Still, chemistry can be complicated. A recent study at the University of Texas-Austin suggests that in dating interactions, men overestimate women's sexual interest, while women underestimate men's willingness to commit. (Evolutionary explanation: when it comes to the reproductive imperative, it's better for males to risk rejection than risk not spawning; for females, it's better to play safe and not be abandoned with cubs.)

Meanwhile, a study at Vienna's Ludwig Boltzmann Institude of Urban Ethology suggests that women don't necessarily send clear rejection signals. A drink in the face is one thing, but we're also conditioned to be polite—or at least to stick around to verify our first impressions.

Best news of all: sometimes we're wrong. My friend Jeanne hated Greg the first time they met, dismissing him as a cigar-smoking, suspiciously tan stockbroker. Of course, Greg is a cigar-smoking, suspiciously tan stockbroker, but after meeting him a second time, Jeanne warmed up to the personality underneath. And eventually married him.

Sometimes first impressions are only second best. Just be yourself. If you're shy, be shy; you'll attract people who like shy. Act loud, and you'll act weird. Even if at first you don't succeed, you will make the right impression on the right person.