A phenomenon called group glee was studied in videotapes of 596 formal lessons in a preschool. This was characterized by joyful screaming, laughing, and intense physical acts which occurred in simultaneous bursts or which spread in a contagious fashion from one child to another.
Science!
See also:
- Radiolab’s 2008 episode on Laughter
- The 1962 Tanganyika ‘laughter epidemic’
- The Giggle Loop
(Via NCBI ROFL, which I guess is appropriately named in this case)