Flirtatious men have a greater chance of "pulling" when a woman is at the peak of her fertility, according to new research released today.
Evolutionary psychologist Dr Edward Morrison, of the University of Portsmouth, asked women to examine various expressions made by men and to rate their attractiveness.
He found that women who were ovulating, the most fertile period of their monthly cycle, showed an increased preference for flirtatious expressions.
Dr Morrison said an awareness of the importance of flirting could help improve a man's chances of pulling at a Christmas party.
He said: "An ability to 'read' and interpret the facial expressions and an awareness of what you are signalling with your own expressions could improve your chances of successful flirting.
"It's difficult to define what constitutes flirtatiousness and much of it may be something we perceive without even realising it.
"But it seems that in the absence of other cues, the 'social properties' of facial movement influences how we judge attractiveness.
"If we wanted to attract someone at the Christmas party, flirting effectively may help to do so."
Published in the Archives of Sexual Behaviour, the study describes how researchers produced several animated facial models whose movement was based on real people.
They then standardised the images by stripping them of any other distinguishing qualities.
These were rated on a flirtatiousness scale by 16 women.
A separate group of 47 women were then asked which of the faces they found most attractive by rating them on a scale of one to seven.
In fertile phases of the menstrual cycle women consistently preferred the faces which had been categorised as more flirtatious, according to Dr Morrison.
By mapping points on the faces to measure their level of movement, researchers revealed that most of the women preferred the faces that were more animated.
But the degree of movement was not the exclusive factor, leading researchers to conclude that women recognised specific "mating-relevant" social cues.
Dr Morrison said: "It demonstrates that attractiveness is not necessarily a fixed property of the face.
"By changing the way the face moves we may be able to increase our appeal to the opposite sex."
Researchers were unable to define which exact movements led the first group of women to interpret which faces were flirting.
Dr Morrison said: "Science is still a long way from discovering the magic formula for what women find attractive in a man."
He added: "The face is where we exhibit our most explicit signals. It's the human equivalent of the peacock's tail.
"But although a smile is fairly easy to interpret, there are more subtle messages going on all the time.
"We use facial movement to interpret people's intentions, such as whether they like us or not. This allows us to allocate our mating effort appropriately.
"For example, there is little point trying to chat up a person we admire if their expression indicates they are not interested."
Dr Morrison said that the research supported previous studies which have found that women's behaviour and preferences vary during the menstrual cycle.
During the period of ovulation women prefer taller men, more masculine faces and deeper male voices.
He said: "By preferring these traits when they are more fertile, women have increased chances of passing them on to their offspring.
"Selecting the most favourable mate is one of the most fundamental aspects of human behaviour."