What's more, women are especially attuned to the importance of kissing during fertile phases of the menstrual cycle. A study by Oxford University researchers suggests kissing helps us size up potential partners and, once in a relationship, may be a way of getting a partner to stick around. "Kissing in human sexual relationships is incredibly prevalent in various forms across just about every society and culture," says Rafael Wlodarski, the DPhil student who carried out the research in the Department of Experimental Psychology at Oxford University. Pucker up 
 
 The latter finding suggests kissing serves a pair-bonding purpose, helping couples show affection and commitment. To understand more, Rafael Wlodarski and Professor Robin Dunbar set up an online questionnaire in which over 900 adults answered questions about the importance of kissing in both short-term and long-term relationships. Rafael Wlodarski explains: "There are three main theories about the role that kissing plays in sexual relationships: that it somehow helps assess the genetic quality of potential mates; that it is used to increase arousal (to initiate sex for example); and that it is useful in keeping relationships together. The researchers report their findings in two papers, one in the journal Archives of Sexual Behavior and the second in the journal Human Nature, both published by Springer. 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. Overall, women valued kissing more than men (no surprise), but guys who rated themselves as more attractive than others or who had lots of girlfriends also placed more emphasis on the smooch. “Mate choice and courtship in humans is complex,” Dunbar said in a statement discussing the work. "It involves a series of periods of assessments where people ask themselves 'shall I carry on deeper into this relationship?' '” The early judgment calls are all based on facial, body and social cues. “Then assessments become more and more intimate as we go deeper into the courtship stages,” he said, “and this is where kissing comes in.” 
 
 How does a kiss determine mateworthiness? (MORE: The Truth About College Hookups) 
 
 Women in the study also ranked kissing as more important for longer relationships, suggesting it was a way of communicating and enhancing affection and attachment. While high levels of arousal might be a consequence of kissing (particularly as a prelude to sex), the researchers say it does not appear to be a driving factor that explains why we kiss in romantic relationships. Other findings included: 
 
 * In short relationships, survey participants said kissing was most important before sex, less so during sex, was less important again after sex and was least important at other times. In fact, more kissing was strongly correlated than more sex with a higher-quality relationship (although the people who had a lot of both had the highest relationship satisfaction). 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. 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. 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.
– A new scientific study delves into that most important of questions: Why do humans kiss? It's a practice carried out by almost no other animals, especially not with the same level of intensity, Time notes. But researchers found locking lips may actually serve an evolutionary purpose: It helps us assess potential mates and weed out the lesser options, LiveScience reports. That explains why women (who are pickier than men when it comes to mating) and people who rate themselves as attractive (who are also pickier) said they found kissing more enjoyable and important than their male and/or less attractive counterparts, Science Daily reports. It also explains why women find kissing especially important when they're at their most fertile. One theory: Kissing might transmit pheromones or chemical information—some of which can be transmitted through smell—about a partner's health, fertility, or genetic and immune compatibility. As one researcher says, "It's just an excuse to get two people who are interested in each other close enough to have a sniff." Says another, "Mate choice and courtship in humans ... involves a series of periods of assessments where people ask themselves, 'Shall I carry on deeper into this relationship?'" Kissing can help answer that question. (And once a partner is found, one writer suggests applying the principles of software code to the relationship.)