The Ikea Effect is a psychological bias first identified by Michael Norton, Daniel Mochon and Dan Ariely. The concept will make perfect sense to anyone who has struggled to put together a bookshelf based on an inscrutable set of instructions. Although the furniture might look like crap – I always have a few leftover screws – the flimsy assembly of molded plywood feels like a masterpiece. (That shelf isn't supposed to be straight, right?)

In one study, the behavioral economists asked people to fold origami and then bid on their own creations. As expected, the subjects were consistently willing to pay more for their own folded paper creations. In fact, they were so enamored of their amateurish designs that they valued them as highly as origami made by experts.

It turns out that the Ikea effect also applies to food, at least in mice. The experiment was simple: Mice were trained to push levers to get one of two rewards. If they pressed lever A, they got a delicious drop of sugar water. If they pressed lever B, they got a different tasting drop of sugar water. (This reward was made with polycose, not sucrose.)

The scientists then started to play mind games with the mice, as they gradually increased the amount of effort required to get one of the sweet rewards. Although the mice only had to press the lever a single time to get the sugar water at the start of the experiment, by the end they were required to press the lever 15 times.

Here's where things get interesting: When the test was over and the mice were allowed to relax in their home cage, they showed an overwhelming preference for whichever reward they'd worked harder to obtain. More lever presses led to tastier water. (The scientists measured these preferences in a variety of ways, including an analysis of "licking microstructure". Preferred foods lead to a faster rate of initial licking and longer duration of "licking bursts.")

The scientists conclude the paper by speculating on why such an effect might exist. They argue that the association of effort and deliciousness would have been an adaptive association back when calories were scarce, and we'd sometimes have to work hard to end up with a rather disgusting dinner:

In evolutionary terms, such mechanisms could benefit survival under conditions of scarcity when the chances of acquiring food are probably related to increased foraging effort. An increase in palatability accruing to lower value foods under such conditions would serve a dual purpose, both to boost consumption and to confer greater learning connected to the prevailing cues in the environment. In the first instance, a change in palatability would prolong meal duration and/or consumption of certain (e.g. less valued) foods that might otherwise be rejected under conditions when food supply is plentiful.

This experiment reminds me of a provocative 2003 paper, "Why Have Americans Become More Obese?" by the economists David Cutler, Edward Glaeser and Jesse Shapiro. They argued that the rise in the weight of the average American in recent decades has been largely caused by a technological shift in food production, which allows us to cook calories with ever increasing ease.* (In 1965, a married women who didn’t work spent over two hours per day cooking and cleaning up from meals. In 1995, the same tasks took about 50 minutes.) The economists illustrate their argument with a parable about the potato:

Before World War II, Americans ate massive amounts of potatoes, largely baked, boiled or mashed. They were generally consumed at home. French fries were rare, both at home and in restaurants, because the preparation of French fries requires significant peeling, cutting and cooking. Without expensive machinery, these activities take a lot of time. In the postwar period, a number of innovations allowed the centralization of French fry production. French fries are now typically peeled, cut and cooked in a few central locations using sophisticated new technologies. They are then frozen at -40 degrees and shipped to the point of consumption, where they are quickly reheated either in a deep fryer (in a fast food restaurant), in an oven or even a microwave (at home). Today, the French fry is the dominant form of potato and America’s favorite vegetable. This change shows up in consumption data. From 1977 to 1995, total potato consumption increased by about 30 percent, accounted for almost exclusively by increased consumption of potato chips and French fries.

Why do the microwave and frozen dinner inexorably lead to obesity? According to the economists, the cheapness of calories (both in terms of price and time) has led us to dramatically boost consumption. Food stops being something we make and create – it doesn't require very many lever presses, so to speak – and becomes something we simply ingest. Eating just gets easier. And then we get fatter.

But maybe we're not just consuming more calories because they're available at such a low cost. Maybe we're also consuming more calories because each calorie gives us less pleasure. The lesson of those lever-pressing mice, after all, is that when we don't work for our food – when it only requires a single press, or a few whirls of the microwave – it tastes much less delicious.

That hypothesis leads me to this study, published in 2008 in Science. The experiment involved giving subjects sips of a chocolate milkshake inside an fMRI machine. The scientists were interested in the activation of the striatum, an area rich in dopamine neurons and involved in the processing of hedonic rewards. (When your striatum is excited, life is good.) Sure enough, obese people tended to have reduced activation in the striatum after sipping the ice cream treat, which led to increased consumption. In other words, they kept on consuming the milkshake in a manic search for satisfaction.

A second study found that, over time, people with a polymorphism that leads to reduced dopamine receptors in the striatum also tended to put on weight, suggesting that obesity is, at least in part, triggered by a shortage of neural pleasure. Of course, this contradicts the popular (and deeply unfair) cultural stereotype of obesity, which assumes that people who are overweight are gluttons, unable to resist temptation. In fact, they are the opposite of gluttons: The reason they eat too much is because they don't enjoy their food enough. They keep on sipping the milkshake precisely because it isn't pleasurable.

Maybe this is why Americans need ever larger portion sizes: Because we didn't make the milkshake ourselves, because that dinner only required a few minutes of work, we need to consume more calories to get the same baseline of satisfaction. The solution to this problem, of course, is simple: We need to take time to make dinner. Instead of buying frozen french fries, make a risotto. The 30 minutes of stirring will make the end result much more delicious.

*Image: Flickr/katybeck

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* Until Cutler, Glaeser and Shapiro came along, one of the leading theories on American obesity had been that of Thomas Philipson and Richard Posner, who argued that changes in the amount of energy spent on the job and commuting to work accounted for the rise in obesity. Yet as the Cutler, et.al. point out, most of the changes into physically passive occupations had taken place by 1970. Furthermore, Cutler, et.al. convincingly demonstrate that changes in exercise patterns over the past 30 years have been fairly minimal. In order for the surge in American obesity to be explained by changes in exercise patterns, Americans would have to have reduced their energy usage by the equivalent of two kilometers of walking per day. No such shift has occurred.