When it comes to kissing, humans globally are hardwired to lean to the right and not left -- a first-of-its-kind finding that may have wider implications for neuroscience and cognitive sciences.
Researchers from universities of Dhaka in Bangladesh and Bath and Bath Spa in Britain also found that the kiss recipients have a tendency to match their partners' head-leaning direction.
While men were about 15 times more likely than women to initiate kissing, over two-thirds of the kiss initiators and kiss recipients turned their heads to the right.
The paper, published in the journal Scientific Reports, is the first to investigate an inherent bias for turning the head to one side while people kiss in a non-Western context -- in Bangladesh.
"Head turning is one of the earliest biases seen in development - even in the womb a preference for turning the head to the right is observable before that of favoring the right hand or foot. Whether this fundamental bias is innate and extends into adulthood is a lingering question for neuroscience and psychology," explained lead author Dr Rezaul Karim from the department of psychology at the University of Dhaka.
For the study, the team invited 48 married couples to kiss privately in their own homes and after kissing, they were asked to go to different rooms, open an envelope and then report on various aspects of the kiss independently of each partner.
Their results highlighted a bias to turn heads to the right when kissing for both the initiator and the recipient of the kiss.
"In addition, the kiss initiators' head-leaning direction strongly predicted the kiss recipients' head-leaning direction. This suggests that the kiss recipients have a tendency to match their partners' head-leaning direction in order to avoid the discomfort of mirroring heads," the authors noted.
The setting for the study was significant as kissing in Bangladesh is a very private behaviour; something censored from television or films.
"So, whereas similar results from the Western countries could be attributed to cultural factors or having learnt how to kiss through influences on TV or film, the same cannot as easily be said for a non-Western country like Bangladesh," the team observed.
It suggests that the act of kissing is determined by the brain splitting up tasks to its different hemispheres -- similar to being either right or left-handed -- specifically the functions in the left cerebral hemisphere, located in the emotion and decision-related areas of the brain.
The researchers suggest different hormone levels (such as testosterone) in each hemisphere and neurotransmitters might be unevenly distributed to each hemisphere (such as dopamine, involved in reward behaviours) as giving rise to a bias to turn right.
This may have implications if you are left handed and your partner is right-handed or vice versa.
According to Dr Michael Proulx of the University of Bath, this study is unique in giving us a look into a private behaviour in a private culture with implications for all people.
"Prior works could not rule out cultural learning due to having Western samples. It turns out, we as humans are similar even if our social values differ," he said.