Given at least 1/20th of a second to look at a man, you can probably guess whether he’s gay — and be correct at least 70 percent of the time. Remarkably, your first split-second assessment would be as accurate as your impression after a full minute.

That was the case, at least, in a study by Tufts University psychologists Nalini Ambady and Nicholas Rule when they asked male and female judges to guess the sexual orientation of 90 faces of gay and straight men (without facial hair or piercings). Regardless of their own sexual orientation, the judges were astoundingly swift and accurate when it came to identifying most men’s sexual orientation. (Women’s faces weren’t tested.)

But the problem with gaydar — a contraction of “gay” and “radar” — is that identifying gay men and lesbians based on physical qualities alone isn’t always reliable. Despite the high accuracy rate for most faces, several yielded no clue of sexual orientation and were consistently misjudged. And when the judges were right, it was tough to pinpoint what clued them in. Was it the set of an expression? The hairstyle, the lips, the lower jaw? The eyes? Nevertheless, the results were surprising because many gays and lesbians attribute their own gaydar to behavioral cues such as the way someone looks at them — a sliding glance or the hint of a wink — or certain postures, gestures, facial expressions, and ways of speaking. As for straight women having gaydar, it might be evolutionarily advantageous — after all, it’s helpful to know who is a potential mate and who is not.

Stripped of cultural influences, there aren’t clear-cut, consistent physical distinctions that would trigger gaydar. Not that researchers haven’t found some very general biological differences between straights and gays.

As I detail in BLONDES:

* Prenatal testosterone levels, which are linked to sexual orientation, also influence traits such as finger ratios. Gay men’s ring and index fingers (of the right hand especially) are more likely to be the same length, like those of straight women, whereas a greater number of straight men have a ring finger that’s longer than their index finger. Women who identify themselves as “butch” lesbians are much more likely than “femme” lesbians or straight women to have a low digit ratio (ring finger longer than index finger) due to higher levels of prenatal testosterone.

* Depending upon the study (one here), gay men have a 34-82 percent greater chance of being left-handed than straight men (note: not all studies detect a bias), and lesbians are up to twice as likely to be lefties than straight women. Bear in mind that even though a gay man or lesbian has a higher chance of being left-handed, the majority of lefties are straight.

* Sniffing t-shirts of gays and straights, gay men prefer the odor of other gay men more than straights of either gender do. (Lesbians generally like the natural smell of other lesbians, straight women, and even straight men.)

* Gay men’s hair whorl (direction of scalp hair rotation) is more likely to be counterclockwise than that of straights. Nearly 30 percent of a sample of gay men had hair whorls that rotate counterclockwise, compared to fewer than 10 percent in a mixed group of men and women.

* A man’s gait might be another clue — in one study, judges identified gay men by their walks with about 60 percent accuracy. The reason is that some gay men have lower waist -to-hip ratios (larger hips compared to waist) than the average straight guy.

None of these traits proves a person is gay or lesbian, nor can their absence prove a person is straight. They’re just trends. Yes, gaydar works, although it’s as much an art as a science.

But someday, if MIT’s Project Gaydar goes as expected, identifying who’s gay may be mere computation.