In experiments first reported in 1998, Baumeister and his collaborators discovered that the will, like a muscle, can be fatigued. Immediately after students engage in a task that requires them to control their impulses — resisting cookies while hungry, tracking a boring display while ignoring a comedy video, writing down their thoughts without thinking about a polar bear or suppressing their emotions while watching the scene in “Terms of Endearment” in which a dying Debra Winger says goodbye to her children — they show lapses in a subsequent task that also requires an exercise of willpower, like solving difficult puzzles, squeezing a handgrip, stifling sexual or violent thoughts and keeping their payment for participating in the study rather than immediately blowing it on Doritos. Baumeister tagged the effect “ego depletion,” using Freud’s sense of “ego” as the mental entity that controls the passions.