In Western cultures, we mark the beginning of romantic entanglement by touching lips. Few actions are as fraught with anxiety and symbolism as that first kiss—and it’s no exaggeration to say that some kisses feel like life or death.
Indeed, a kiss can kill, in the most medical, un-romantic, non-metaphorical sense. Sticking your tongue in the mouth of another person for ten seconds can transmit 80 million bacteria, says one 2014 study. As if to drive this point home, last week the Brazilian government warned pregnant women to refrain from kissing for fear of passing the Zika virus.
But romantic kissing isn’t universal, not even close. In fact, many cultures consider kissing on the lips to be repulsive—a perfectly sound conclusion, given how much disease can be passed between mouths. The most authoritative cross-cultural analysis of kissing, published last year in the journal American Anthropologist, reviewed studies of 168 societies and found that less than half showed evidence of “sexual-romantic kissing,” as the authors call it.