Attention Restoration Theory
Attention restoration theory holds that directed attention is a resource spent down over the course of a day, and that natural environments replenish it in a way built environments do not. It was formalised by Rachel Kaplan and Stephen Kaplan. The theory splits into two claims that are doing very differently, and the vault treated them as one until 2026. Nature reliably makes people feel better. It does not reliably make them think better.
What it is
The mechanism the Kaplans proposed is a distinction between two modes. Directed attention is effortful, goal-driven and depleting. Soft fascination is involuntary and cheap, the kind of holding that a moving stream or a canopy of leaves does without being asked, and because it runs on the automatic system it leaves the directed system alone rather than asking it to work harder somewhere prettier.
The older line of evidence is Roger Ulrich's 1984 study, in which surgical patients recovering in rooms with a window view of trees needed less pain medication and left hospital sooner than patients facing a brick wall. The sample was small and the finding has been conceptually replicated since. It is a claim about recovery rather than about attention.
The study the attention half of the theory rests on is Marc Berman, John Jonides and Stephen Kaplan's 2008 experiment, in which participants walked either a wooded arboretum path or a busy city centre and were retested on working memory. Several books in this library now build on that one modest experiment, which is a good illustration of why cross-source corroboration is not independent evidence.