Why Shouldn’t You Shop While Hungry? Ego-Depletion and the Brain as a Muscle

I’ve heard it a few times: Don’t shop when you’re hungry, you’ll spend more and buy unhealthy food! There is more than a grain of truth to this, and plenty of empirical research to back it up. Yet, it is worth asking why this phenomenon occurs in the first place. You probably have an intuitive notion of why, but let’s look at some studies to gain a better understanding of what happens at grocery stores when we’re hungry.

The first study comes from Baba Shiv (now a professor at Stanford). Shiv gathered 165 undergraduates for his experiment. He created two groups, one which was instructed to memorize a seven digit number (high processing-resource condition) and another which was instructed to memorize a two digit number (low processing-resource condition). Shiv then instructed participants to walk one by one to another room where they had to recite their number. Right before arriving at the second room, however, they encountered a table full of chocolate cake and fruit salad. Shiv told them that the food was there as a thank you, and the participants were instructed to pick between the two options. After the participants decided on their snack (which they received at the end of the study) they proceeded into the second room where they recited their numbers. Shiv didn’t care about the numbers though, he was focused on the cake and fruit.

He found that the participants who were instructed to memorize a seven digit number (high processing-resource condition) almost always picked the chocolate cake while the other participants (low processing-resource condition) almost always picked the fruit salad. What explains this? In the battle of the rational brain versus the emotional brain the participants in the high processing-resource condition were at a severe disadvantage – their rational brain was distracted. While the rational brain was busy remembering seven digits, the emotional brain took over the decision-making and chose the much more yummy, though less healthy, chocolate cake. On the other hand, the folks with a lighter cognitive-load – those with only two digits floating around in their prefrontal cortex – had enough brain power to resist what their emotional brain really wanted and make the more rational choice.

Shiv’s experiment was not very consequential (unless you count extra calories). But sometimes how “distracted” your rational brain gets carries serious repercussions. Consider a recent report out of the Proceedings of the National Academy of Sciences. Unlike psych studies, this one did not involve undergraduates in search of extra credit points, quite the opposite actually. The participants were eight parole judges in Israel – judges who spend their entire day either granting or denying paroles. (Some background on the parole hearings: cases are presented at random and an average of six minutes is spent on each one. The default decision is to deny parole and only 35 percent get approved.  The exact times of each decision is recorded, as are the three food breaks taken by the judges – these were the key data points).

The researchers plotted the approval ratings of requests against the time since the last food break and found an interesting pattern. The percentage of paroles granted spiked to 65 after each meal – a 30 percent increase from the average rate – but gradually decreased to about zero just before their next meal. What does this mean? Like those who had to remember seven digits and choose between chocolate cake and fruit salad, “tired and hungry judges tend to fall back on the easier default position of denying requests for parole.”

I would feel remorse if I didn’t mention the work of Roy Baumeister. Baumeister has spent most of his career studying self-control. His latest book, which he co-authored with New York Times columnist John Tierney, outlines how and why will power operates like a muscle. Baumeister (along with Mark Muraven) coined the term ego depletion, which describes how the brain is less willing and able to exert self-control or complete a task when it is tired – it has a limited amount of cognitive resources in other words. Here’s one of his studies to illustrate this point. Baumeister and his team had volunteers interpret the body language of a women by watching her being interviewed in a short silent film. To induce ego depletion they imposed a string of words that rolled across the screen during the interview (think a really distracting news ticker) and told the volunteers to try hard to ignore them. Afterwards, they were given a “task in which they needed to overcome an intuitive response to get the correct answer.” As predicted, Baumeister found that they did much worse compared to a control group that was not distracted by the string of words. When the brain is tired it under performs.

But Baumeister incorporated a twist. Before participating in the task the volunteers were asked to drink lemonade; half were given lemonade with Splenda and the other half lemonade with glucose (glucose is a an important source of energy). How much did this matter? Baumeister found that the glucose drinkers did not make nearly as many errors as the Splenda drinkers. A shot of glucose, it turned out, kept their brains fresh and sharp – more rational.

And this brings me back to groceries.

These studies demonstrate that food greatly influences how we decide; the more nourished we are the more rational we tend to be. Similarly, our irrationalities increase the longer we go without food. This helps us explain our grocery store tendencies better: a hungry brain has a hard time focusing on the healthiest and cheapest options because doing so requires mental energy it doesn’t have. The opposite happens when we shop on a full stomach; a glucose rich brain has a much easier time seeing what is best. (There is a fairly straightforward evolutionary explanation to this. As one Psychologytoday.com article explains, “If you’re starving, you better not be distracted by the latest designer doodad on the way to the nearest food source. The only reward that matters now is one that will fuel your physical survival.” This idea is a whole other post though.)

Aside from the don’t-shop-while-hungry-message, there is a more general intellectual nugget to draw from these findings. Brains don’t have an infinite amount of energy. Like muscles, they can get tired and underperform. This is why we sometimes describe ourselves as being mentally “drained” or “fried.” (Just think about how your brains feels after finishing the SAT.) The psychological research outlined here nicely describes how these metaphors are more than mere literary devices. So listen to your mother the next time she reminds you to eat well.