While there are a variety of reasons for smiling, happiness is generally thought of as the most common cause. In a Canadian study (via CBC News), participants were first shown the same member of the opposite sex in four distinct poses — happy, proud, ashamed, and neutral — and asked to rate their attractiveness. Happy was illustrated with a smile, proud with arms raised above the head, ashamed with downward-facing eyes, and neutral with a vacant stare. In the end, the male participants rated smiling (“happy”) women as the most attractive.
Although the women who participated in the study actually found “proud” men to be the most attractive, “happy” men were only “about one point lower” in attractiveness. Out of all the female participants, younger women rated smiling men the lowest whereas older women found “happy” and “ashamed” men to be pretty much equally attractive. Nevertheless, happiness — and its hallmark sign, the smile — are undeniably attractive.