It seems most people turn their heads to the right when they pucker up, according to the German researcher who conducted a three-continent study of the subject.
The study, appearing in the Feb. 13 issue of Nature, shows a trend that actually starts while you are still in your mother's womb, says Onur Güntürkün, who is in the psychology department at Bochum-Ruhr University.
Most babies turn their heads to the right during the final few weeks in the womb and the first six months of life. This bias is believed to influence the later development of perception and motor preferences by skewing visual orientation to the right side, he adds.
Indeed, even though this right-side bias disappears at about six months, it appears to re-emerge years later. Most humans are right-footed, right-eared, right-eyed, and right-handed, Güntürkün says.
Similar processes are at play in the animal kingdom. For instance, in birds, "a right-turn of the head before hatching results in a higher amount of light stimulation in the right eye," Güntürkün says. "As a result, the maturing brain is transformed into asymmetrical functioning, inducing motor, visual, and cognitive asymmetries," he adds.
"I never believed that our head-turning preference to the right disappears, but I needed to test adult humans in a situation where they have to decide for a side," he continues. "I was never interested [in studying] kissing because of kissing," he says. "Instead I wanted to understand the rules that transform our brains into asymmetrically functioning entities."
Güntürkün set out to observe couples in the act. He watched 124 couples -- ages from about 13 to 70 -- kissing in airports, train stations, parks, and beaches in the United States, Germany and Turkey.
The methodology was impressively low-budget and low-tech.
"During my flights, I often had to wait for hours for my next connection flight," he explains. "I started to spend my waiting time at the arrival halls of the airports. The same was true for large railway stations or public places like beaches or parks. I tried to be as discrete as possible, and I guess most couples didn't sense that they were observed. As soon as they had kissed, I walked away."
In order to be included in the study, kissing couples had to be facing each other, succeed in making lip contact, and not be holding anything in their hands, which might bias the kissers to one side or the other. If the scene consisted of multiple kissing, only the first smack was counted.
Sampling was conducted in three different continents to reduce the impact of cultural factors. "There is a lot of kiss-kiss going on, and they show a tremendous cultural diversity," Güntürkün says.
Among the 124 kissing pairs, 80, or 64.5 percent, veered to the right while only 44 (or 35.5 percent) veered to the left. Most of the subjects showed no hesitation at all when it came to positioning their head.
"I guess the couples I observed did not have their very first kissing encounter in my presence, so they were used to [kissing] this special person and probably had developed a habit how to do it," Güntürkün notes.
More importantly, because the subjects represented such a wide cross-section of people and displayed the same ratio of right-to-left preference as for foot, eye, and ear, it seems that the head-motor bias does in fact persist into adulthood and may even set the tone for overall "sidedness."
"The results of this little study may open a way to understand the common concert of right-sided asymmetries of various motor and sensory functions," Güntürkün concludes. "In the end it might be possible that there are only very, very few initial asymmetries governed by genetic factors. The rest is secondary."
Güntürkün fears he has no good advice for lovers, however."I'm pretty sure that the romance doesn't depend on kissing direction," he says. "But kissing is one of these subtle habits that tell us something about the very dawn of each of us."