In a Dark-Arts-y way, glasses?
(A brief search indicates there are several studies that suggest wearing glasses increases percieved intelligence (e.g. this and this (paywall)), but there are also some that suggest that it has no effect (e.g. this (abstract only)))
There definitely exists a stereotype that people that wear glasses are more intelligent. The cause of this common stereotype is probably that people that wear glasses are more intelligent.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
(I plan to make these threads from now on. Downvote if you disapprove. If I miss one, feel free to do it yourself.)