Evolutionary design could be the reason why women are more sexually fluid than man, a new theory suggests.

In a proposal published to Biological Reviews, evolutionary psychologist Dr. Satoshi Kanazawa of London School of Economics suggests ladies being more relaxed about their sexual preference stems from when they had to cope with avoiding conflict in "polygynous marriages" - where a man is allowed more than one wife.

While both genders can be sexually fluid, not conforming to one sole sexual identity, Dr. Kanazawa has found women are more likely to switch between identifying as homosexual, bisexual, or heterosexual. This would have allowed them to sleep with their co-wives as well as with their husband to reproduce back in the day, creating benefits to the set-up.

"The theory suggests that women may not have sexual orientations in the same sense as men do," the expert explained.

"Rather than being straight or gay, to whom women are sexually attracted may depend largely on the particular partner, their reproductive status, and other circumstances."

When it comes to measuring sexual orientation, the doctor noted that brain and genital responses are the "most objective and accurate" ways to determine it.

He distinguished three different aspects: nonexclusively, change, and variance, after looking into the National Longitudinal Study of Adolescent Health (Add Health). The findings measured the sexual orientation of American youths in four different ways, with the study going on for over 10 years.

One stage measured self-identified sexual feelings among a given set of labels - 100 per cent straight, mostly straight, bisexual, mostly gay, 100 per cent gay - when participants were between 18 and 28. In another section, participants were asked to describe their sexual identity aged been 25 and 34, with Dr. Kanazawa able to construct measures of "adult sexual attraction" from this information.

Overall, it found women are more fluid in their sexuality. The expert also found that ladies who are more sexually fluid have more children, and that those who marry or have kids early in adult life are less fluid.

"Since friendships and alliances can have reproductive benefits, sexual fluidity that facilitates such friendships and alliances among women is expected to be evolutionary selected," the researcher said, of how this theory explains how women form close bonds and alliances in their group.

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