Did you know that there is a name for the scientific study of kissing? It is called Philematology, from the ancient Greek word philos = earthly love. Scientists found out that we use up to 34 muscles for intensive kissing, that kissing increases levels of endorphins and dopamine, and that we exchange as many as 10 million to 1 billion bacteria during kissing (don't worry, 95% of them are not pathogenic for immunologically competent people). You might be aware of that, but have you ever wondered why you nearly never bump your partner’s nose or head when you kiss?
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| The Kiss by Auguste Rodin, Musée Rodin, Paris |
Onur Güntürkün observed 124 kissing pairs in public spaces in three countries and documented that 65% turned their heads to the right and only 35% to the left, resulting in a 2:1 ratio for right-kissing [1]. Where does this head-turning asymmetry come from? Is it due to brain laterality or due to a motor bias?
Romantic Theory about the Influence of Emotion
One hypothesis is that head-turning asymmetry during kissing is linked to brain laterality. In popular psychology the left hemisphere is often said to be involved in analytical thinking, whereas the right hemisphere processes emotions. At least for posing behavior in portraits, it was shown that the emotional context has an effect on lateralisation bias. In emotive conditions, individuals show the left cheek, i.e. turn the head to the left, whereas when posing for an impassive scientific portrait they show the right cheek [2]. Is head-turning to the right during kissing, like posing behavior, also influenced by the emotional context?
To study the association of right-kissing and emotions, subjects were asked to kiss a life-sized doll head without emotion. The right turning ratio was compared to that of kissing couples. The results showed a similar right turning ratio for both the doll kissers and the couples. As no preferential difference between kissing couples and doll kissers was observed, the emotion theory was dismissed. Kissing the doll does not involve emotions like kissing a partner, but the head-turning ratio was still similar [3].
Reappearance of a Neonatal Right-Side Preference
Another possible explanation for the preference of right-kissing is the persistence of a motor bias seen in neonates: During the final weeks of gestation and during early infancy, most humans have a preference for turning the head to the right. This motor bias persists into adulthood and has effects on various asymmetries. The preferential head turning direction in infants can even be used as prediction for later handedness [4]. However, the prevalence of right-handedness is 8:1 [5]; thus not consistent with the 2:1 kissing ratio for the right side observed by Güntürkün. Thus, the kissing asymmetry is not a simple result of right-handedness. If the asymmetry during kissing is caused by a motor bias, how is it related to handedness and other lateral preferences?
To test whether head-turning preference is related to other lateral preferences, the handedness, footedness, and eye preference of volunteers who were asked to kiss a symmetrical doll face were determined. The study revealed that right-kissers were more likely to be right-handed and right-footed than left-kissers [6]. This relation could not be shown for eye preference, but as the data structure was the same the authors speculate that their questionnaire was not detailed enough. However, the study showed that head-turning preference during kissing is indeed due to a motor bias and related to handedness and footedness.
Dominance of Right-Kissers
As kissing always takes two and both kissers are always influenced by the partner’s head-turning preference, scientists were interested in what happens if a right-turner kisses a left-turner. Therefore Van der Kamp and Canal-Bruland analyzed the consistency of the head-turning bias in kissing by using a doll head rotated in different orientations that were either compatible or incompatible with the participant’s head-turning preference. In the study, right-kissers were more likely to persistently turn their head to the right even if the doll's head was turned as if kissing on the left side [7]. Because the head turning bias among right-kissers is stronger than among left-kissers, two people with different head-turning preferences are more likely to turn their heads to the right during kissing. Furthermore, the results support the hypothesis that behavioral asymmetries are stronger for individuals with a lateral preference pattern for the right than for the left side [8].
In conclusion, the observed asymmetries during kissing can be explained by a motor bias rather than by the emotive context.
[1] Güntürkün, Nature, 2003
[2] Nicholls et al, Proc Biol Sci, 1999
[3] Barrett et al, Laterality, 2006
[4] Michel, Science, 1981
[5] Corballis, Psychol Rev, 1997
[6] Ocklenburg and Güntürkün, Laterality, 2009
[7] Van der Kamp and Canal-Bruland, Laterality, 2011
[8] Searlman and Porac, Brain Cogn, 2003
By Claudia Willmes, PhD Alumni, AG Eickholt and AG Schmitz
This article originally appeared 2014 in CNS Volume 7, Issue 2, Neuroscience of Love
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