A scientist who refused to believe bisexual men exist has proved they in fact, do.
Gerulf Rieger of the University of Essex once authored a study which concluded men who claimed to be sexually aroused by both men and women were probably lying.
"Regardless of whether the men were gay, straight or bisexual, they showed about four times more arousal" to one sex or the other, he told the New York Times in 2005, concluding most were gay but didn't want to admit it.
But he's now changed his mind. He and a team of British and American scientists took numerous studies conducted over the past 20 years, weeding out poor quality data, "potentially invalid statistical tests" and "inconsistent findings" and combined them for a larger, more comprehensive look at male sexuality.
"Although some men identify as bisexual and have sexual experiences with men and women, the extent to which this reflects an underlying bisexual orientation has been questioned," their new study, published in the Proceedings of the National Academy of Sciences this week, reads.
"Early sex researchers... believed that bisexual behavior and identification occurred primarily among monosexual (ie, either heterosexual or homosexual) men for reasons other than a bisexual orientation. For example, some homosexual men identify as bisexual, or engage in sex with women, due to social pressures that favour heterosexuality."
Others perhaps doubted the existence of bisexuals because they found it easier to understand monosexuality - either totally gay or totally straight, "because both have strong sexual attraction to one sex and virtually none to the other".
"Furthermore, bisexual individuals may be mistrusted and stigmatised by both heterosexual and homosexual people, and perceived as untrustworthy, promiscuous and unable to commit."
One early sex researcher who understood bisexuality was Alfred Kinsey, who developed the Kinsey scale - which goes from zero (exclusively heterosexual) to six (exclusively homosexual). In between are the bisexuals - of which Dr Kinsey was one.
The new study looked at data from several hundred cisgender men (ie.men whose gender identity matches their sex assigned at birth, whether gay, straight or in between) collected between 2000 and 2019. It found that men between two and four on the Kinsey scale definitely showed signs of physical arousal when shown images of both men and women, much more so than men who identified as straight or gay - proving they are truly bisexual.
"It has always been clear that bisexual men exist in terms of self-identity and behaviour, but many, including myself, were sceptical about their ability to be sexually aroused to both men and women," Dr Rieger said.
"Now, with this exceptional number of participants, we have clear proof of their bisexual arousal. This reshapes our entire understanding of male sexual orientation."
Lead author Jeremy Jabbour, a PhD candidate at Northwestern University in Illinois, said bisexual men don't need scientists to "validate or justify their lived experience to others", but he hopes "findings such as ours will continue to help the public see the many shades of gray that exist in human sexuality".
The researchers said not much research has been done into female bisexuality because its existence "has been less controversial".