What’s in a kiss? Well, hopefully not this. But physiologically speaking, a lot of subtle science goes into a lip lock—something that science writer Sheri Kershinbaum explores in her book The Science of Kissing.

She wrote up an excerpt for the Washington Post in honor of our upcoming kiss-centric holiday:

Humans have evolved to use a number of signals—including taste, smell and possibly silent chemical messengers called pheromones—to help us figure out whether someone is a suitable partner and a good person to reproduce with. A kiss means getting close to someone—close enough to suss out important clues about chemistry and genetics. At this range, our noses can detect valuable information about another person’s health and perhaps even his or her DNA. Biologist Claus Wedekind has found, for instance, that women are most attracted to the scents of men with a different set of genetic coding for immunity than their own. This is probably because when there is greater genetic diversity between parents in this area, their children will have more versatile immune systems. The assessment occurs at a subconscious level, yet a bad initial kiss may be a result of a genetically star-crossed pair. Don’t like ads? Become a supporter and enjoy The Good Men Project ad free

Kirshenbaum goes on to talk about the chemicals that could potentially be released at the strike of midnight. Oxytocin—a favorite subject around these parts—is released if a kiss is going well, and “a bad kiss, alternatively, can lead to chemical chaos. An uncomfortable environment or a poor match can stimulate the ‘stress hormone’ cortisol, discouraging both partners from continuing.”

Perhaps a little clinical but the research often lines up with our conventional wisdom. For instance, the fact that a first kiss is unforgettable.

Psychologist John Bohannon of Butler University and his research team surveyed 500 people to compare their recollections of a variety of significant life experiences—such as a first kiss and the loss of virginity—to find out what made the most dramatic impression. A first kiss trumped everything: It was the most vivid memory in the minds of those being surveyed. In fact, when asked about specifics, Bohannon reported that most people could recall up to 90 percent of the details of the moment—where they were, who made the first move—no matter how long ago the exchange took place.

But honestly, guys, no pressure.

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