Full Idea
Man is the only animal that laughs, so a starting point for all enquiries into laughter must be the hypothesis that it is an attribute of reason (though that gets us no further than our definition of reason).
Gist of Idea
Since only men laugh, it seems to be an attribute of reason
Source
Roger Scruton (Laughter [1982], §1)
Book Reference
Scruton,Roger: 'The Aesthetic Understanding' [Methuen 1983], p.152
A Reaction
I would be inclined to say that both our capacity for reason and our capacity for laughter (and, indeed, our capacity for language) are a consequence of our evolved capacity for meta-thought.