Main idea

Three volunteers are overseeing a building's renovation. One thinks everyone understood the aerial lift had been pushed back. Another believes the fifth-floor leak was already mentioned by email. The third assumes his three-week reading backlog hasn't made him miss anything important.

Three reasonable assumptions, and three ways to make a bad decision at the next meeting. Each member holds a different piece of the case, and nobody knows what the others don't know: the gap is visible from nowhere, since each person files it in their own head as something obviously shared.

Why it matters

The flaw comes from no individual negligence. It arises from the distribution itself: the more information travels through personal channels, the more each person overestimates what the others know. That's why good-practice reminders — "keep each other posted" — don't fix it.

This gives a criterion: a system for collective follow-up is judged by its ability to make visible what one person knows and the others don't, not just by its ability to store what each person knows.

Nuances and limits

Some of this asymmetry is healthy: not everyone needs to know everything. The risk lies in what a shared decision depends on.

Open questions

• How can you spot, before a meeting, the information each person believes is shared and isn't?