cognitive psychology
Zeigarnik effect
We remember - and keep feeling pulled towards - tasks we've started but not finished. An open loop sits in working memory and nags for attention until you close it; the moment you complete it, the mind quietly lets go.
The demo
A tiny three-step setup. Start it, then mark each step done - but watch what the one unfinished step does to your attention while it's still open.
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What this demo shows (text version)
You're given a three-step setup with a progress ring, and you mark each step done in turn. As you go, a running message names how many steps are still open.
The thing to notice is the unfinished step: while it's open it keeps pulling at your attention, where the steps you've already completed go quiet the instant they're done. The list itself becomes a small open loop that wants closing. That tug is the Zeigarnik effect - the mind holds an unfinished task in working memory and keeps prompting you about it until the loop is closed, which is why a half-filled progress bar is so much harder to walk away from than an empty or a full one.
That itch while one step sat unfinished - the way your eye kept sliding back to it - was the whole effect. The steps you'd already done went silent; only the open one tugged. Close the loop and the pull vanishes, which is exactly why progress bars and "profile 80% complete" nudges work so well.
The dark turn is easy to feel once you've named it: the same pull that helps someone finish a useful task is what keeps them scrolling a feed that's built never to close. Zeigarnik is a tool, and like most things in here it can be aimed at the person instead of for them.
My rule: open a loop only when closing it serves the person, not just the metric. A checkout progress bar earns its nag - it's helping you finish what you came to do. A streak you'll lose unless you open the app tonight is the same psychology pointed the other way.