Faced with a portion of food twice as big as what you normally consume will lead you to eat about a third more food than usual. This portion-size effect helps explain how growing serving sizes may be contributing to the growing prevalence of obesity.
There's a lot of research showing larger portion sizes lead people to eat more. And with food now available at unit prices that beggar belief, never before has so much been obtainable for so little. But as portion sizes grow, so do our waists; the price of plenty is obesity.
Plenty just keeps getting bigger
Do you remember when coffee came in just one small size? Today, there are multiple sizes with Starbucks featuring a range from Tall (350 millilitres) through to Trenta (920 millilitres).
Coca Cola portions have grown from the original 1915 bottle with 192 millilitres to today's variety of single portions that range from the recently introduced 200-millilitre and 250-millilitre "mini cans" through to 600 millilitres.
The fact is, portion sizes have been growing for millennia. To the extent that art imitates life, an analysis of 52 paintings of The Last Supper shows the painted portions of bread and wine have been steadily getting bigger over the last 1,000 years.
The pull of plenty
If the road to hell is paved with good intentions, then plenty and portion sizes are the horses taking us that way. The problem with expanding portion sizes is that we seem ill-equipped to say "enough" and to stop.
If we're served a larger portion, we consume more - as my colleagues and I show in our review of 88 existing studies that manipulated portion size and measured consumption. We found that doubling a portion leads to an average 35% increase in consumption.
In addition to being substantial, the effect is robust, even pernicious. Larger portions lead to greater consumption even across conditions of bad food, where the portion size is not visible, and among people who should know better.
The portion-size effect has been demonstrated with stale popcorn, for instance, and in a bottomless bowl of soup (one that's ingeniously fitted so that the contents are continually refilled surreptitiously). It's been shown in a dark restaurant where the waiters are blind and diners eat in complete darkness, and among nutrition experts.
Bigger eyes than stomachs
Why does this happen? One likely contributor is a "consumption norm" whereby individuals typically consume a fixed proportion of the portion served.
One version of this rule is eating everything on your plate. But a similar proportional rule is reflected when we leave 10% of the portion because we consider it polite to do so, or consume just half the portion because we're on a diet.
A problem with consumption based on a fixed-proportion rule is that it will inevitably increase with increasing portion sizes; a doubling of portion size will lead to double the consumption. But this is not what we observed in our review.
Instead, our research shows the effect of portion size is larger for smaller portions and declines with increasing sizes.
We suspect that fixed-proportion consumption norms operate for smaller portion sizes, but people begin to heed internal cues with increasingly bigger sizes. That is, our body groans in protest and we slow down or stop rather than follow the fate of M. Creosote of Monty Python fame.
Mind your portions
The best way to not eat more than necessary is to control your portion sizes, and don't let them control you.
We're all subject to this bias, and so we need to retrain ourselves to smaller portions - use smaller plates, choose the entrée size, don't eat at smorgasbords.
The second step is to train children to listen to internal cues and not be misled by external cues.
Curiously, our research revealed that children display a smaller portion size effect than adults. So, it's probably less a case of training good habits, and more a case of not training bad habits.
Don't encourage children to "eat everything on your plate" and "eat some more vegetables before you can have dessert". This encourages them to follow a rule and ignore satiation.
The third step is to have food marketers limit portion sizes. Increasing portion size makes an offering more attractive, but when everyone does this in order to be competitive, all available offerings become large. This leads to unnecessarily large portions and unnecessarily large people.
So make your portions smaller. You'll save some money and save your waistline.
Stephen S Holden is an Associate Professor, Marketing at Bond University. He does not work for, consult to, own shares in or receive funding from any company or organisation that would benefit from this article, and has no relevant affiliations.