Last month, Sen. John McCain dropped by “Saturday Night Live,” drawing laughs from his promise, if elected president, to fight expensive federal projects -- such as, he spoofed, a Department of Defense device to “jam gaydar.”

That was a joke. But some scientists are, in a way, working on gaydar, the supposed ability to discern whether a person is homosexual by reading subtle cues from their appearance. Just don’t refer to it that way. The preferred term is “sexual orientation correlates.”

These scientists are searching for innate traits that might not appear to be related to sexual orientation or even to standard clichés. So measuring a subject’s shoe size is permissible; asking about ownership of Barbra Streisand albums would be cheating. Some inborn traits might be expected if homosexuality is -- as most scientists believe -- rooted in biology, and they might provide clues about the biological origins of sexual orientation.

Finding and solidifying these links isn’t easy. Studies contradict each other, and some promising paths don’t pan out. (A link between male homosexuality and finger lengths isn’t holding up, and a claim that gays have distinctive fingerprint ridge patterns is largely discredited.) Scientists don’t always agree on how to interpret the results, and more progress has been made with regard to men than to women.

* Big brothers. Study after study -- including one of 87,000 British men published last year -- has found that gay men have more older brothers than straight men do. Only big brothers count. Lesbians don’t show such patterns.

The numbers: Each older brother will increase a man’s chances of being gay by 33%, says Ray Blanchard of the University of Toronto, an expert on the “big-brother effect.” That’s not as dramatic as it might sound. A man’s chance of being gay is pretty low to begin with -- perhaps as low as 2% (lowered from 10% by researchers in the early 1990s). So having one older brother ups the chance to only about 2.6%.

What it might mean: Psychological influences are probably not at work, because the pattern holds even for gay men who weren’t raised with their older brothers. Instead, the mother’s womb might be key. After giving birth to a boy, her immune system might create antibodies to foreign, male proteins in her bloodstream. Subsequent sons in the womb could be exposed to these “anti-boy” antibodies, which might affect sexual development in the brain.

Accordingly, you’d expect the percentage of gay men in a society to vary depending on demographic differences in family size: One study calculated that a one-child-per-family law would reduce male homosexuality by about 29% from current levels.

* Left hand vs. right hand. The hand you use to sign your name might have something to do with what gender you are drawn to.

The numbers: More lefties -- or at least more somewhat-ambidextrous folks -- crop up in the gay population than among straight people, several studies have shown. An analysis of more than 23,000 men and women from North America and Europe in 2000 found that being non-right-handed seems to increase a man’s chances of being gay by about 34%, and a woman’s by about 90%.

What it might mean: One guess is that different-than-normal levels of testosterone in the womb -- widely theorized to play a role in determining eventual sexual orientation -- could nudge a fetus toward brain organization that favors left-handedness as well as same-sex attraction.

Another theory is that development of a fetus might be disturbed by factors such as a mother’s illness, steering the fetus into being less than strictly right-handed -- and, in some cases, less than strictly heterosexual.

It’s a politically sticky idea, says Qazi Rahman of Queen Mary-University of London. “It’s essentially saying that homosexual preference . . . is some kind of biological error,” he says. (It might tick off the left-handed folks too.)

* Hair whorl. How does your hair grow? This might reflect your sexual orientation.

The numbers: A 2004 study of nearly 500 men -- 272 on Delaware’s Rehoboth Beach, popular with gay men, 200 on a beach without that reputation -- found that hair on the heads of men on the gay beach was 3.5 times more likely to grow in a counterclockwise direction. (Scalp hair typically resembles a clockwise-rotating typhoon.)

What it might mean: One theory is that a single gene might influence hair-whorl direction, left-right brain organization and, somehow, sexual orientation. Exactly how it would do all this, however, is anyone’s guess.

The study, although intriguing, suffers from a lack of scientific rigor. The author walked around while on vacation, collecting hair-whorl observations on men from a discreet distance. He didn’t know anyone’s sexual orientation for sure, and didn’t objectively examine any scalps up close. Rahman’s group is attempting to replicate the results in the lab.

* Penis size. If exposure to testosterone in the womb influences sexual orientation, scientists reckon that straight and gay people would differ in body parts strongly affected by testosterone, such as the penis.

The numbers: Anthony Bogaert of Brock University in Ontario and his colleagues re-analyzed data on 5,000 gay and straight men from sexologist Alfred Kinsey’s famous files, collected from the 1930s to the 1960s. The results, published in 1999, showed that gay men had longer, thicker penises than did straight men: on average, about 6.5 inches long and 4.95 inches around when erect, versus 6.1 inches long and 4.8 inches around for straight men.

What it might mean: Scientists don’t really know. One guess is that gay men could have been exposed to an odd mix of hormones in the womb. Testosterone levels might peak early, causing enhanced penis growth, then drop off later in pregnancy -- leading to some feminine characteristics.

There’s one catch: Kinsey asked his subjects to measure themselves at home and mail a postcard recording their dimensions. It is within the realm of imagination that not every man reported the perfect truth. If everyone lied, the essence of the results wouldn’t change. It’s a problem only if gay men were more factually creative than straight men.

Bogaert says that all the measures -- length and circumference, erect and flaccid -- seem to plausibly line up, which probably wouldn’t be the case if the men had tacked on a vanity half-inch or so. Also, a smaller, 1960s study (in which a physician did the measuring) backs up the findings. As to whether gay or straight men are more likely to exaggerate about penis size, “It would be an interesting master’s thesis project,” Bogaert muses.

However, the next frontier in this kind of research seems to lie elsewhere -- with subtle differences in how gay and straight brains navigate new cities, respond to erotic movies and react to the scent of sweat and urine.