Does Self-Control Deplete Over the Course of the Day? Maybe Not, Says New Study

For months now, I’ve been trying to cut out sugar from my diet. I’ve read about all the ways my sweet tooth will be the death of me, and I’ve resolved to give it up. And yet, even as I write this, my long-term goal to eat healthy is losing out to my eternal desire to eat M&Ms at my desk. Is it because it’s the end of the day, and I’ve been trying to make choices for eight hours already? Or is it something else?

A new study in PLOS One pushes back on the popular theory known as "ego depletion," which hypothesizes that self-control is a finite resource that depletes throughout the day, much like energy levels. Instead, researchers from the University of Toronto and the learning technology company Cerego found that people's self-control depletes when it comes to doing one task for a long period of time, but that self-control fatigue isn't a factor when you're switching tasks. In other words, it's hard to say no to the box of cookies all day long, but saying no to the box of cookies won't impede other acts of self-control, like your ability to focus on your homework instead of turning on the TV.

The study used data from Cerego, which publishes online study materials, examining the study behaviors of two groups of college students using the Cerego system as part of semester-long psychology courses. The researchers looked at data from two groups of users, one group of 8700 students and one of almost 8800, focusing on how long they worked during each session and how well they performed at the memory tests within the curriculum.

If self-control really is a finite resource, it should be depleted by the end of the day, after people presumably have spent many hours resisting their first impulses in one way or another. But the researchers found that this wasn't true. Overall, students didn't do any better if they used the program earlier in the morning. Instead, performances peaked around 2 p.m., and people logged in to use the software more and more as the day went on, suggesting that the motivation to learn doesn't fall off at night (though that may also be because that's when college students do their homework in general).

However, mental resources did seem to be drained by doing the same task for a long period of time. The researchers found that after a certain point, students' performance dropped off, peaking at about 28 minutes of work. They made about 5 percent more mistakes 50 minutes into the session compared to that peak.

When it comes to the idea that we exhaust our store of self-control, the authors write, "the notion that this fatigue is completely fluid, and that it emerges after minutes of self-control, is under considerable doubt."

The notion of ego depletion comes from a 1998 study in which researchers asked participants to hang out in a room full of fresh-baked cookies, telling them to eat only from a bowl of radishes, leaving the cookies untouched. Then, those volunteers worked on an impossible puzzle. Volunteers who had spent time avoiding the delicious pull of cookies gave up on the mind-boggling task an average of 11 minutes earlier than a group of volunteers who were brought into the same room and allowed to eat as many cookies as they wanted. (Lucky them.)

Since then, the idea has taken off, leading to hundreds of subsequent studies and even influencing the habits of people like Barack Obama, who told Vanity Fair in 2011 that he only wore blue or gray suits in order to cut down on the non-vital decisions he had to make throughout the day.

This current study isn't the first to challenge the theory’s veracity, though. In 2016, a 2000-person replication study by some of the same authors (with scientists in 23 different labs) pushed back on the theory of ego depletion, finding that short spurts of self-control didn't have any effect on subsequent tasks. This study just adds to the evidence against the well-established idea.

So it's looking more and more like ego depletion isn't a good excuse for my afternoon vending-machine habit. Perhaps the true secret to excellent self-control is this: Just be a raven.