At a basic level, kissing is a biohazard. What is love then, if not the willingness to expose yourself to a host of nasty diseases lurking in your partner's mouth?
But could kissing also be a tool with a purpose?
Psychology graduate student Rafael Wlodarski, from the University of Oxford, wanted to find out. Results from his experiments supported two of the existing hypotheses about why we kiss. First, we kiss to assess potential mates. Second, we kiss the mate we've found to maintain attachment.
And while kissing may result in arousal, he says, it's probably not a driving reason why people in romantic relationships kiss each other. Killjoy.
So, how did Wlodarski figure this out? He posted a questionnaire on an online repository for psychology studies. For the chance to win an online shopping voucher, 900 people in about 20 countries responded to questions about how they value kissing in various romantic situations. Most of them were in the U.S. and the U.K.
Wlodarksi concluded from the response that men and women value kissing for different reasons. "Men are more likely to initiate kissing before sex, when it might be used for arousal purposes, whereas women are more likely to initiate kissing after sex, where it might better serve a relationship maintenance function," he writes.
A lot of good kissing was associated with a better relationship and satisfaction with the amount of sex. Lots of sex, however, wasn't related to the quality of the relationship. "There seems to be something special about kissing," Wlodarksi tells Shots.
Wlodarksi also found that, overall, women rated kissing as more important than men did. Women were also more likely than men to feel a change in attraction after a first kiss. Long-term partners said kissing was more important than did those in short-term relationships.
In a companion paper published in the journalHuman Nature, Wlodarski speculates that the hormone progesterone, which fluctuates with menstrual cycle, may guide a woman's attitude toward kissing.
Past research showed that when a woman's progesterone levels are high, she may prefer healthy-looking men with feminine faces, men who look like them and men with lower voice pitch. But birth control pills end up flattening out the natural monthly ebb and flow of female hormones.
Female study participants gave the date of their last periods and indicated if they were on birth control or not. If they took the pill or had irregular periods they were excluded from the analysis.
"Within naturally cycling women, there's actually a very regular pattern of hormonal release, especially of estrogen and progesterone," says Wlodarski, who was able to analyze results for about 170 women. "It correlated very highly with their attitudes toward kissing."
When a woman kisses a man, she might be subconsciously detecting and scrutinizing his chemical signals, generating some sort of instinctual health profile of the guy. Especially if she's in the stage of her menstrual cycle when she can get pregnant most easily.
Kissing could all boil down to smell, acting mostly as a way of getting people close enough to take a deep whiff of each other.
Past research — like a famed experiment that involved sniffing sweaty T-shirts — has shown that by smelling someone's odors, animals and maybe people can get a sense of a mate's health and genetic compatibility.
Mouth-to-mouth contact might give the tests an extra boost with taste tests of skin oils and saliva compounds.
"More often than not, kissing involves some sort of face-to-face contact, often that allows people to get close enough to smell each other," says Sheril Kirshenbaum, author of The Science of Kissing, a book about the history of kissing.
Kissing hasn't always been popular, and it hasn't always looked the same. When the Black Death struck in the Middle Ages, she says, kissing lost popularity. She says early explorers and tradesmen noticed alternative behaviors to kissing during their travels — like nipping eyebrows, licking or sniffing.
One of the earliest written mentions of kissing appears in the Vedic scriptures dating back to at least 1,500 B.C. "Kissing was initially described as inhaling the other person's spirit," says Wlodarski. And what's known as an Inuit kiss "involved placing your nose on a loved one's cheek or forehead and inhaling deeply."
In the end, kissing may be a good tool, but a good face sniff could work just as well.