“Strongest evidence yet” for ego depletion – the idea that self-control is a limited resource

For years, "ego depletion" has been a dominant theory in the study of self-control. This is the intuitive idea that self-control or willpower is a limited resource, such that the more you use up in one situation, the less you have left over to deploy in another. It makes sense of the everyday experience of when you come home after a hard day at the office, abandon all constructive plans, and instead binge on snacks in front of the TV.

The trouble is, the theory has taken some hard knocks lately, including a failed joint replication attempt by 23 separate labs. Critics have pointed out that most supportive studies – and there are over 200 of them – are small and underpowered. A meta-analysis that corrected for a positive bias in the existing literature concluded that ego depletion is not real. A study in India – where there's a cultural belief that exercising self-control is energising – even found evidence for "reverse ego depletion".

It's not easy to weigh the evidence for and against, but perhaps the science is tipping back in favour of ego depletion. Two new studies, made publicly available on the PsyArXiv preprint website, provide what the researchers at Texas A&M University, led by Katie Garrison, described as "the strongest evidence yet of the ego depletion effect".

The studies involved relatively large samples and were preregistered, meaning the methods and hypotheses were made publicly available prior to data collection, a practice that reduces the risk of false-positive or spurious results.

The standard format in most research into ego depletion is that one group of participants perform a particularly challenging version of a mental task, designed to drain their self-control, then they perform a second task. A control group performs an easy version of the same initial task, then performs the exact same second task as the first group. Inferior performance on the second task by the "ego depleted" group compared with the control group is taken as evidence for ego depletion.

Garrison and her colleagues followed this format. In the first study with 657 student participants, the first task involved either writing for five minutes about a recent trip (easy version) or writing about a recent trip without using the letters "A" or "N" (i.e. a more difficult version requiring more self-control). The writing task was followed by one of two versions of the Stroop task: either participants had to name the ink colour of colour-denoting words, such as the word "red" written in blue ink, or the ink colour of emotional or neutral words. It takes a degree of self-control to ignore the meaning of colour words, or emotional words, and focus on the ink colour.

Participants who completed the more difficult version of the writing task responded just as fast, but made more mistakes on the Stroop tasks than the control group. "This pattern represents unambiguous evidence for poorer attention control under ego depletion," the researchers said.

A second study was similar. Over 350 participants completed either the easy or difficult writing task and then the Attention Network Test, which involves repeatedly indicating the direction of target arrows on a computer screen, while ignoring the direction of adjacent, distracting arrows, which either face in the same or a different direction (the task is trickier when they face in a different direction). Participants who completed the harder writing task made more errors on the Attention Network Test, which is again consistent with ego depletion theory.

There is a complication with the second study. When the researchers removed outlier participants, as they said they would in their preregistered plans (for instance because participants were particularly slow or fast to respond, or made a particularly large number of mistakes), then there was no longer a significant difference in performance on the Attention Network Test between participants who'd performed the easy or difficult version of the writing task.

However, it's notable that more participants who'd completed the difficult writing task were excluded for making excessive amounts of errors on the attention test, which is again as would be expected based on ego depletion theory. Moreover, combining results from the two studies, there was an overall small, but statistically significant, ego depletion effect even after removing outlier participants (and this was after only a five-minute self-control challenge, so you can imagine the effects being larger after more arduous real-life challenges).

If it can be established that ego depletion is a real thing, a question remains about how exactly it occurs, such as whether it's due to running out of self-control, or perhaps more to do with motivation or conserving energy. "The answer remains to be seen," the researchers concluded, "but the current experiments provide new evidence that this is still a question worth asking".