On Valentine’s Day this year, I was taking a leisurely (and lonely) stroll down Lygon Street in Melbourne. There were couples aplenty on the footpaths, cheerfully celebrating yet another year of mutual affection. And, as part of this romantic ritual, they were all doing the same thing, displaying a very unusual behaviour. You’ve never thought of it as being particularly odd, but it is: kissing.
What possible use could kissing have? If anything, swapping saliva should be wildly disadvantageous, the perfect way to spread germs from one person to the other. So, with my science senses tingling, I dove headfirst into the scientific literature…
… and came up with surprisingly little! Despite being one of our commonest social behaviours, not many scientists have seriously looked into the question of why we kiss. But, fear not, because there are benefits that potentially outweigh the costs of kissing:
It feels good
Our lips have an unusually large density of touch receptors, so lip-to-lip contact is felt very strongly by the brain. This tactile sensation causes the release of oxytocin and dopamine, two hormones that make us feel reaaaally gooood. But these hormones are also released in response to other physical contact like hugging or hand-holding, so why even bother involving the tongue?
It reduces stress and cholesterol levels
A recent study at Arizona State University divided a sample of ordinary couples into two groups: those who were asked to increase their frequency of romantic kissing, and those who were given no such instructions. Six weeks later, the frequent-kissing group were found to have significantly lower stress levels and increased relationship satisfaction. They also had reduced blood cholesterol: not because kissing is particular hard work (even the most passionate tongue-lashing only costs 2-3 calories per minute), but as a result of the lower stress levels.
It provides immunity against a form of herpes
This sounds counter-intuitive, but bear with me. Human cytomegalovirus (HCMV) is a type of herpes that you’ll find lurking in the epithelial cells of the salivary gland, which means that it’s readily passed on through salivary exchange.
Most of the time, this virus is a reasonably benign presence, but it can be deadly to unborn children in the early stages of pregnancy. By getting herself infected with HCMV in the earlier stages of a sexual relationship (for example, by swapping spit with her partner), a prospective mother can gradually build up an immunity that will protect any future pregnancies.
It developed from mouth-to-mouth feeding
Here’s a theory that will ruin your next romantic moment: French kissing is thought to have evolved from the practice of mothers transferring pre-chewed food into the mouths of their infants. This “kiss-feeding”, or pre-mastication, is observed in some animal species such as chimpanzees, and also several human cultures. I’m assured that, in both behaviours, the tongue movements are exactly the same, minus the chewed-up food.
All of a sudden, Casablanca doesn’t seem quite so romantic to me anymore.
For an audio companion to this article, check out the February 24 podcast of “The Science Hour”, in which I discuss (with substantially less eloquence) this very topic.