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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 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.

  • The cake and fruit photos were taken from Wikipedia Commons. I took the graph from Google images. I do not have permission to use this photo. I will gladly remove it if the authors of the study or the journal say so.
  • Here’s an interesting question I just thought of: Do people listen/enjoy shittier music the more their ego’s are depleted? Creed came up on my Pandora while I was writing this (I have no idea how, btw). I hadn’t eaten all day and part of me like it.

Gailliot, M., Baumeister, R., DeWall, C., Maner, J., Plant, E., Tice, D., Brewer, L., & Schmeichel, B. (2007). Self-control relies on glucose as a limited energy source: Willpower is more than a metaphor. Journal of Personality and Social Psychology, 92 (2), 325-336 DOI: 10.1037/0022-3514.92.2.325
Shiv, B., & Fedorikhin, A. (1999). Heart and Mind in Conflict: the Interplay of Affect and Cognition in Consumer Decision Making Journal of Consumer Research, 26 (3), 278-292 DOI: 10.1086/209563

17 Comments Post a comment
  1. Kuze #

    ” Creed came up on my Pandora while I was writing this (I have no idea how, btw). I hadn’t eaten all day and part of me like it.”

    Ordinarily your brain has sufficient energy to supress your love for Creed but your resolve was temporarily weakened and thus it came bubbling to the surface. Had you gone much longer without food you may have broken through what neuroscientists refer to as ‘the nickelback barrier’.

    In all seriousness I bought Baumeister and Tierney’s book but haven’t got around to reading it yet (I wish I was making this up).

    November 22, 2011
    • sammcnerney #

      Hahahahaha Brilliant.

      Yeah, I haven’t read it either – I just know Baumeister’s stuff through his papers. I’ll get around to it soon.

      November 22, 2011
  2. MNP #

    We actually do tend to shop when hungry, (or at least I do, wifey usually has breakfast) but we make our shopping list right after we eat. Then it’s just a matter of sticking to the list.

    November 25, 2011
    • sammcnerney #

      Good strategy!

      November 25, 2011
    • Lopetin tuon Baumeisterin lukemisen te4he4n ktaaohn:Roy Baumeister cherry picks references in an attempt to explain why men dominate the culture..Jos joku on noin tutkalla, he4nen juttujaan ei ole syyte4 lukea enempe4e4.- Vortacp.s. Harmi, kun te4sse4 ei ole editointimahdollisuutta, niin joutuu poistamaan viestin, jos tekee virheen. Se selitte4e4 tuon “puolestaan”-sanan ke4ytf6n edellisesse4 viestisse4ni, joka oikeastaan siis oli varsinaisesti je4lkimme4isempi viesti alunperin.

      October 26, 2013
  3. clr #

    Thank you.
    Do you think our brains are effected similarly, but when it comes to doing a work project & being ‘hungry’ for being paid?
    I personaly prefer to be paid for a project, once it has been completed… that way, if I do lose inspiration along the way – then the financial gain / survival instinct kicks in and makes me ‘hungry’, so to speak, to complete the task.

    I also find that, as a woman, going for shopping, once in a blue moon… a wonderful creative experience, at first… but once i’ve been dragging myself around from one shop to the other after a couple of hours (trying on many outfits – but talking myself out of buying any of the lovely outfits)… I eventualy lose all interest, in what first started out as a ‘wonderful creative experience’… then becomes ‘cross eyed’ and a sense of feeling disappointed.
    Do you think I needed some glucose intake at this stage of my journey?

    I’m curious to understand how different environment / tasks, effect our brains..

    March 13, 2012
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    February 11, 2013
    • , for physical work we have mcluses that burn calories and mental work does tire us, but we should not conclude that we have some analogical mental mcluses that burn mana points.One can use up a good bit of his waking life reading this stuff and not gain any mana points at all for it.;-)

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      October 25, 2013
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  5. Kim #

    , for physical work we have mulcses that burn calories and mental work does tire us, but we should not conclude that we have some analogical mental mulcses that burn mana points.One can use up a good bit of his waking life reading this stuff and not gain any mana points at all for it.;-)

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