite bars were reduced from 400 g to 360 g and 170 g bars to 150 g.
However, the Mondelēz cited its motivations as compensating for the higher costs of ingredients while making the product affordable.
“Manufacturers need to be careful because consumers think that increasing price is fair game, but that decreasing size is not. A size reduction is best when it occurs simultaneously with a package redesign, formulation change or any other positioning change,” said Dr Pierre Chandon, professor of marketing at INSEAD in a response given to this site in January.
“If everything else is constant, but only the size is lower – and it’s very clear [if], say, only the height was reduced – then some consumers will find out and feel cheated.”
Source: Psychological Science
Published online ahead of print: doi.org/10.1177/095679761769204
“Psychologically Informed Implementations of Sugary-Drink Portion Limits.”
Authors: Leslie John, Grant Donnelly and Christina Roberto