So falling in love costs us friends? Good – most of us need to cull

Friendship is fine, but now it's turned into a social monster that won't stop growing, says Tim Lott

I was anything but surprised when a study from Oxford University last week showed that falling in love costs you at least two close friends. Personally I think it's an underestimate – most people shed friends like leaves in an autumn storm the moment they get hitched.

The study doesn't attempt to explain the phenomenon, beyond suggesting that you have less time for your friends so you're more liable to drift apart. This ducks the central reality – that the friends who tend to be for the chop are those your new partner doesn't like – or who don't like your partner. Or perhaps your friend's partner doesn't like your partner, or vice versa. The possibilities are endless. But at the root of it, someone doesn't like someone else.

Friend-shedding is not necessarily a bad thing. Every life needs editing now and again. Most people can afford to lose a few buddies. In fact, most people by the time they hit the age when they want to settle down would benefit from a wholesale cull. Because friendship has got out of control. It's become the primary relationship of contemporary life, a social monster that won't stop growing.

It started with Friends Reunited and the revelation that all the friends that you thought had been flushed down the toilet of history were still out there. Furthermore, they still wanted to talk with you – in fact some of them wanted to sleep with you. At a stroke you had an entirely new – albeit old, as it were – circle of acquaintances. Recycling had been taken into an entirely new arena.

When social networking sites such as Bebo and Facebook were launched, there was a new tidal wave of virtual buddies – or "imaginary friends" as I prefer to call them – all chattering away over the electronic garden fence, even though a good proportion of them had never even met you.

It's not only the number of friends that has grown. It's the range of functions that they fulfil. My father, when he got married – rather as the Oxford study predicts – put away his friends, as he put away other childish things. The focus of his life became his wife and family. He rarely went out for a drink with mates, and neither he nor my mother would have dreamt of going out on a one-on-one meeting with a member of the opposite sex. The friends they did have were kept within strict boundaries of what it was appropriate or inappropriate to discuss.

That world is now archaic. Wives and husbands nowadays are perfectly likely to form intimate friendships with members of the opposite sex. And the meaning of the word "friend" has changed beyond recognition. Nowadays, it can mean nothing more than someone with whom you exchange a message on the internet. At the same time, the primacy of the couple/child/family relationship is shrinking as the tentacles of "friendship" spread everywhere. Once marriage was for life – now many people have abandoned that hope and given themselves over to the far less demanding relationship of friendship.

The high level of divorce is one of the causes of this new emphasis. Once, you threw all your chips in with your partner, in the near-certainty that you would spend the rest of your lives together. Now there is an evens chance that you will end up as a single parent.

It makes sense to maintain a network of friends so there will be support if you end up on your own. And while divorce is one of the causes of the primacy of friendship, it is simultaneously one of the effects. For if you no longer believe that your marriage is any more important, or qualitatively different, from any other relationship, then it may seem equally soluble.

Women tend to be particularly good at creating a network of friends after a relationship breakdown – especially if there are children involved, since it gives mother-friends something powerful and binding in common. And the friendification of the world, I would suggest, is more female than male driven. I have recently joined Facebook, and the vast majority of the posts seem to be female friends chatting. There are photos of children, tales of holiday mishaps, complaints about school admission procedures.

I find it all deadly dull, but the fact that Facebook – which is really the shop window of what I call the New Friendship – seems to revolve around the minutiae of daily life seems to appeal largely to female sensibilities. Maybe it's the company I keep, but I have not seen a single post about the outcome of a big football match or the Olympic capacity of this or that geezer to drink beer or pull women.

Whatever the gender split, this increasing emphasis on friendship is also connected with a modern obsession with choice. In a long-term relationship, one other adult is unlikely to be able to sustain your interest over what is going to be a much longer active life than was once the case. Friends bring variety, new perspectives and the potential for fun and intimacy outside the daily domestic routine.

Which is all to the good – but it has its perils. Your husband or wife may not appear to stack up so well against your friends, since you see your partner close up every day, and your friends only now and then, and at a distance. This means that they can present a highly edited version of their lives, so you are liable to imagine that they are happier, or freer, or more fulfilled.

I am not against friends – on the contrary, I am fascinated by them. When I was in my 20s, I was so in love with my friends that I had a gallery of Polaroids with every single one of my "close friends" on it. There were about 30. Only a few remain now. The photographs languish and fade in a remote attic box. I can't even remember the names of one or two. I have learned that friends are much more dispensable than you imagine when you are young – which brings us back to the possibility that there was something valuable in the archaic, exclusive attitude to friendship that my parents' generation shared.

For although their lives were limited by their choices, they understood that the hard work in a marriage required real commitment and that concentrating on it and all its difficulties produced a long-term benefit. Friends enriched their lives. But they were in no doubt about what really mattered. Friendship was the icing on the cake. It tasted sweet, but it melted easily.

Of course that generation went too far along that road, and the lack of perspective that friends might have given them doubtless narrowed their lives. However there is a middle ground between having hardly any friends, having intense friendships that are given too much precedence, and simply having too many friends to be able to call them friends in the first place. It seems to me all too common that New Friendships fall into one of these last two categories.

For the inexorable rise of the New Friend means that friendship has become wide and shallow rather than deep and narrow, or inappropriately intimate when it should know its place. This is a form of decay. A good friendship, like a good marriage, has at its heart an understanding of boundaries.

I find the sheer number of friends that people now have inhuman and inauthentic. Ditching a couple of friends when you fall in love strikes me as eminently sensible – as does getting the friends that remain into proper perspective.

The Oxford Study, as I have said, identifies not a regrettable falling off but a healthy tendency. Less is more. And, in any case, you probably stopped having anything in common with most of your friends years ago and just never did anything about it.

Friends are a joy and a blessing and a comfort. But it's easy to forget that they are also more degradable than you imagine. Fortunately, when you fall in love, your partner is almost certain to remind you.