WASHINGTON — So a scientist walks into a shopping mall to watch people laugh. There's no punchline. Laughter is a serious scientific subject, one that researchers are still trying to figure out.

‘LAUGHTER’ is a word, we are told by the philologists, that is a distant cousin of the Greek κλωσσϵιυ, ‘to cluck like a hen,’ and also of κρáζϵιυ, ‘to croak.’ But we need not go any further than our ...