From a New Yorker interview with Sheril Kirshenbaum, author of The Science of Kissing in 2011.
In a good kiss, our pupils dilate, which is one of the reasons we close our eyes, our pulse quickens, and our breathing can deepen and become irregular. But we’re also hard at work on a subconscious level. Scent plays a really powerful role in whether it’s a good kiss or not. Women are actually most attracted to the natural scents of men who have a different set of genes called the major histocompatability complex that codes for immunity. We’re most attracted to people whose MHC genes have a lot of diversity from ours—the advantage of that would be if you reproduce, that child’s probably going to have a stronger immune system, and so be more likely to survive to pass on their genes. This isn’t something that we’re consciously aware of, but we do seem to know if something feels off. And actually, more than half of men and women—fifty-eight per cent of women, fifty-nine per cent of men—report ending a budding relationship because of a bad kiss.
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Over and over, around the world in different studies, kissing was reported as much more important to women. I ended up calling Gordon Gallup, an evolutionary psychologist at SUNY Albany, and talking to him about why this existed. When we got down to it, it has a lot to do with reproductive biology, because men, from a strictly biological standpoint, are a lot less picky when they’re choosing a partner, whereas women invest much more time and energy into reproduction. We’re fertile for a much smaller period of our lives, so it’s really important to figure out whether someone would be a good genetic match. So when we’re up close and personal with someone, kissing them, we’re actually more sensitive to things like taste and smell and really engaging all of these senses as we kiss.
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