Oxford University’s professor Robin Dunbar said “too many casual acquaintances make you infertile. You see it in primates.”
It also reportedly disrupts a woman’s monthly cycle.
“The size of relationships place huge stress on endocrinology (hormone levels). It can destabilise the menstrual cycle and cause infertility,” he said.
“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.”
Instead, he told the American Association for the Advancement of Science in Boston that “shoulder-to-cry-on” friends or a tight knit group can boost fertility.
He based the idea on research conducted into primates which found female monkeys lacking a close group of allies are more likely to be infertile.
Professor Dunbar had previously conducted research into why our brains limit us to having a maximum of 150 friends, deeming the figure the “dunbar number”.
He then studied the notion that women who phone each other regularly remain friends. However, men need to physically see each other to maintain friendship.