A fascinating sociological study of profile photos:
Now, you’re always told to look happy and make eye contact in social situations, but at least for your online dating photo, that’s just not optimal advice. For women, a smile isn’t strictly better: she actually gets the most messages by flirting directly into the camera [... However,] flirting away from the camera is the single worst attitude a woman can take. Certain social etiquettes apply even online: if you’re going to be making eyes at someone, it should be with the person looking at your picture.
Men’s photos are most effective when they look away from the camera and don’t smile:
2006-2011 archives for The Daily Dish, featuring Andrew Sullivan