The researchers found that subjects who believed that smiling reflected actual happiness reported feeling better when they smiled, because, it follows, they were actually happy when they were smiling.
However, for those who believed that smiling reflected fake emotions, or was just an attempt to improve mood, smiling actually made them feel less happy. So, forcing a grin made them feel worse than they otherwise would have.
While previous research has suggested that smiling in itself can actually make you feel better, these results show that when it comes to putting on a happy face, intention matters. If you're not flashing your teeth out of genuine joy, chances are you'll end up feeling like a big fat phony. And, there's the added downside that whoever you're flashing 'em at can probably see right through you, anyway. Something to consider next time you're about to grin and bear it.