That makes sense, say the researchers, since both women and good-looking guys tend to be more picky about their mating partners. For women, bearing and raising children is a huge investment of their life and health, so they want to choose the right co-parent. As anthropologist Helen Fisher told U.S. News and World Report, “When you get a highly intelligent, pair-bonding species that requires years to raise a baby, you evolve more and more brain mechanisms to weed out the losers.” The hottie guys are picky because, well, they can be; they have a lot of choices, so they need a way to winnow them down. Kissing, says the Oxford team, may therefore be a way of trying on potential partners.

“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? It’s not really clear, but some philematologists (those are the people who study kissing) believe that it has to do with smell. In her book The Science of Kissing, Sheril Kirshenbaum cites Claus Wedekind, who she says found that “women are most attracted to the scent of men who have a very different genetic code for their immune system in a region of DNA known as the major histocompatibility complex.” Having different DNA from the individual you are kissing heightens the chance of having healthy offspring should the kissing lead somewhere. And juxtaposing two orbicularis oris muscles in a state of contraction is usually more fun than having a genetic test.