'The caricature is, if you move away, girls will be on the phone or Facebook with each othe.' File picture: Reuters

Boston - Having five close friends may boost a woman’s fertility, a leading psychologist claims.

That is because being in a close-knit group acts as a buffer against stress, according to Robin Dunbar, professor of evolutionary psychology of Oxford University.

Those lacking a circle of confidantes are more likely to see their monthly cycle disrupted and thus will find it harder to conceive.

And the stress of maintaining too many casual acquaintances – rather than a core of close friends – will also disrupt a woman’s hormones, Professor Dunbar claims.

He told the American Association for the Advancement of Science’s annual meeting in Boston that women attempting to have children will find a circle of close friendships helpful.

He bases his theory on research in primates that found that female monkeys lacking a close "coalition" of allies are more likely to be infertile. Professor Dunbar said: "The size of relationships place huge stress on endocrinology [hormone levels]. It can destabilise the menstrual cycle and cause infertility. Having too many casual acquaintances makes you infertile. You see it in primates.

"There is a suggestion that your core friends – the shoulder to cry on friends – can bolster your fertility. A core of five best friends are crucial in buffering you against the stress of other people.

"If you do not have these friends, you are more likely to have your menstrual cycle and endocrinology disrupted." The professor is famous for coming up with the Dunbar Number – a theory that our brain limits us to a maximum of 150 friends.

His latest research suggests women can maintain their friendships by chatting with each other regularly by telephone.

By contrast men need to meet – such as going to the pub or playing football – to maintain friendships. Oxford researchers found a gender divide when it comes to friendships. Professor Dunbar followed 30 teenagers as they left home to start university or to pursue a career.

He looked at why some friendships lasted, while others faded.

"What determined whether they survived with girls was whether they made the effort to talk more to each other on the phone," he said. "The telephone was only of any use to the girls though.

"Talking had absolutely no effect on boys’ relationships at all. What held up their friendships was doing stuff together.

"The caricature is, if you move away, girls will be on the phone or Facebook with each other.

"With guys, it is 'out of sight out of mind'. They just find four more guys to go drinking with."