Historian Gary Willis tries to puzzle it out:
Everyone has noticed by now the non-laugh laugh of Mitt Romney, a kind of half-stifled barking. But what does it mean? It is blurted out as abruptly as it is broken off. Is it a kind of punctuation, part comma, part full stop, part interrogatory mark? What, if anything, is it trying to convey? Why does it seem more like coughing or burping than laughter?
Willis ends up comparing Romney’s “comic rictus” to Dostoevsky and The Idiot.
Andrew Sullivan goes deeper.