It was eventually discovered that laughter provides significantly more power than screams. This revelation led the primary power company to transition from a "Scaring" floor to a "Laugh" floor, fundamentally changing monster society. Franchise History & Timeline

The original 2001 film where the duo discovers that human children are not dangerous after a toddler named "Boo" accidentally enters their world.

In the Monsters, Inc. universe, the city of Monstropolis is a thriving society powered by the energy harvested from the human world.