fear response ... kinda like you're around a group of people...you tell jokes to break the ice... but everyone is afraid of the alpha male ahole who holds the upper hand psychologically ... or whomever is perceived in the power position ... or ... you're in a very nervous situation and you laugh uncontrollable...but you're really scared out of your mind ... or ... someone is talkiong to you and they laugh after nearly every sentence, even though its not funny ... or isn't necessarily a funny topic ... yeah ...nervous laugh..I call it the , what the h--- is funny here, smile and laugh for nothing idiot...but at least they're smiling instead of grumping/ yeah, I can see that response being from fear :) and gorillas do have facial expressions ... some humans do too ...less and less with all the botox being injected.

Humankind has not woven the web of life. We are but one thread within it.
Whatever we do to the web, we do to ourselves. All things are bound together.
All things connect......Chief Seattle, 1854