According to Glasgow-based Go Group, a smile, which appears too swiftly and does not extend to the eyes, can sends signals that you lack sincerity.
(They found that people should avoid three types of grin:
The Enthusiast - A huge smile showing all of your teeth and wide eyes can be overbearing.
The Big Freeze - A constant, practised smile lacks sincerity.
The Robot - A tiny, polite smile doesn't display warmth.
It seems there's more to smiling than meets the eyes, that said, the more you practise, the less sincere you look.)