So we set up our experiment to pit these three against each other. At a lab meeting, Dianne Tice, my wife and longtime colleague, suggested confronting people with radishes and chocolates and telling them, "We really need you in the radish condition" to deplete their willpower before they solved some puzzles. Essentially, we had people engage in one self-control task and, afterward, had them do something quite different that still required self-control.

What made your study stand out?

This sort of theory was very far from the reigning models and theories in psychology when we started this work in the mid-1990s. Everyone was talking about cognitive processes and using information-processing models. Nobody talked about energy. In fact, when I first began to present this work to scientific audiences, I used to joke, "Energy models are so out of fashion that we're not even opposed to them anymore!" They were associated with obsolete Freudian theories about clashing instincts and the like. Our group was actually not very happy about these findings, because they really suggested we needed a theory based on energy, and we knew these would be tough to sell. When I was looking for a term to describe these effects, I settled on "ego depletion" as homage to Freud, because as far as I could ascertain he had been the last major theorist to say that the human self was made partly of energy and worked by energy processes.

As a researcher on self and identity, a common refrain I've heard at conferences in the late 1980s and early 1990s was, "We really don't understand the so-called agent aspect of the self." There was plenty of research on self-concepts and self-esteem, and even some on how people manage the impressions they make on others. But how the self does things was by all accounts still a deep mystery. I think that was because one needs energy theories to understand it, and, as I said, nobody in mainstream psychology was using energy theories.

As luck would have it, the 1990s saw psychological theorizing shift away from emphasizing information processing toward incorporating more biology-based ideas. So the basic, immediate skepticism that greeted our early findings gradually subsided. Nowadays, energy theories are becoming respectable again, and it's nice to think that our findings on ego depletion have contributed to their greater acceptance. By now, however, they've been replicated and extended in many different laboratories, so I am confident they are real. In fact, there was recently a meta-analysis, which I had nothing to do with, confirming the reality of ego depletion across dozens of studies.

How far has our knowledge of willpower come since, and what questions remain?

There have been several leaps forward, adding wholly new dimensions to our basic finding. For one, we found that willpower gets depleted not just by acts of self-control but also by other key things the self does: making choices and decisions, exerting initiative, perhaps planning and executing plans. This has led us to cast about for an even bigger umbrella than self-control, and it has gotten us into the interdisciplinary debates about free will. Some new work being published this year also showed that people suffer from ego depletion on an almost daily basis, outside the laboratory, and away from experimental manipulations, in the context of their everyday lives and normal activities. This has been a very welcome extension.