Being highly sexed changes men's and women's sexual orientation in startlingly different ways, a major academic study has concluded.

For most women, a high sex drive increases their sexual attraction to both men and women. The opposite occurs in men, where a high sex drive simply exaggerates existing sexual orientation.

Researchers are finding evidence that there is a key biological difference at play between the sexes, rather than sociological factors alone.

The TV sex therapist Tracey Cox says: "Bisexuality is going to be very interesting - something to watch, particularly with women. They've done experiments where they wire up people and get them to watch porn, woman on woman, man on man and hetero, and women were aroused by all three.

"Nearly all the sex therapists I know, if I ask what's the top fantasy for women, [will say] sleeping with another woman."