Ask an economist why obesity rates are rising across the rich world, and you’ll likely get two answers. First, the shift away from occupations requiring serious amounts of physical labour means that the typical worker burns fewer kilojoules on the job than in the past. Second, advances in agriculture and food technology have made kilojoules cheaper and yummier than ever before. (Empirically, it turns out that the former explained weight gain until about 1970, and the latter is the key factor since then.)
But would you trust an economist to tell you how to lose weight? Strikingly, part of the growth of behavioural economics has involved the dismal science turning its analytic toolkit to understanding why many of us overeat, and how we can reduce our food consumption.
The economic approach to estimating which kinds of dieting strategies succeed involves identifying cognitive biases, which are then tested in randomised experiments in the laboratory or real world. Coupled with this is the evaluation of policy changes, to see whether initiatives such as kilojoule labelling help or hurt consumers.
A trio of papers in the May 2009 issue of the American Economic Review summarises some of the new research in this field. Analysing eating patterns across the day, Marianne Bertrand and Diane Schanzenbach find that while Americans spend less time eating as their primary activity, it has been increasingly common to eat while doing something else (I hope you’re enjoying that sandwich, by the way). About a tenth of US kilojoules are consumed while watching TV. And while people tend to eat less when they have consumed more in the previous six hours, this rule doesn’t apply to television eating: people eat just as much in front of the television regardless of what they have recently consumed.
So could just turning off the television make a difference? To test this, Brian Wansink and co-authors ran a horse race between ten dieting strategies. Randomly assigning 1,000 participants from a weight-loss website to one of the strategies, they were able to monitor compliance and weight loss.
The upfront finding - not surprising to anyone who has ever gone on a diet - is that very few people stick to the plan. No dieting strategy garners more than a 40 per cent compliance rate.
Yet the successful strategies are not always those that are easiest to stick to. The best ways to lose weight are to use 25cm plates (an average 0.9kg weight loss in a month), avoid eating while watching television (0.7kg weight loss), and eat fruit before snacking (0.5kg weight loss). If you’re munching through The Biggest Loser, you may be missing the point.
These results are consistent with the notion that we eat more with our eyes than our stomachs. Indeed, other evidence shows that those who decide when to stop eating based on external cues (of the kind “I stop eating when my drink runs out”) are more likely to be overweight than people who say they stop eating when they feel full.
Can public policy help too? One popular solution among politicians is to require restaurants to label the kilojoule content of each dish. In principle, one might think that more information can only benefit consumers. But by surveying customers before and after New York City’s 2007 adoption of such a law, Julie Downs and co-authors conclude that the rule did not reduce food consumption. Indeed, the authors find some evidence that dieters consumed more kilojoules after the labelling law came into effect, which they attribute to the fact that dieters tend to overestimate the amount of energy in a meal. If the legislature’s intention was just to increase information, then the law succeeded; but if they had hoped that it would help people lose weight, it was an abject failure.
Although behavioural economics is still a relatively young field, research such as this may be helping to bring a little more rigour into the notoriously unscientific diet industry. With obesity being one of the leading health concerns in developed nations today, even modest advances in our knowledge can have a substantial payoff. Who knows: perhaps the “behavioural economics diet” will be coming soon to a bookshop near you?
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