Pajiba Dirty Talk: A Kiss is Just a Kiss
Many of us enjoy kissing, both as a pleasurable act in and of itself and as a prelude to more intense forms of pleasure, and yet it is kind of a strange habit when you think about it. We mash our lips together and maybe stick our tongues in each other's mouths and strangely enough, it feels awesome (well, not always). You might think this is some bizarre practice invented by early humans that caught on (and you'd be partially right), but in fact we aren't the only animals that practice the whole lips-mashing thing, though it isn't always sexual. (They don't French though, that does in fact seem to be a human invention, and a western one at that).
There actually isn't a whole lot out there, in terms of big research studies that focus on kissing, and much of the research that has been done is fairly recent, so this field is basically in its infancy. One study (which unfortunately I haven't been able to find published - only reported on) looked at the levels of the hormones oxytocin and cortisol in 15 heterosexual couples before and after holding hands and kissing. Oxytocin is often referred to as the "love hormone," because it's associated with bonding, but in fact it seems to be more of a general emotion-heightening hormone than one associated solely with love. Cortisol on the other hand, is a stress response hormone. And of course both hormones, like all hormones, act in conjunction with many other factors in the body to produce different responses. The study found that cortisol levels decreased for both men and women after kissing, and oxytocin levels rose in men but fell in women. The results imply that kissing is a stress-reducing activity (well, assuming you're over 16 and/or this isn't your first kiss), but the oxytocin levels are hard to interpret. It could be that the men actually do find kissing an even more bonding experience than the women, but since oxytocin plays various roles and doesn't really act alone, that's a stretch. And of course the sample size of that particular study is incredibly small.
For the book, Kirshenbaum also collaborated with Dr. David Poppel at NYU to conduct her own study on kissing - in this case on the brain response to it using a magnetoencephalography (MEG) machine. Unfortunately, you can't fit two people in an MEG machine, so instead subjects were shown images of two people kissing, including same sex couples, and their responses to the images were monitored. The study is presumably explained in more detail in the book, but the most surprising result, according the Kirshenbaum, was that people responded most strongly to the homosexual images. She explains this as a result of the "frequency effect," - i.e. we're used to seeing heterosexual couples kiss but we don't often see homosexual kissing. I can think of a few other possible explanations myself - like that the responses measured weren't pleasurable ones, or that the subjects were just more interested in seeing two people of the sex they're attracted to. (Of course without seeing the details of the study I can't really conclude anything.)
While the physiological aspect of kissing is still not well studied, there are several studies on the sociological aspect of kissing. One survey of 1041 college students looked at sex differences in men and women's approach to kissing. In that study, respondents were asked to rate the importance of kissing before, after, and during intercourse as well as other kiss-related preferences. Women were less likely to be willing to have sex without kissing at all (only 7% versus 52.8% of men) and rated kissing more important than men at all stages of intercourse. On the other hand, 70% of men assumed that a kiss would be an effective way to end a fight within a relationship, while only 58% of women agreed (chances that these results indicate biology and not socialization: slim to moderate). My favorite part of this particular study is the question of how "wet" participants preferred their kisses to be and how much tongue should be involved. Men apparently like it sloppy, whereas women only like lots of tongue and lots of saliva when the kiss is from a long-term partner. The researchers explanation for this is that kissing allows us to detect chemicals and hormones that might help us determine mate selection, and men having less sensitivity to such things need more saliva to detect whether the woman they are kissing is, for example, fertile (ah, we meet again, Evo Psych). There's also a theory that by transferring their testosterone-laden saliva into a woman's mouth, a mad can get her aroused. Kissing is near universal in the human experience (it occurs in 90% of cultures), but romantic kissing with tongue is in fact a newer, mostly western invention, which makes the saliva-swapping theories a little suspect.
So why do we like to kiss? Our lips are one of the most sensitive parts of our bodies. They are sensitive not necessarily for sexual purposes but because they are a gateway to our stomachs - the sensitivity may have helped our ancestors (and other species for that matter) detect dangerous foods before they ate them. The sexual aspect of touching each others' lips may in fact only be a byproduct of that - one of many types of touch that serve not only useful purposes but also give us pleasure.