Oh blog, I'm sorry you've been abandoned! Of course, in an ironic twist (just one of ten zillion ironic twists that I'm beginning to think define the human condition), my lack of writing indicates that there is far more to write about than it's possible to conceive... Alas.
To fill this interim of emptiness/infinite fullness that is my current Springtime-at-Carleton existence, I'm copying an essay that I wrote for my Behavioral Neuroscience class last term. In it, I research and overview various explanations of the evolution and neurobiology of kissing.
Why Do We Kiss? The Evolution and Neurobiology of Osculation
I. Introduction Kissing is an unmistakable expression of affection in our culture: parents gently kiss their children goodnight, friends exchange pecks as an amiable salutation, and lovers lock lips in fits of passion. However, when you step back and think about this behavior, it seems odd, to say the least. What impels us to brush our lips across the skin of those we care about? Why do we long to share saliva with those we love?
Although the literature surrounding the bizarre phenomena of kissing (or “osculation,” in scientific terms) is somewhat scattered, there are several prevailing theories that hypothesize as to its evolutionary origin. Additionally, some interesting studies have been conducted that examine the effects of kissing on the brain. The following essay is an overview of the current scientific consensus (or lack thereof) on the evolutionary history and neurobiological underpinnings of this strangely essential human behavior.
II. Ancestral Hunger?Before discussing the evolutionary history of kissing, it is important to note that, despite the fact that over 90% of human societies partake in this behavior (Fisher, 2004), it is not universal among human societies. For example, the 20th century scientist Cristopher Nyrop studied isolated Finnish tribes in which families bathed together but found the practice of kissing repulsive and indecent (Nyrop & Harvey, 1901). Also, Chinese culture is reported to have regarded mouth-to-mouth kissing as “horrifying” at the turn of the 20th century. Therefore, it remains a debate among scientists whether kissing is a matter of genetic programming or cultural conditioning.
A popular evolutionary explanation of kissing that does implicate our genes for the compulsion to kiss was developed by the famous British zoologist Desmond Morris (Morris, 1969). According to Morris, kissing is a “relic gesture” that reflects maternal behavior of our distant ancestors. Based on his observations of chimpanzees, Morris hypothesized that kissing has its evolutionary origins in mothers feeding their babies by chewing food and then depositing it into hungry mouths, lips puckered. Lip contact may then have evolved to simply comfort hungry children, and then eventually to express romantic passion (Foer, 2006).
Another hypothesis is that romantic kissing evolved as an adaptive courtship strategy that functions as a mate assessment technique, a way of arousing sexual interest and receptivity, and a bonding behavior for long-term couples. A study of college students conducted by Hughes, Harrison, and Gallup in 2007 suggests that this might be the case.
Vaughn Bryant of Texas A&M University, however, has challenged arguments such as these—Bryant posits that kissing is not universal or genetic, but rather based on cultural norms (Foer, 2006). Bryant hypothesizes that kissing was “invented” in India, for the first recorded kiss dates back to early Vedic scriptures (~1500 B.C.). Bryant says that, although some women may have fed their babies in the way that Morris describes, they did not practice mouth kissing until after contact with Indo-European cultures.
However, it can be argued that even cultures that abstain from lip-to-lip contact brought their faces together to express endearment. Often these “kiss-substitutes” involve smelling. In fact, the early Vedic scriptures that Bryant cites in his theory begin by describing people “sniffing” with their mouths before they mention “setting mouth to mouth”(Foer, 2006). Additionally, Mongolian fathers smell their sons’ heads in lieu of kissing them, and Eskimo “kisses” in fact allow loved ones to smell each other’s cheeks (Walter, 2006). It is interesting to note as well that in “some dialects of Arabic the words for ‘kiss’ and ‘smell’ are said to be cognate”(Chamberlain, 1906). This connection between kissing and smelling could have interesting implications for some of the current research regarding the neurobiological effects of kissing, discussed below.
III. Kissing and the Brain Whether kissing was invented in India or is somehow programmed into our “animal” instincts, there is no question that, nowadays, it is an integral component of most romantic relationships. Kissing can light the fire of romance, or, in some circumstances, drive partners apart. Why is it that lip-to-lip contact evokes such powerful sensations and emotions? The answer is not found in our hearts, but in our heads. Both the structure of our brains and the chemicals that flow through them affect the way we perceive kissing.
We sense kissing as a rich tactile experience because a relatively large portion of our somatosensory and motor cortices is dedicated to the lips (notice their size in the somatosensory homunculus pictured above right). Although this brainpower dedicated to our lips may primarily play a role in eating and facial expression, but it also allows the experience of kissing to be nuanced and profound.
And this intense bonding experience seems to play an important biological role. Gordon Gallup completed a survey in 2007 that asked the question, “Have you ever found yourself attracted to someone, only to discover after kissing them that you were no longer interested?”(Hughes et al., 2007). In response, 59% of males and 66% of females answered in the affirmative. We are left with the question, therefore, of how, neurobiologically, the simple act of kissing can make such a profound difference.
One suggestion, maintained by Hughes et al., holds that kissing might activate “evolved mechanisms that function to discourage reproduction among individuals who could be genetically incompatible”(2007). Such a mechanism could potentially be explained by the fact that sebaceous (or oil-secreting) glands are located in high density on the face and are regulated by sex hormones (Durham, et al, 1993; Hoshi, et al., 2002; Rosenberg, 2002; Service, 1998). Additionally, the taste and smell of someone’s mouth can also indicate to your brain that your partner might have underlying health problems (Durham, et al., 1993; Rosenberg, 2002; Service, 1998).
Other subtle information could be conveyed via mouth-to-mouth contact. For example, it is possible that kissing can provide males information about a female’s reproductive status since saliva and breath odor change across the menstrual cycle (Hughes et al., 2007; Fullager, 2003). In addition, with salivary exchange, males could introduce hormones or proteins into women’s mouths that may influence her mating psychology, and even make them her more sexually receptive. Studies have shown that the mucosa membrane inside the mouth is permeable to hormones such as testosterone that are found in saliva (Dobs, Matsumoto, Wang, & Kipnes, 2004). (According to scientists like Helen Fisher, this may explain why men prefer “wetter, open mouth kisses”(2004).)
Another hormone that may play a role in kissing is oxytocin. It is known that oxytocin levels are stimulated by skin-to-skin contact (i.e. Brizendine, 2004; Nicholson, 1984), but few studies have been conducted that specifically examine the effects of kissing on oxytocin levels. One such study was conducted by Wendy Hill of Lafayette College in 2008 (Saey, 2009). Researchers took blood samples of couples to measure oxytocin, and then asked the volunteers to either kiss for fifteen minutes or to hold hands and talk for the same amount of time. Afterward, blood oxytocin levels were measured once more.
As per Hill’s prediction, women had naturally higher levels of oxytocin than men at the beginning of the experiment. However, the results after kissing/hand-holding were completely unexpected: after kissing, men’s levels of oxytocin increased, but women’s levels of the hormone dropped. The hand-holders showed a similar pattern of oxytocin levels, but to a lesser degree than the kissers. Although Hill suspects that the unexpected drop in female oxytocin levels was due to the clinical environment of the experiment, these results demand further investigation into the matter of kissing, oxytocin, and the differences between women and men.
One final property of kissing that I will touch upon here is its reported anxiolytic effect. Studies have shown that kissing lowers cortisol levels in both females and males, thereby reducing stress and increasing quality of life (Fisher 2004).
IV. Conclusions… Kissing, as with all behavior, is a complex amalgam of brain chemistry and social influences. It exemplifies the way in which evolutionary history intertwines with neurobiology—an understanding of one can help us understand the nuances of the other. At this point there are many competing theories and mysteries surrounding the kiss, and further research is needed to determine the mechanisms that underlie this simple pleasure.
