A new study concludes that the large majority of men purporting to be bisexual are actually gay, while the rest are more likely to be heterosexual.
Senior author Michael Bailey, from Northwestern University in Chicago, said: "Bisexual male behaviour certainly exists, but the study suggests that a bisexual orientation, an actual sexual preference for both men and women, does not exist in men. If such men exist, they are certainly very rare and we didn't find them."
Researchers recruited 101 young adult men, 33 identifying themselves as bisexual, 30 straight and 38 homosexual.
Gay men were aroused by images of men, while heterosexual men were aroused by women. But psychologists said those claiming to be bisexual were only aroused by one or the other - 75per cent by men and the rest by women.
Dr Bailey said some men claim to be bisexual as it is easier than admitting to being gay, while others might consider it some kind of achievement and be proud to swing both ways.