Although often interpreted as a "go" signal by the man, these represent only an instinctive attempt by the woman to assess him and work out whether he is worth pursuing. All potential mates are subjected to the same subconscious interrogation.

The Proteus effect - named after the Greek river god who constantly changed his form to evade his enemies - is one of a number of courting rituals analysed in a study of flirting to be published by Britain's Social Issues Research Centre, an Oxford-based organisation supported by the anthropologist Desmond Morris.

Others signals include the "copulatory gaze" and the phenomenon of "courtesy flirting" - a practice that works well with Europeans but that can go badly wrong if tried in the US.

Kate Fox, the centre's co-director and author of the report, said women tended to blitz men with protean signals in the first minute or so of meeting them.

"By sending erratic and ambiguous signals in the early stages of an encounter, women manipulate men into showing their hand," Fox said.

"It's not entirely surprising, given the levels of ambiguity and deception to which they are subjected, that males tend to become confused."

Fox and her researchers questioned more than 1000 people and organised a series of focus groups to test their findings.

Flirting proper was found to begin with what the researchers call the copulatory gaze, where intense eye contact is broken momentarily with occasional lifting or lowering of the eyes. This is followed by smiles, synchronisation of body movements, coy looks and head-tossing by the female. Men sometimes thrust their chests out like apes.

"Human flirtation involves sequences of gestures and expressions not unlike the courtship dances of birds and other animals that we see on wildlife programs," says the report.

Surprisingly, the researchers found two-thirds of flirtatious encounters are initiated by women. They share the characteristic - known as "female proceptivity" - with other species, such as chimpanzees.

"Chimpanzee females, for example, actively solicit sex with males, going so far as to pull a resting male to his feet and insist on copulation," the report says.

"Among humans, female proceptivity is much more subtle. In fact, female solicitation is done so unobtrusively that most people think men take the initiative in making the advances."

However, the tactic seems to work. The researchers found women are not simply initiating flirtatious encounters - something they are always thought to have done - but are often going one step further and asking men out.

More than 30 per cent of women who took part in the survey said they had initiated dates, and only 1 per cent reported being turned down.

Of dates initiated by women, 51 per cent lead to relationships lasting more than six months - a far better success rate than for those initiated by men.

The research found people often flirt for the sake of politeness. But this so-called "courtesy flirting", where the aim is to flatter rather than find a mate, can be misinterpreted, often with embarrassing results.

"It is mainly practised by men, who engage in mild flirtation with women as a form of politeness," the report says. "It is particularly common in Britain and Europe, and can be confusing for foreigners, particularly Americans, who mistake it for the real thing."

Frequency of flirting was also measured in the study. Possibly reflecting the fact that men misinterpret protean signals for the real thing, 45 per cent of the men surveyed reported experiencing flirting "in the past week", compared with 37 per cent of women.

The Australian