Eli, who doesn't resemble James Spader, emailed me this:

"If you ever thought you might want to pick up the New York Review
of Books, now is the time; the most fun I had all week was reading
Joan Acocella's article in the December issue, about a study of
Italian hand gestures:"
"...you must contextualize [the 'mano cornuta'] with the
appropriate facial expression, so that you send the correct message.
When your daughter is going to the dance in a new dress, you want
your 'mano cornuta' to be signaling 'amulet' [against bad luck] and
not, for example, 'I will poke your eyes out' or, God forbid,
'phallus.'"