According to a ScienceDaily article here, (Link) suggest that individual that underwent rejection can spot insincere smile easier than those who didn’t, like gut feeling. Socially rejected, off course, they experience so little genuine smile that made them suspicious to all. If they make a guess that all smiles are faked, sadly, they will get it correct as it is true. We are surrounded by them. It is the real one that is hard to decent.
Hold you trust till you see the deeds.