Each decision costs you something. You compare the options, picture what happens next and put your name to one of them. Make enough of those in a row and the next call gets a thinner version of that work, so it drifts towards whatever is quickest to settle.
The idea grew out of Roy Baumeister's ego-depletion research in the late 1990s, which treated self-control as a limited store that use runs down. Baumeister and Kathleen Vohs later applied it to choosing, arguing that deciding draws on the same store. That leaves open whether decision fatigue is real, a question the evidence hasn't settled.
From the inside, a fatigued choice usually comes with a sound reason attached, such as keeping the plan you already have or approving a draft that is nearly there. Nothing about it looks careless, and on another day it might even be right. Because the reason is real, the drift is easy to miss.
The practical tell is the count. Before a call that matters, ask how many decisions you have already made that day. A long list is your cue to slow down, or to take the call when the list is shorter.
Tom emailed a strong new media kit to twenty advertisers, and not one replied. When I asked why he hadn't phoned them first, he couldn't say, so I asked what he'd already decided that day. He'd spent most of the morning and early afternoon in and out of meetings, each one needing a call from him, and by the launch he sent the kit whichever way cost him least. When he rang round, most hadn't noticed it in their inbox, and he pulled a few sales back.