David Epstein shares (in Inside the Box) the story of a famous playground study:
Teachers were directed to take preschool children to local playgrounds that had no designated boundary, and then to playgrounds that had a clear boundary marked by a fence. At the no-boundary playgrounds, the children clustered around their teacher. At the fenced-in playgrounds, the children felt safe to explore, so they left the teacher and roamed widely. Clear boundaries liberated them to experiment.
Unfortunately, in following up on the suggestions to check out that study, I concluded that it doesn’t exist. I traced the story back to an undergraduate project on public spaces that won a student award. That student is now a professor of landscape architecture, and when I asked him about it, he said that he had (understandably) taken the word of two child psychologists about it back in his college days, but had never been able to find an actual study and regretted perpetuating it.