Strictly heterosexual people don't exist, according to a psychologist who claims more and more men are better defined as "mostly straight".

While most societies promote heterosexuality as the "norm", a leading researcher at Cornell University has found most of us get aroused by both genders, the Daily Mail reported.

The paper brings into question strict definitions of sexuality, and posits that instead of categories we should see it as a spectrum.

Lead author Ritch C Savin-Williams, a psychologist specializing in gender studies, warns we still struggle with the concept of bisexuality - particularly when it comes to men.

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Strictly heterosexual people don't exist, according to a psychologist who claims more and more men are better defined as "mostly straight".

While most societies promote heterosexuality as the "norm", a leading researcher at Cornell University has found most of us get aroused by both genders.

The paper brings into question strict definitions of sexuality, and posits that instead of categories we should see it as a spectrum.

Lead author Ritch C Savin-Williams, a psychologist specializing in gender studies, warns we still struggle with the concept of bisexuality - particularly when it comes to men.

Savin-Williams' new book Mostly Straight: Sexual Fluidity Among Men finds younger generations are increasingly open to "looser boundaries", and interviews 40 men who insist they are "straight" but dabble in liaisons with other men.

It builds on his study, published in the Journal of Personality and Social Psychology, Savin-Williams examined volunteers who identified as either male or female.

They showed them each porn involving men, and porn involving women, and measured the dilation of their pupils - an indicator of sexual arousal.

Women's eyes dilated watching men with women, and watching women with women.

Men's eyes dilated watching women masturbate, and watching men masturbate - regardless of their stated sexual preference.

"We used to think [bisexuality] was only a female phenomenon," he told Broadly. His research assessing both men and women shows that is hardly the case.

In a previous study, Savin-Williams found between two and 11 per cent of adults had reported experiencing homosexual feelings.

However, he believes that figure woefully understates the fluidity of sexual arousal among all people.

He warns our strong cultural narrative that women can be sexually fluid, and not men, means most males fear expressing those feelings.

"Men have gotten so much cultural crap put on them that even if a man does have some sexual attraction to guys, they would never say it," he told Broadly.