Men seem to prefer women with a low waist-to-hip ratio. Women prefer men with optimally long jaws. For reasons we don't entirely understand, humans find symmetrical faces consistently bewitching. A 2005 study found women can accurately guess the symmetry of a man's face just by smelling his tee-shirt. These calculations are made with breathtaking speed. We decide whether we like a face in no more than 13 milliseconds, according to a 2005 study. (That's 30x faster than an average blink.)