Humans are hard-wired to favour leaning to the right while kissing romantic partners, an international study by psychologists and neuroscientists has found.
The research, by the universities of Dhaka, Bath and Bath Spa, found that kiss recipients have a tendency to match their partners' head-leaning direction.
Experts built on work from Western countries to investigate kissing behaviours in a non-Western context, including a bias for turning the head to one side.
Their work, published in the journal Scientific Reports, studied 48 married couples in Bangladesh, where romantic kissing is not typically observed in public.
The study was building on work from Western countries to investigating kissing behaviours in a non-Western context
Couples were asked to kiss privately in their own homes, then go into different rooms and independently report back on various aspects of the kiss.
Men were about 15 times more likely to initiate kissing than women, and both partners showed a bias for turning their heads to the right.
Dr Rezaul Karim, from the department of psychology at the University of Dhaka, said: 'This is the first study to show sex differences in the initiation of kissing, with males more likely being the initiator, and also that the kiss initiators' head-turning direction tends to modulate the head-turning direction in the kiss recipients.
'Based on our prior theoretical work we are also able to make new hypotheses about the underlying neural basis for these behaviours.'
The study found that more than two-thirds of kiss initiators and kiss recipients turned their heads to the right. Men accounted for 79% of the kiss initiators.
The study found that that men were about 15 times more likely to initiate kissing than women
A person being left or right handed predicted their head-leaning direction but this was only the case if they initiated the kiss.
The head-leaning direction of the kiss initiator also strongly predicted the head-leaning direction of the kiss recipient.
This suggests that the kiss recipients have a tendency to match their partners' direction in order to avoid the discomfort of mirroring heads.
People who mirrored each other's head movements on request for a kiss reported that they felt discomfort kissing in that way.
'This further suggests the underlying cognitive mechanisms of the act of kissing and head turning,' the authors said.
'Though this action tends to be performed intuitively, a decision must be made about the direction to which the partners should lean to kiss each other.'
The setting for the study was significant as kissing in Bangladesh is very private and censored from television or film, they added.
Kiss recipients have a tendency to match their partner's direction in order to avoid the discomfort of mirroring heads
Results from Western countries could be attributed to cultural factors or learning to kiss through influences on TV or film but this cannot be said for countries like Bangladesh.
Previous studies have involved couples kissing in public places such as airports, railways stations, beaches or parks.
Dr Michael Proulx, from the department of psychology at the University of Bath, said: '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.'
The research suggests that the act of kissing is determined by the brain splitting up tasks to its different hemispheres, similar to being either right of left funded.
This is specific to the functions in the left cerebral hemisphere, located in the emotion and decision related areas of the brain.
Hormone levels, such as testosterone, might be unevenly distributed in each hemisphere, causing a bias to turn right.
It is hoped the findings will feed into further studies of neurophysiological mechanisms of such behaviours.