“There are all kinds of gay people,” begins the first edition of Free Your Mind, a 1996 support book for gay, lesbian, bisexual youth and their allies. “No one is sure what percentage of the population is gay or lesbian, and there is considerable ongoing debate. ”

Ultimately, the book suggests, the percentages don’t matter. Being gay or lesbian just means being attracted to someone of the same sex. “Gay and lesbian,” written in one breath, are just words for two gendered versions of the same thing.

This is roughly the same story I heard in my first real conversation about homosexuality in grade school. “Anyone can be gay,” my best friend told me with wide-eyed sincerity over the vinyl cafeteria table. And it happens to one in 10 people, she added, probably unwittingly quoting 1940s sexologist Alfred Kinsey.

Even Kinsey, though, did not find that 10 percent of people were gay. He found that about 10 percent of men, in his small sample, were exclusively homosexual. Only about half as many women behaved the same way.

Since Kinsey, the science of gay demography has progressed dramatically. We now know that far fewer than 10 percent of people are likely gay, but that answer changes dramatically depending on exactly what question you ask.

Gary Gates, an LGBT demographer at the Williams Institute, estimated in one review that 11 percent of the American population has experienced same-sex attraction, but only eight percent have acted on it in their adult life, and only 3.5 percent identify as gay, lesbian or bisexual. But like Kinsey, Gates found the numbers were sharply divided by gender. In an average of studies across the United States, he found that while 2.2 percent of the adult male population identified as gay, only 1.1 percent of women identified as lesbian: precisely two to one.

But those data are from the United States. What about here in Canada? The Canadian Community Health Surveys in 2003 and 2005 reported that 1.4 percent of its male sample was gay, compared to only 0.8 percent of women. Once again, about two gay men to one lesbian. The UK’s Integrated Household Survey? 1.5 percent gay men to 0.7 percent lesbians. Two to one. Norway? Ditto. Australia? Similar.

Reading these numbers, I was curious whether the same ratio held true in gay and lesbian communities. Unfortunately, I had to resort to much less scientific means. I got my hands on a marketing study prepared for Xtra in 2009. It found that 76 percent of our readers were male and 21 percent female, more than three to one.

Our readership may be a slanted sample, so I collected five years of photographs of Vancouver’s Pride parade and counted every male- or female-presenting person I could see. All told, 69 percent were male and 31 percent were female: about two to one.

Despite what Free Your Mind claims, we actually know quite a bit now about the composition of gay and lesbian populations, and the results are puzzling. It would be comforting if all cross sections of society were equally, independently, immutably and democratically gay. But they aren’t. If anyone can be gay, then where are all the lesbians?

***

Between 1998 and 2001, Meredith Chivers, a Canadian graduate student at Northwestern University, invited 121 gay, straight and bisexual men and women into her lab in Chicago and asked them to watch videos of couples having sex. Chivers had the subjects record their feelings of arousal and, at the same time, monitored their genital blood flow — and thereby physiological arousal — with a device called a plethysmograph.

What Chivers discovered was a profound difference between how men and women get turned on. The men, for the most part, behaved as advertised. The straight men were aroused whenever a woman was on screen; the gay men when they saw a man. Also, their subjective records of their own arousal matched the increase in blood flow to their genitals.

The women, on the other hand, showed hardly any “category specific” arousal at all. Everything turned them on: men with men, women with women, men with women, and even bonobos with bonobos. In addition, what the women said they liked did not necessarily match their own physical reactions.

At the time, and after Chivers’s study was popularized in Daniel Bergner’s book What Do Women Want?, some scientists took her research to suggest that all women were basically bisexual and that gay and straight women were more of a socially constructed phenomenon.

Chivers, however, was not satisfied. What if, she reasoned, the women were reacting to something in the videos that “trumped” their sexual orientation? In 2007, she ran the experiment again at the University of Toronto but this time included videos of solitary men and women masturbating or exercising naked. The second experiment confirmed the results of the first, with one difference: the lesbians in the study were more aroused by the solitary women and less by the solitary men. The straight women, however, remained turned on by everything.

Not to be discouraged, Chivers, now a professor at Queen’s University, tried one more time to find gender-specific reactions in straight women. In a third study, Chivers had her subjects look at still photographs of aroused genitals — erect penises and engorged vulvas — with no context, storyline or other sexual content. This time, finally, the straight women came through. Looking at the still images, their reactions to erect penises were significantly stronger than to female genitalia.

None of this means, Chivers is quick to clarify, that straight women or gay women don’t really exist. The formula of social, genetic and environmental factors that produces sexual behaviour is, she says, “more complicated than I or anybody would care to imagine.” Straight women obviously prefer men, and those preferences guide their sexual choices. What it does mean, however, is that “sexual orientation” means something fundamentally different for men and women.

***

While Chivers studied women, the male side of the equation was picked up by Northwestern University sexuality researcher Michael Bailey, who collaborated on Chivers’s first experiment (you might remember him fondly from a media firestorm following his presentation of a live “fucksaw” demonstration in an optional, after-class lecture).

While female bisexuals seemed ubiquitous, things looked grim for the existence of male bisexuals. The male self-identified bisexuals in Chivers’s study reacted physically only to one gender or another, not both. Most reacted only to men. When Bailey tried the same thing in his own lab, he found the same monosexuality. Most men who said they were bisexual seemed to be physically aroused only by other men.

Bailey also persevered, however, at the urging of the American Institute of Bisexuality. In the first studies, all the male bisexuals were self-identified. Instead of relying again on identification, which seemed like a poor predictor of actual arousal, Bailey looked for male bisexuals who were acting bisexually. He recruited for a new study from Craigslist ads of men looking to have sex with couples: men who were displaying their bisexuality up front. This time, the men in the study were turned on by both men and women: the male bisexual was vindicated, at least partly.

Some bisexual men do react to both male and female stimuli, Bailey concluded, but many do not. “You have to ask yourself,” he says, “which one of those two is most common.”

While Chivers discovered that female bisexuals are likely much more common than anyone imagined, Bailey’s research suggests that while male bisexuals exist, they are a rare breed.

In speaking to Chivers and Bailey it becomes increasingly clear that, while men and women both have a broad range of sexual orientations, male and female homosexuality are not at all the same thing. What we usually call “sexual orientation,” an exclusive pattern of arousal focused on one gender, is a trait mostly possessed by men and, to a lesser degree, lesbians. Non-lesbian women, on the other hand, are aroused by so many different things that gender can become an afterthought.

“I have asked, not in jest, whether women have something like a sexual orientation,” Bailey says. “Men’s orientation is a guide, a motivation to find people to have sex with. Women’s is not that.”

***

In light of Chivers’s and Bailey’s research on the flexibility of female and the specificity of male sexuality, gay demographics make a lot more sense. Take Gates’s review of American surveys that showed only 1.1 percent of women are gay compared to 2.2 percent of men. The same review showed 2.2 percent of women identify as bisexual, to only 1.4 percent of men. Add up the totals, and you get 3.4 and 3.6 respectively. About the same.

The Canadian Community Health Survey shows a similar pattern. While gay men, at 1.4 percent, are twice as numerous as gay women, once you add in bisexuals, you get 2.1 percent gay and bisexual men to 1.7 percent lesbian and bisexual women, considerably closer.

This helps to clear up the mystery of the missing lesbians. It isn’t that fewer women are gay; it’s that gay women are less monosexual than their male not-quite-counterparts.

***

One criticism of Chivers’s and Bailey’s research is that the physiological reactions they study might not represent true sexual identity. Just because a broad range of stimuli get women’s blood pumping doesn’t mean they are really attracted to all those things. Shouldn’t we pay more attention to what people say about their identities, instead of what shows up in a lab?

Qazi Rahman, a researcher in the neurobiology of sexuality at King’s College London, thinks not. He recently asked 3,000 subjects about their identity, behaviour, fantasies and turn-ons and found no real relationship between identity and behaviour in the non-straight group. He believes that attraction patterns are determined before we’re even born and that identity has very little bearing on behaviour or desire.

The differences between male and female homosexuality, Rahman theorizes, may be related to different fetal hormone levels in utero. Because all fetuses start out female, he says, the hormonal process of turning a fetus male is more complex — the same reason men suffer from more developmental disorders. Somewhere in the fraught process of making a male fetus, the gay switch is more easily flipped. This could partly explain why more men turn out gay than women.

Even if the genetic causes of gayness are the same between men and women, Rahman says, the different hormonal cocktails that produce male and female fetuses may cascade in different directions. The result is that while gay men are attracted to other men, they retain the same monosexuality as straight men. Gay women, on the other hand, are attracted mostly to women but retain at least some of straight women’s more fluid sexuality.

“Humans are multilevel creatures,” Rahman tells his students. “We all basically have multiple personality disorder.”

Biology, and the possibly hormonally determined flexibility of female sexuality, does not explain everything, though. If the demographers are correct, then gay and lesbian communities should be awash in female bisexuals — nearly as numerous as gay men. But you don’t need to be a demographer to know that isn’t true.

If the dearth of lesbians is explained by a wealth of bisexuals, then where are all the bisexuals?

***

Denise Penn, the vice-president of the American Institute of Bisexuality, was giving a presentation to a lesbian reading group in Orange County, California, when one listener made a startling accusation. “I was telling them about bisexuality, and one of them said, ‘Why do you want to jump on the bandwagon?’” she says. “I thought, we’ve been on the bandwagon from the beginning.”

Penn says most bisexuals have faced discrimination within gay and lesbian communities, and many leave because of it. Gays and lesbians, she says, often see bisexuals as greedy, indecisive, promiscuous, incapable of commitment or downright wrong about their own sexuality.

“The gay and lesbian community will assume, sometimes, that if you’re hanging out with them you are gay. And then when a person comes out, it’s seen as ‘Wait a minute, we thought you were one of us,’” Penn says. “Especially bisexual women are seen as someone who is afraid to come out. It’s sometimes seen as a phase, as someone who is hanging on to ‘heterosexual privilege.’ In other words, ‘Oh, you can’t come play with us and then go back to the safety of your heterosexual life.’”

Not surprisingly, many bisexuals either leave gay or lesbian communities or remain silent about their sexual preferences. But if lesbian communities continue to lose bisexuals, the future could be a very different place. The Canadian Health Survey shows that more than a third of bisexuals, but only 10 percent of lesbians, are under the age of 24. More than two thirds of lesbians are over 35.

“When I talk to high school students, there are no lesbians there,” says Ritch Savin-Williams, a Cornell psychologist who studies the sexuality continuum. “There are lots of bisexuals.”

Williams says that among young women, the term “lesbian” is diving in popularity and is being replaced by queer, bisexual or just “not quite straight.” And if biologists like Chivers and Bailey are right, women, especially, may be more accurately describing their own behaviour and desires by embracing such terms.

The future demography of gay and lesbian-bisexual populations will likely hinge on how warm a welcome these new generations of women receive in traditional gay and lesbian communities. Without bisexuals to bolster their ranks, lesbians may dwindle, even as a steady stream of gay men tips the gay population scale decidedly in its favour.

Two days after I discussed the future of bisexuality with Savin-Williams, he sent me an excited email. By chance, a study had crossed his desk for blind peer review. As an unreviewed, blind paper, it needs a grain of salt, but I could see why he wanted me to see it. The study asked a large group of female Canadian psychology students about their sexual orientation. The results were as follows:

Straight: 70 percent. Mostly straight: 25 percent. Bisexual: 5 percent. Mostly lesbian: 1 percent. Lesbian? 0 percent.