There’s probably some scientific evidence to back this up somewhere.
If you smile at someone with a genuine, heartfelt smile – they will smile back.
It works 9 times out of 10. I don’t know if it’s an inborn trait or cultural conditioning. I know there are people out there a lot smarter than me who can answer all the questions about “why” this works. All I have to go on is my daily anecdotal experience.
Oh…and I should mention here – it seems to work with frowns as well.