Laughter's Function

1) Laughter is a waste of breath.  To laugh is to show, simultaneously, that we don't need the breath for something else, mainly running

2) Laughter shows vulnerability.  When we want to signal comfort, we can use laughter to do it.  We do not have to use breath to run away = we are comfortable

3) However, laughter also can show dominance; it can be used as a show: I don't need to use this breath to run because I'm not scared of you.

When we are nervous, we often laugh to try for a return laugh, which could signal comfort.  We are also trying out a little dominance; we are testing the waters for a response.