Can men and women ever be just friends? It is the eternal question that When Harry Met Sally set out to answer in 1989, and endless romcoms have continued to debate in the decades since (Friends with Benefits, 13 Going on 30 and so on).

Now a new study has shed light on the age-old question, but instead of focusing on the "sex part" that Billy Crystal's Harry was so preoccupied by, it looks at what I've always suspected to be the real problem between the genders: how they define friendship.

While women view their best friends as something in between sisters and soul mates, men see theirs purely in terms of convenience. Oxford University's evolutionary psychologist Professor Robin Dunbar discovered this when exploring how people's friendship circles changed when they left school for university.

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

He went on to look at the differences between the sexes: "Women clearly have much more intense close friendships. Guys tend not to have that relationship. They tend to have a group of four guys that they do stuff with. That is much more casual. With guys it is out of sight out of mind. They just find four more guys to go drinking with."

A close male friend

This will be familiar to any woman who has had both the pleasure and disappointment of a close male friend - particularly in your twenties and thirties. At first, things are wonderful. Life is full of great catch ups, and laughter over pints of beer. They listen to your woes, and you help them with their woman problems. It's the friendship neither of you knew you needed.

Only then, something changes. They start a new relationship. They move to a new postcode (really, it doesn't have to be far). They get a new job with a ready-made social life. They get a new flatmate. Whatever the change, they suddenly have a replacement for you, and can't seem to find the time to meet you for that coffee.

You, naively, keep trying. You call them, you send jokey pictures and do exactly what you'd do if a girl friend was growing distant: bombard her with the attention you wish that she was showing you. Except with a male friend, it just doesn't work.

A platonic relationship

Whether it is nature or nurture, most men simply do not view friendships in the same way as women. Sex is hardly the issue - it is the practical problem of how much time and effort they are able to put into a platonic relationship. As the study says, there's always another drinking buddy around the corner.

Men reading this may feel unfairly judged. So I put it to a colleague and current male friend (let's be honest, the second we stop sharing a desk, I'll never hear from him again). "Some of my best friends are women," he says vaguely.

When I ask him if the effort he makes with friends is comparable to that his girlfriend makes with hers, he begrudgingly accepts that he could not spend hours on the phone with his pals - of either sex.

I've lost count of the male friends who have disappeared over the years. Often, it coincides with the arrival of a new girlfriend. But contrary to popular interpretation, it is rarely to do with that new girlfriend's jealousy. As one former male friend charmingly told me: "I have to hear all my girlfriend's emotional stresses now. I don't have time for yours."

With male mates like that, it's probably no surprise that all my efforts go into my female friendships. And now the academics have backed my choice.