The possibility that an individual could be equally or nearly equally attracted sexually to both men and women gained some degree of support in the mid-1940s, when pioneering sex researcher Alfred Kinsey devised a 7-point scale to measure an individual's sexual orientation. While a rating of zero suggested that an individual was "exclusively heterosexual" and a 6 rating was classified as "exclusively homosexual," the middle numbers on the scale indicated an attraction to both sexes.
Kinsey's scale has since been the target of criticism from some researchers, but it may have helped open a new avenue into the study of the phenomenon, which would become known as bisexuality.
But much about this category of sexual orientation remains mysterious. Only relatively recently have researchers begun to unlock some of the clues to the bisexual puzzle. And one thing they have found is that, for some reason, women may be more likely to describe themselves as being attracted to both sexes.
"Over the years, a number of large-scale representative surveys have suggested that bisexual patterns of attraction are more common among women than among men," Diamond says. "In addition, some laboratory research has suggested that women have more flexible patterns of erotic response to sexual imagery than men."
She says this means women are more likely to be aroused by both same-sex and opposite-sex sexual imagery -- regardless of whether they identify themselves as straight or lesbian -- whereas gay men tend to respond only to same-sex imagery, and heterosexual men tend to respond only to opposite-sex imagery.
Bailey says his previous research corroborates the possibility that the very nature of sexual orientation may be quite different for women than for men.
"Women don't do sexual orientation the same way men do," he says. "It could be that men have evolved to be very specific as to who it is that turns them on, because in particular, the male role has been to go out and find someone that turns them on. Women have evolved differently, for whatever reason, and are more fluid with their sexuality. That's one theory."
But Diamond is quick to point out that just because reports of male bisexuality are sparser does, it does not necessarily mean that male bisexuality is not a myth.
"This is not to say that bisexual men 'don't exist' -- a claim that some researchers have made, but with which I disagree -- but simply to say that we need separate studies of male bisexuality in order to determine its own unique features," she notes.
While the new findings could have an impact in the public's perception of sexual orientation, Diamond says that she hopes her research also has an impact in the professional arena.
"Psychological professionals who encounter such individuals should not prompt them to 'resolve' their attractions in a heterosexual or lesbian direction but should respect the legitimacy and stability of their bisexuality, and help them cope with any stigmatization they may encounter," she says.
Bailey agrees. "I think that it would suggest that counselors should not make the assumption that women who are bisexual are going to eventually decide one way or the other," he says. "There is no reason to think that they would be especially likely to come down on one side or the other."