You've got to kiss a lot of frogs to find your prince, as the saying goes. New research suggests the cliché is true on an evolutionary level.
Kissing might have evolved as a way to assess the quality of potential mates, according to two new studies. Women, who tend to be pickier about romantic entanglements than men, also care more about kissing in the first phases of a relationship, suggesting that make-outs may weed out duds. What's more, women are especially attuned to the importance of kissing during fertile phases of the menstrual cycle.
Kissing exists in virtually every culture on Earth, said study researcher Rafael Wlodarski, a doctoral candidate at the University of Oxford. Some of the oldest records left by humanity, including the Hindu Veda and ancient Egyptian wall murals, depict kissing.
"Because it's so common," Wlodarski told LiveScience, "it might serve a purpose."
The evolution of make-outs
Theories about why kissing matters fall into three categories. Some believe kissing evolved to help people assess potential mates, perhaps by transmitting pheromones, or chemical signals that could carry information about health or immune compatibility. [50 Sultry Facts About Sex]
"It's just an excuse to get two people who are interested in each other close enough to have a sniff," Wlodarski said.
No particular compound has been proven to be a human pheromone, but there is evidence that scent carries information. One study published in April 2013 found that women prefer the scent of men who have high levels of the masculine hormone testosterone.
Kissing also may have evolved to keep romantic pairs bonded, or to increase arousal prior to sex. To test these theories, Wlodarski and his colleagues recruited 902 American and British adults to answer questions about their attitudes toward kissing.
The participants rated how important they considered kissing at various stages in relationships. The approximately half of participants who were in relationships also reported how much they and their partners kissed, and how satisfied they were in the relationship.
The results gave little support to the notion that kissing evolved to ease the way to sex (even if it may often be used that way). People in short-term relationships saw kissing as most important right before sex, but there was no other indication that people use kissing primarily as a sexual warm-up act. In fact, people in relationships closely associated the amount and quality of their kisses with relationship satisfaction. The more kissing, the happier they were. The amount of sex, on the other hand, wasn't related to relationship satisfaction at all.
The latter finding suggests kissing serves a pair-bonding purpose, helping couples show affection and commitment. But kissing also seems to help people gauge relationship potential.
If kissing is a way to assess mates, the pickiest people should place the highest importance on kissing. This appears to be the case: Women, who take on the risk of gestating, birthing and caring for a child when they have sex, are generally more choosy about mates than men. They're also more likely than men to rate kissing as important, and more likely to say that an initial kiss had changed their attraction to another person, Wlodarski and his colleagues found. [Busted! 6 Gender Myths in the Bedroom & Beyond]
People who rated themselves as attractive — and thus who likely can afford to be picky — were also the most interested in kissing and the most likely to say that a kiss could sway their perceptions of attraction. Wdolarski and his colleagues report these findings in an upcoming issue of the journal Archives of Sexual Behavior.
A second study by the researchers, this one published in the September issue of the journal Human Nature, examined only female attitudes toward kissing. If kissing communicates some information about health, fertility or genetic compatibility, the thinking went, women who are at risk of conceiving are more likely to think kissing is important — after all, they might end up with a baby if the romance goes well.
The researchers surveyed 84 American and British women, asking them to report the dates of their menstrual cycle and to answer questions about how important kissing is in various stages of a relationship. Fifty of the women were in the luteal, or less-fertile, phase of their cycle, and 34 were in the late follicular phase, the point at which fertility peaks.
The most fertile women were more likely than the least fertile women to say that kissing in the early stages of the relationship is important, lending credence to the idea that they might be subconsciously sniffing out the best genes for their potential offspring. Both groups were equally likely to say kissing later in a relationship is important, potentially pointing to kissing's bond-cementing role.
"At different times in the relationship, [kissing] is used for different things," Wlodarski said. He next plans to move beyond kissing into even murkier depths.
"I'm interested in doing more research on what love is in humans," he said. "What is it that makes us so intimately attracted to one specific person?"
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Stephanie Pappas is a contributing writer for Live Science, covering topics ranging from geoscience to archaeology to the human brain and behavior. She was previously a senior writer for Live Science but is now a freelancer based in Denver, Colorado, and regularly contributes to Scientific American and The Monitor, the monthly magazine of the American Psychological Association. Stephanie received a bachelor's degree in psychology from the University of South Carolina and a graduate certificate in science communication from the University of California, Santa Cruz.