Guwahati, Sept. 29 -- The morning sun at Tokiwa Park in Ube City, Yamaguchi Prefecture, does more than illuminate the emerald waters of Tokiwa Lake. It falls on a kind of social contract so smoothly that it barely registers as one. Families spread mats across lawns that look tended by a jeweller rather than a gardener. Children chase the park's well-known pelicans, their laughter carrying through a sculpture garden that hosts the Ube Biennale. But for a visitor arriving from the streets of Guwahati, what stands out most is what is not there: there are no litter bins.

Hundreds of people picnic, walk and play here on any given day, yet there is not a stray candy wrapper or discarded PET bottle to be found. People simply tuck their own wast...