We've all heard people say they have a "gaydar".

It is the alleged ability to know whether someone is gay or straight based on their appearance.

The idea of gaydar got a scientific boost from a 2008 study that concluded people could accurately guess someone's sexual orientation based on photographs of their faces.

But a new study challenges what it calls the "the gaydar myth", saying it isn't accurate and is actually a harmful form of stereotyping.

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"Most people think of stereotyping as inappropriate," said Professor William Cox at the University of Wisconsin-Madison.

"But if you're not calling it 'stereotyping', if you're giving it this other label and camouflaging it as 'gaydar', it appears to be more socially and personally acceptable."

Cox and his team questioned the validity of the previous research, citing differences in the quality of the photos used for the gay and straight people featured in the study.

The gay men and lesbians, according to Cox's studies, had higher quality pictures than their straight counterparts.

When researchers controlled for differences in photo quality, participants were unable to tell who was gay and straight.

Cox says another reason people's judgments of sexual orientation are often wrong is that such a small percentage of the population - five per cent or less - is gay.

"Imagine that 100 per cent of gay men wear pink shirts all the time, and 10 per cent of straight men wear pink shirts all the time," he said. "Even though all gay men wear pink shirts, there would still be twice as many straight men wearing pink shirts. So, even in this extreme example, people who rely on pink shirts as a stereotypic cue to assume men are gay will be wrong two-thirds of the time."

In one of the studies, Cox and his team manipulated what participants understood about gaydar by providing different explanations of it to three groups. The researchers told one group that gaydar is real, another that gaydar is stereotyping, and did not define gaydar for the third group.

The group that was led to believe gaydar is real stereotyped much more often than the other groups, assuming that men were gay based on the stereotypic cues - statements such as "he likes shopping".

"If you tell people they have gaydar, it legitimises the use of those stereotypes," Cox says. He hopes his research counteracts the gaydar myth and exposes it as something more harmful than most people realise. "Recognising when a stereotype is activated can help you overcome it and make sure that it doesn't influence your actions."

- Daily Mail