The following TED Talk gives us a lesson on how improving the appearance of an environment can influence quality of life. I myself have noticed that graffiti tends to be only on badly-kept or abandoned properties, even to the point where an old railroad embankment where murals had been painted had the artwork surrounded — but untouched by — graffiti.
This also gives us hints about shaping our society without force or other coercion. “Perception is reality” is often a phrase used in politics, and we humans live and breath in a truly subjective world.
We may as well use it for good.
You may wish to check out the Situationist International’s early work on the concepts of psychogeography (the psych-emotional effects of the built environment on individual and collective consciousness), best achieved by derives (spontaneous, unplanned, and unguided journeys through the city (in the SI’s case, mainly Paris).