The New York Post highlights some key ideas in Sheril Kirshenbaum's new book The Science of Kissing: What Our Lips Are Telling Us, which explores the history and biology behind mankind's most recognizable gesture of affection.
So why do we kiss?
[T]he most important and obvious reason why we kiss is that it facilitates reproduction. Women, who according to studies place more emphasis and importance on a kiss, use the mouth-to-mouth moment as a way to judge the taste of the tongue, lips and saliva to see if she is with an adequate mate.
Sense of smell doesn’t just provide a window into hygiene habits, it also gives women access to the unseen DNA of their chosen mate. According to recent studies, women can smell when a man’s group of genes that manage the immune system, called MHC, are matched well to her own. Scientists theorize that kissing may be so ubiquitous because it gives women an instant check on if there is chemistry, literally (or less poetic terms, if they would make good children together).
That's a biological explanation for why we kiss, but in human culture, the kiss serves a host of other functions as well. In early Greek and Roman history, kisses had as much to do with master-slave relationships as with affection. In the middle ages, illiterate people signed their names on official documents by drawing an "x" and kissing it to make it authentic—hence the symbol of "x" for a kiss in "xo." The book also examines everything from the kiss's first appearance in writing in 1500 B.C.'s Vedic Sanskrit texts in India to Steve Carell and Tina Fey's five-minute marathon embrace beneath the closing credits of the movie Date Night.