I was at a public lecture and later saw photos of it. In one photo I held my index finger across my lips. A guy sitting before me, whose face I couldn't see at the time, did the exact same gesture, and the resulting photo looks pretty funny, as if we're all consciously posing to signal that we're thinking real hard.

There are other poses of this sort, the thinking monkey meme, Rodin's Thinker statue... even the sidebar of this very sub has a picture of a lemur whom we recognise as "thinking" because it has its hand on its mouth.

Are these gestures just culturally conditioned? But regardless of whether they're just cultural or universal – why do they exist?