David Harding|Metro13 April 2012

Cupid's arrow strikes more than the heart when couples fall in love.

It can also bring on remarkable gender-bending changes, researchers claimed yesterday.

While the male hormone testosterone - which increases aggression and sex drive - drops in men who have found love, in women it goes up.

The effect was discovered by Italian scientists who measured the hormone levels of a dozen couples who had recently fallen in love.

Results were compared with those of another 24 men and women who were either single or in stable long-term relationships.

Donatella Marazziti, who led the team from the University of Pisa, told New Scientist magazine: 'Men, in some way, had become more like women, and women had become more like men.

'It's as if nature wants to eliminate what can be different in men and women, because it's more important to survive and mate at this stage.'

But it was not all good news for lovers. Scientists also found increased levels of the stress hormone cortisone in couples, indicating courtship can be a difficult time.

The hormonal changes last only while people are in love. Two years later, hormones returned to normal.

However, the findings have been questioned. Andreas Bartels, from University College London, said the hormonal changes could simply be the result of increased sexual activity.

But Ms Marazziti pointed out that, in her study control group, participants had sex just as often as those in the 'in love' group. Furthermore, other studies suggested testosterone levels in men rose as sexual activity increased.

If the hormonal changes were just the result of sex, testosterone levels would be expected to rise in men, not fall, she said.

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