Gaydar (a portmanteau of gay and radar) is a colloquialism referring to the intuitive ability of a person to assess others' sexual orientations as gay, bisexual or heterosexual. Gaydar relies almost exclusively on non-verbal clues and LGBT stereotypes. These include (but are not limited to) the sensitivity to social behaviors and mannerisms; for instance, acknowledging flamboyant body language, the tone of voice used by a person when speaking, overtly rejecting traditional gender roles, a person's occupation and grooming habits.
The detection of sexual orientation by outward appearance or behavior is frequently challenged by situations in which masculine gay men who do not act in a stereotypically "gay" fashion, or with metrosexual men (regardless of sexuality) who exhibit a lifestyle, spending habits, and concern for personal appearance stereotypical of fashionable urban gay men.
Scientific research 
A number of scientific studies have been conducted to test whether gaydar is real or just a popular myth. Perhaps the earliest study asked people to judge sexual orientation from video clips, with results concluding that it was a myth. A later and more rigorous study published in a prestigious psychology journal showed that people could accurately judge sexual orientation. This study asked people to indicate their sexual orientation using the Kinsey scale and then had others view very brief silent clips of the people talking using thin-slicing. The viewers rated their sexual orientations on the same scale and the researchers found a significant correlation between where the people said they were on the scale and where they were perceived to be on the scale. Sexual orientation was therefore perceived accurately just from nonverbal behaviors. Later studies have repeated this finding and have even shown that home videos of children can be used to accurately judge their sexual orientation later in life.
Later studies found that gaydar was also accurate at rates greater than chance for judgments just from the face. Study participants use gendered facial cues and stereotypes of gay people to make their judgments, but reliably misjudge sexual orientation for people countering stereotypes. The race, ethnicity, and nationality of neither the person making the judgment nor the person they are judging seems to make a difference when making judgments from faces. Even individual facial features (just the eyes) can sometimes give enough information to tell whether a man or woman is gay, straight, or lesbian. One study showed that judgments of men’s and women’s faces for about 1/25th of a second was enough time to tell whether they were gay, straight, or lesbian. People’s judgments were not any more accurate when they had more time to make their judgments. Follow-up work to this suggested that gaydar happens automatically when someone sees another person and that seeing someone’s face automatically activates stereotypes about gays and straights. People seem not to know that they have gaydar, though. Gay men have better gaydar than straight men and women have better gaydar when they are ovulating. One study hypothesized that this might be because homosexual people are more attentive to detail than heterosexual people are, apparently as an adopted perceptual style aiding in the recognition of other homosexual people.
Other studies have found that men and women with body shapes and walking styles similar to people of the opposite sex are more often perceived as gay. The study, by UCLA assistant professor Kerri Johnson, found that observers were able to accurately guess the sexual orientation of men 60 percent of the time — almost a coin toss; with women, their guesses didn’t exceed chance. But what’s most interesting to researchers is understanding how that snap judgment can unleash a series of stereotypes. Contrary to hype surrounding the study, the results suggest that walking styles and body shapes do not give away sexual orientation. The study was intended to reveal information about the perception of the observer, but has been misinterpreted as conveying reliable information about the sexual orientation of the participants. Gender-specific body movements are not reliably associated with a person’s sexual orientation This is true of face shape, but surprisingly not for voices, even though people think they do. A handful of studies have investigated the question of gaydar from the voice. They have found that people can tell who is gay and straight from their voices but have mostly focused on men. Detailed acoustic analyses have highlighted a number of factors in a person’s voice that are used, one of which is the way that gay and straight men pronounce "s" sounds.
Further reading 
- Gaydar by Donald F. Reuter (ISBN 978-0-609-61102-9)
See also 
- Biology and sexual orientation (physical differences)
- Fruit machine (homosexuality test)
- Linguistic profiling#Gender and sexual orientation
- Gay lisp
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- http://colorado.edu/linguistics/CRIL/Volume22_Issue1/paper_ZIMMAN.pdf[full citation needed]
- Public Radio Program on gaydar
- The Science of Gaydar: The new research on everything from voice pitch to hair whorl