Dr Chivers, a research fellow at the Centre for Addiction and Mental Health at the University of Toronto, says she has data to support this.
She recently published results of a study in which she showed people video clips of naked men and women in various sexual and non-sexual situations and measured their genital arousal.
Heterosexual women, Dr Chivers and her colleagues found, were no more excited by naked men doing yoga or tossing stones into the ocean than they were by the control footage: pans of the snowcapped Himalayas.
When straight women viewed a video of a naked woman doing calisthenics, on the other hand, their blood flow increased significantly.
What really matters to women, Dr Chivers said, at least in the somewhat artificial setting of watching movies while intimately hooked up to a device called a photoplethysmograph, is not the gender of the actor, but the degree of sensuality. Even more than the naked exercisers, they were aroused by videos of masturbation, and more still by graphic videos of couples making love.
Women with women, men with men, men with women: it did not seem to matter much to her female subjects, Dr Chivers said.
"For heterosexual women, gender didn't matter. They responded to the level of activity," she said.
Dr Chivers' work adds to a growing body of scientific evidence that places female sexuality along a continuum between heterosexuality and homosexuality, rather than as an either/or phenomenon.
"She's pinpointing what's kind of obvious, and yet unexplored: that women are so fluid in their sexuality," co-director of Bi the Way, Josephine Decker, said.
Women's responses to images of coupling extends even to other species, Dr Chivers found. In a 2004 experiment, and again in the recent study, published in the December 2007 Journal of Personality and Social Psychology, Dr Chivers and her colleagues found women slightly but significantly aroused by footage of bonobo chimps mating. Men showed no such response.
And when Dr Chivers asked her subjects to rate their own arousal to the videos they watched, the women, whether gay or straight, tended to give higher ratings to films showing women. Straight and gay men, as well as lesbians, were more predictably aroused by images of their preferred sex, Dr Chivers found.
"To conclude that women are bisexual on the basis of their sexual responding overlooks the multidimensionality of female sexuality," Dr Chivers said. She did allow that the apparent flexibility of women "may be related to greater potential for bisexuality in women than in men".
"Either way, it's clear that young people are redrawing the map of sexuality," co-director of the film, Brittany Blockman, said.
Blockman, 27, who holds a masters in medical anthropology from Harvard, said she got the idea for the film when she channel-surfed across The OC and saw Mischa Barton's character kiss another young woman. "When did two girls making out on mainstream teen shows become acceptable and cool?" she asked.
"I felt like I'd missed some kind of cultural shift."
Co-director Decker, 27, seemed a little embarrassed by her own limited experience.
"The sad thing is, I desperately need to get with a girl," she said
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