The real reason people close their eyes when they kiss is because the brain can't deal with more than two things at once.
That is the finding of academics who have explored one of the great mysteries of the human psyche.
When people pucker up the brain finds it difficult to process the sensation if cognitive power is also being used to analyse what we see, said psychologists from Royal Holloway, University of London.
'Tactile awareness depends on the level of perceptual load in a concurrent visual task,' said Polly Dalton and Sandra Murphy, from the university, in the Journal of Experimental Psychology: Human Perception and Performance.
The cognitive psychologists reached their conclusion without studying people actually kissing.
Instead people were asked do visual tests while their response to something touching their hands was measured.
The academics found that the visual field overrides tactile responses - meaning people would struggle to continue kissing if what they were looking at became more difficult to process.
Dr Dalton told The Sunday Times: 'If we are focusing strongly on a visual task, this will reduce our awareness of stimuli in other senses.
'It is important for designers to be aware of these effects, because auditory and tactile alerts are often used in situations of high visual demand, such as driving a car or flying an aircraft.'
The findings could also explain why people often shut their eyes whilst reading braille or when dancing.
Dr Dalton added: 'These results could explain why we close our eyes when we want to focus attention on another sense. Shutting out the visual input leaves more mental resources to focus on other aspects of our experience.'