The advice at the heart of nearly every productivity system is the same: get it out of your head and write it down. It’s not just good advice, it is a critical step. But it comes with an unforeseen cost: the list grows, and grows. And a growing list doesn’t just get longer. It gets harder to manage.

Here’s why. Your working memory, the mental workspace where you actively hold and compare information, is small. Startlingly small. Cognitive scientists have been measuring it for decades, and the ceiling keeps coming in at just a handful of items at a time. That’s the entire workspace you have for weighing options against each other.

Now picture what scanning a 200-item task list actually asks of that mental workspace. To choose well, you’d need to compare each task against the others: is this more important than that? Is this one blocked? When is that one actually due? Every comparison loads more into a workspace built for a handful of items, and past its limit, the workspace doesn’t stretch, it spills. You don’t hold more; items start to fall out, and you never notice which ones. It’s like trying to judge a talent show where two hundred contestants perform at once, on the same stage, all singing over each other. The evaluation doesn’t get harder in proportion to the list, it collapses entirely. So your brain does what brains do when a task is impossible: it substitutes an easier one. Instead of “which of these two hundred tasks matters most,” you answer “which of these tasks do I remember, or notice, or feel like doing.”

And there’s a second cost stacked on top. Every one of those what-should-I-do-next moments is a decision, and decision quality is not a fixed resource. It degrades with use. This is decision fatigue, and it’s been observed everywhere from consumer research to a famous study of parole judges, whose rulings got measurably harsher as the session wore on and snapped back after a break. Whether or not tired judgment works exactly the way early researchers thought, the practical pattern is one you already know from your own life: by late afternoon, “what should I work on” gets answered with “whatever’s easiest.” And fatigue doesn’t just lower the quality of the choice, it changes how you choose at all. A tired brain stops weighing options and starts reaching for shortcuts: the task already top of mind, the one you were just working on, whatever keeps you on your current path. Not the important thing, the findable thing. A fat list doesn’t just make each decision harder. It makes you pay that harder price over and over, dozens of times a day, from a budget that runs down as the day goes on.

Put those two together and you get the fat list’s signature trait: the more faithfully you capture, the more the list demands exactly the cognitive resources it was supposed to protect. Past a certain size, opening the list costs more than it gives. So you stop opening it because the list is no longer helping you decide what to do next. You are. And once you’re the one deciding, the important tasks start getting lost in the noise, so you stop relying on the list and start relying on your gut about what needs doing. The list is still there getting more and more out of sync with reality.