News in, its official. It appears that people find you more attractive if members of the opposite sex smile at you. According to British psychologists it is because we subconciously take account of our sexual competitors, when judging a potential partner. This has been shown by studies taken at Aberdeen University. In the test, women saw men as much more attractive when they had been shown images of other women smiling at them. It is believed by psychologists that men respond in the same way as women have been exposed to behave. 
Ben Jones, author of the paper in the Royal Society's biological research journal says," so when you're out in a bar, there is an advantage to being nice to everyone, because if they are smiling at you, its going to make you more attractive."
Our opinion on the matter is that even if your only being nice to people for superficial reasons and your own benefit the world will still essentialy have ugly and attractive people just in differetn proportions and possibly those less fortunate could get with the person of their dreams if they just smile. So smiles could get you 'air' miles.