TIL there are gendered differences in the sexual attractiveness of smiles among heterosexuals: while men are more likely to prefer women who look happy & least attracted to those who appear proud, women are less likely to be attracted to happy/smiling men & prefer those who look proud or brooding.
http://news.ubc.ca/2011/05/24/happy-guys-finish-last-says-new-study-on-sexual-attractiveness/