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.
