After the recent hullabaloo about whether ego depletion was a real phenomenon, I decided to finally read Roy Baumeister and John Tierney’s Willpower cover to cover (I’ve only flicked through it before).
My hope was that I’d find some interesting additions to my understanding of the debate, but the book tended into the pop science/self-help genre and there was rarely enough depth to add anything to the current debates (see Scott Alexander on that point). That said, it was an easy read and pointed me to a few studies that seem worth checking out.
One area that I have been interested in is the failure of the mathematics around glucose consumption to add up. Baumeister’s argument is that glucose is the scarce resource in the ego depletion equation. Exercising self control depletes our glucose, making us more likely to succumb to later temptations. Replenishing glucose restores our ego.
As plenty of people have pointed out - Robert Kurzban is the critic I am most familiar with - the maths on glucose simply does not add up. The brain does not burn more calories when making a quick decision. Even if it did (say, doubling while making a decision), the short time in which the decision is made means the additional energy expenditure would be miniscule.
Baumeister and Tierney indirectly dealt with the criticism, writing:
Despite all these findings, the growing community of brain researchers still had some reservations about the glucose connection. Some skeptics pointed out that the brain's overall use of energy remains about the same regardless of what one is doing, which doesn't square easily with the notion of depleted energy. Among the skeptics was Todd Heatherton....
Heatherton decided on an ambitious test of the theory. He and his colleagues recruited dieters and measured their reactions to pictures of food. Then ego depletion was induced by asking everyone to refrain from laughing while watching a comedy video. After that, the researchers again tested how their brains reacted to pictures of food (as compared with nonfood pictures). Earlier work by Heatherton and Kate Demos had shown that these pictures produce various reactions in key brain sites, such as the nucleus accumbens and a corresponding decrease in the amygdala. The crucial change in the experiment involved a manipulation of glucose. Some people drank lemonade sweetened with sugar, which sent glucose flooding through the bloodstream and presumably into the brain.
Dramatically, Heatherton announced his results during his speech accepting leadership of the Society for Personality and Social Psychology … Heatherton reported that the glucose reversed the brain changes wrought by depletion, a finding he said, that thoroughly surprised him. … Heatherton’s results did much more than provide additional confirmation that glucose is a vital part of willpower. They helped resolve the puzzle over how glucose could work without global changes in the brain’s total energy use. Apparently ego depletion shifts activity from one part of the brain to another. Your brain does not stop working when glucose is low. It stops doing some things and starts doing others.
In an hour of searching, I couldn’t find a publication arising from this particular study - happy for any pointers. (Interestingly, Demos is author of a paper on a failed replication of an ego depletion experiment.) I’m guessing that the initial findings didn’t hold up.
Given the challenges to ego depletion theory, it seems Baumeister is considering tweaking the theory (I found an ungated copy here). If you want a more recent, although not necessarily balanced view on where the theory is at, skip Willpower and start there.
For another perspective on Willpower, see also Steven Pinker’s review.