Evolutionary psychologist Professor Robin Dunbar said: "These were a bunch of secondary school kids. They all went off to university. Couldn’t go back see friends at weekend as used to. We followed them through their first year.

"We looked at what happened to the original set of friendships - they deteriorate really rapidly over a matter of months. The churn was phenomenal. Forty per turnover every six months.

"What determined with original friendships whether they survived with girls was whether they made effort to talk more to each other on the phone.

"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. Going to a football match, going to the pub for a drink, playing five a side. They had to make the effort."