For example: If you show most men a picture of an attractive man and an attractive woman, Savin-Williams says, the guy is drawn to the opposite sex. Repeat the test with women, and the results are quite different: Just like the men, they're checking out the chick.
Obviously, women might be admiring the other girl's haircut -- or, in classic catty style, assessing how they match up. But another study provides stronger evidence that there might be something else going on. Perhaps even attraction.
That study, published in the journal Psychological Science, measured the genital arousal of men and women watching porn.
What the researchers found was, in many ways, to be expected: Gay men like watching men have sex with other men. Straight men like watching women have sex with other women, and they also like watching guys and girls together. Men banging men? Never.
The surprise was in the women's reactions.
Sure, lesbians liked watching women together -- but those same lesbians were almost as turned on by watching a guy with a girl. And while straight women liked the girl-on-guy stuff, they were just as excited about watching girls with other girls.
Taken as a group, the difference between the reactions of gay and straight women was almost imperceptible.
The study's authors are quick to explain that their study doesn't mean all women are bisexual. But the only evidence they offer is a generalization.
"The large majority of women in contemporary Western societies have sex exclusively with men," they write, as if that explains it. "A self-identified heterosexual woman would be mistaken to question her sexual identity because she became aroused watching female-female erotica; most heterosexual women experience such arousal."
Meaning: If all women are closet cases, it must be okay.
That explanation may comfort the uptight, and might have passed muster in 1950, but in 2005 it hardly offers a final verdict. Times are changing, and complicated desires are no longer easily dismissed.
Indeed, the study seems to raise two real questions: What if many heterosexual women are actually dying to get with a woman? And what if most lesbians wouldn't mind sleeping with a man?
While the researchers don't go there, their conclusion is still interesting. A woman's pattern of sexual arousal, they write, doesn't constrain her behavior to nearly the degree that it constrains a man's.
In other words, a gay man feels compelled to sleep with a man because that's what excites him. But a woman may be physically turned on by women and still have a happy, healthy sex life with a guy. Her body is mostly bisexual, and her decision to be straight or gay, or neither, isn't biology so much as choice.
To some extent, the research wipes out the biological argument for gay rights. If orientation is a preference, and not a fact of DNA, a lesbian isn't born. Society may push her one way, and circumstance another, but it's still ultimately her decision.
That fluidity is reflected in a 2003 study that Diamond did with 80 lesbian college students.
At the beginning, all 80 women considered themselves gay. (Roughly half were lesbian, while the rest were bisexual or unlabeled.) Five years later, 49 percent had changed their label. A full 12 percent decided they were straight after all.
While the "Lesbian Until Graduation" concept is old news, the way the women explained the experience was revealing. The students who'd decided they were straight still reported being attracted to women. Every single one said that she'd be open to hooking up with a woman in the future.
Their attractions didn't change. What changed was how they thought about them, and who they chose to be with.
Researchers are now debating whether women even have a sexual orientation. The question has become so hot that one academic journal has decided to devote an entire issue to the subject, says Cornell's Ritch Savin-Williams.
"You'd never ask a question like that for guys," he says, laughing. "It's clear some women do have a sexual orientation: They're so into women, or into men, that they could never consider going another way. But the interesting thing is that there's a big group in the middle. And now we're wondering: Maybe it's not that their orientation is fluid -- but that they just don't have one."
It's easy to see conservative Christians seizing on such findings. If orientation is fluid, after all, it shouldn't be hard to straighten everybody out, one Exodus program at a time. Indeed, one of Diamond's studies now pops up on a Web site that pushes therapy to help gay people re-orient as straight.