Should we be surprised that friendship isn’t always mutual? That is one of the findings of a team of researchers at Tel Aviv University who’ve just published a paper in an academic journal. They asked several hundred students to identify which members of their peer group they considered to be ‘friends’. On average, half the people included in this category by each respondent did not feel the same way about them.

According to the researchers, this news would come as a shock to most people. The students in the survey thought that 95 per cent of the people they regarded as ‘friends’ would identify them as ‘friends’ too. But I can’t say I’m surprised. In fact, a 50 per cent reciprocity score strikes me as suspiciously high. The researchers cite another friendship survey in which the score was only 34 per cent. That seems about right to me.

I haven’t always been so cynical. Before I got married, I was a fully signed-up member of the friendship cult. Like many young men, I regarded my close friends as a kind of substitute family, with all the accompanying ties and responsibilities. If one of them was in trouble, you did everything in your power to help them and if you were in trouble you could expect the same of them. As far as I was concerned, we had a lot in common with the Mafia, save for the need to do something unspeakable before you were admitted. Loyalty was the supreme virtue, with any other quality coming a distant second.

It was on my stag weekend 15 years ago that the scales fell from my eyes. There were about ten people I placed in the innermost circle — my own personal Cosa Nostra — and I invited them all to Malaga a week before I got married. Or rather my best friend invited them, having volunteered to organise the trip. He promised a whistle-stop tour of the most glamorous nightclubs in Marbella and enlisted the help of a well-connected local DJ to smooth our passage. I didn’t think of this as an opportunity for a final blowout with my nearest and dearest, since it didn’t occur to me that I’d be seeing any less of them after I got married. Innocent that I was, I thought of marriage as adding another person to my intimate circle rather than the substitution of one for the other.

I experienced a brutal reality check when only four of the ten honoured guests appeared at the Spanish hotel on the Friday evening. The no-shows included my best friend, the organiser of the festivities. He left a message on my phone explaining that he’d been held up by an ‘emergency’ and might be a few hours late — needless to say, he never made it — but he’d fully briefed another member of the group and he was more than happy to take the reins. Unfortunately, that ‘friend’ didn’t materialise either. We ended up spending the first night in an ‘English pub’ watching West Ham lose 2-0 to Leeds United.

The low point was the ‘activity’ on the Saturday – a scuba-diving trip to some local caves which my best friend had persuaded me to pay for on the understanding that everyone would pay me back. They might have, too, if they’d bothered to turn up.

In the event, only three of us made the trip, with the other two refusing to get out of bed for the early morning start. It made no odds anyway, because the scuba instructor decided to cancel the dive at the last minute on account of the heavy rain. He gave me a partial refund but kept the deposit, which, if memory serves, was around £500.

As we puttered back to shore in the leaky fishing boat, the rain lashing our wetsuits, I had a moment of clarity. My belief in the unbreakable bonds of friendship was a sentimental illusion. The true test isn’t when you’re in trouble — it’s relatively easy to stand by your friends in their hour of need, although, come to think of it, plenty of my friends have failed that test, too. It’s whether they’re prepared to inconvenience themselves for your benefit, particularly if it involves getting on a plane and shelling out a few hundred quid. Turned out 60 per cent of the people I regarded as my closest friends weren’t.

In retrospect, it was a good lesson to learn just before getting married. After that, whenever there was a conflict between loyalty to Caroline and loyalty to my friends, I was never in any doubt about who came first.

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We had a lot in common with the Mafia... loyalty was the supreme virtue

Toby Young is associate editor of The Spectator.