- The OED keeps things unhelpfully general: "a look or expression implying derision, contempt, or scorn".
- Merriam-Webster Online is slightly more specific: "an expression on a person's face that is like a smile but that shows dislike and a lack of respect for someone or something".
- But it's the Free Online Dictionary that goes into most detail: "A scornful facial expression characterized by a slight raising of one corner of the upper lip."
Neither Merriam-Webster nor FOD mentions the eyebrow, which - as the word "supercilious" suggests - is also an important component of the sneer, at least in my book.
By way of
a) The largest group conforms to the FOD's "asymmetric" sneer theory.
Interestingly, all but one appears to sneer on the left side of their mouths. I wonder if this is related to handedness? [ETA: I've checked, and Angelina Jolie, the one exception, is left-handed. Go figure.]
b) Next come the snarly sneers:
Hello again, Mr Rumsfeld.
c) Next, a small clutch of disgusted sneers:
d) An incredulous sneer:
e) And finally, the one picture that I would call a True Sneer:
I'd be interested in which of these you would describe as sneers. Is there a transatlantic difference, I wonder?