Matthew Parris in today's Times:

"You know that funny thing the Prime Minister does with jaw and tongue while talking: dropping the jaw between words, as though tongue were being plunged into nether gums? I’d never observed the habit in any other human being until doing my homework for this week’s Great Lives programme on Radio 4. Our chosen (fallen) hero was the 20th-century American playwright Tennessee Williams; and, though I love Williams the playwright, I knew little about the man. So I watched a 1970s television interview, where a young and fresh-faced David Frost flattered and prodded the sadly burnt-out writer.

And guess what? Tennessee Williams had exactly the same jaw-dropping, tongue-diving facial tic. This looks like a nervous rather than physical complaint.

Something in Williams’s demeanour sparked in my imagination a hypothesis as to the possible psychological cause. Could the gesture be an aborted sticking-out of the tongue? The reaction of an insolent boy who’s been threatened with a smack the next time his tongue pokes through his lips, and so contorts it instead? Try it. Push the tip of the tongue between your top and bottom teeth as though about to stick it through your pursed lips — then abort the procedure at the last minute by diverting the tongue down inside the bottom lip, into the lower jaw. That’s what both men seem to be doing.

Basically, they’re sticking out their tongues at us."