You might think you know how much you ate for dinner last night - but you could be wrong.
It turns out that modern diets are custom-designed to trick us all into overindulgence. Most people have no idea how many calories they're consuming. Even dietitians get hoodwinked: just tell them it's low-fat and they blithely underestimate just like the rest of us.
Consumers should know what they're getting their teeth into. And to help them the New York City Department of Health now compels large chain restaurants to own up to calorie contents on menus, boards and labels. Other states may soon follow suit.
But is posting calories enough to make us rethink that mint chocolate chip frappuccino with chocolate whipped cream? How are people really using the information?
One possibility is that they use it to compare all the items on the menu and home in on the choice with the lowest number of calories. This is an example of a general psychological principle - the ‘comparison level of alternatives'. We evaluate things based on the benefits they offer over the other options available to us. This theory has frequently been applied to mate choice. Of course, most of us don't select a candidate from our available mating pool based purely on numbers. Liking and enjoyment are crucial, and this applies equally to food.
Debra Zellner gave people full-strength fruit juice followed by some watered down, wimpy versions. But she told half of them that the wimpy versions were ‘fruit juices', and the other half that they were ‘commercial drinks'.
People in the first group were unimpressed. If they were meant to be comparing the new drinks with fruit juices then why did these alternatives taste so insipid and disgusting? In contrast, those in the second group liked the new drinks much better - bad experiences within the commercial drink category had evidently lowered their standards enough to make them feel they were getting a tasty deal.
This tells us that our liking for something depends on what we imagine our alternative options to be. If we compare it to things we like, we like it less. If we compare it to things we don't like as much, it starts to look more appealing. The same ‘context' effect works for fruit, coffee, flowers and beer.
So what are the implications for calorie posting? Well if we think consuming less energy is desirable, our rational side should reluctantly pick the lower calorie menu item. For a while we'll compare it in our head with previously permitted options (pepperoni pizza with extra cheese, cheese-stuffed pizza crust...), and feel thoroughly cheated. The smugness that comes from making the ‘healthy' choice may not be enough to compensate for the self-denial.
But Zellner's finding also opens up an intriguing possibility: could we persuade our brains to imagine a different set of available alternatives - one that might make the healthy choice seem a bit less wimpy? Maybe if we used the calorie information to limit our options to lower calorie items (e.g. diet soda, regular-size portions), we could recalibrate our food evaluation system - and even start to relish healthy choices. Was that 'fat chance' I heard you say?
Mind you, there's another problem with all this talk of context. If food choices emerge from comparison between available options, what happens when all of them are as bad as each other? My colleague Eva went to a popular chain restaurant at the weekend and, despite an intensive search, found almost nothing on the menu below 1000 calories.
Nutrition education may help. If we know how many calories we need in one day, it doesn't take a genius to work out that 1150 calories for a burger and fries is a tad over the top. NYC's health champions are wise to this and are using posters to break the bad news to straphangers on their morning commute.
Naturally, none of this changes the fact that tasty, high calorie options will always make the mouth water. And there's no reason to deny yourself your favorite meal. You can always share it with a friend or take half home to eat tomorrow.
You could even treat the dog - but I expect veterinarians would have something to say about that.
