This is a fascinating lecture by Dr Bruce Lipton on the biology of belief.
“Our beliefs are altering our biology at all times, in every way.”
“Under stress you are reduced to reflex behavior = your intelligence is reduced.”
“How I see things is affecting who I am. Beliefs act as a filter between your environment and your biology.”

This will give you the science behind the quote “stress kills, laughter heals.”

Part 1

Part 2

Related link: http://www.laughteryogaamerica.com/read/blog/world-peace-laughter-idea-potentially-biological-fact-time-changing-706.php