You can alter your attraction to the opposite sex simply by looking straight at them and smiling, research suggests.
The best look to make yourself attractive
A study of hundreds of volunteers at Stirling and Aberdeen Universities found averting the eyes even a fraction can make you appear less attractive.
In the Royal Society's Proceedings B journal, they say the direction of gaze plays a role alongside a symmetrical face or healthy skin.
An expert said it may stop people wasting energy on pointless courtships.
The study used pictures of male and female faces which had been subtly digitally manipulated.
In one picture, a woman might be looking straight at the camera, while in the next, a tiny adjustment meant she would be looking marginally to the left or right.
The difference was so small that it was not immediately obvious to the viewer.
However, after these pictures were shown to 460 men and women, who were asked to rate them for "attractiveness", it became clear that it was having a pronounced subliminal effect.
In some pictures, there was an eight-fold difference in ratings between the "straight to camera" and averted gazes.
While many studies have found links between face shape, expression and other physical "cues" to attraction, this is one of the first to look in more detail at the direction of gaze.
The researchers wrote: "Mating effort is a finite resource that should be allocated judiciously, and preferences for direct gaze in opposite-sex faces would increase the likelihood of allocating mating effort to potential mates who are most likely to reciprocate."
One of the paper's authors, Dr Claire Conway, said: "People prefer faces that appear to 'like' them, showing that attraction is not simply about physical beauty."
Professor Ruth Mace, a researcher into evolutionary anthropology at University College London, said that while this seemed an obvious principle, it could be a sign of evolution at work.
She said: "It's a pretty clear signal whether a person is interested in you or whether you are wasting your time.
"But it suggests that how attractive you find someone is governed partly by how likely you are to be successful."