In today’s large, impersonal cities, there seem to be three ways to make friends: at work, at the school gate and by walking the dog. I have personal experience only of the first two. I do not own a dog but I have noticed, during my walks through London’s parks, that dog owners enjoy a chatty solidarity denied to the rest of us.

The decline in the number of places to make friends was detailed by the US academic Robert Putnam in his influential book Bowling Alone. Mr Putnam noted the demise of previously vibrant organisations such as the Glenn Valley, Pennsylvania, bridge club, the Charity League of Dallas and the Roanoke, Virginia, chapter of the National Association for the Advancement of Colored People and concluded that people of whatever colour increasingly did things alone.

Now Mr Putnam has returned to the news, with an article in the journal Scandinavian Political Studies describing his research into how well 30,000 people across 41 US localities get on with their neighbours. The places studied ranged from Los Angeles and San Francisco, “among the most ethnically diverse human habitations in history”, to a rural South Dakota county, where “celebrating ‘diversity’ means inviting a few Norwegians to the annual Swedish picnic”. The conclusions are bleak: the more ethnically diverse the place, the less people trust each other. Not only do they distrust those of different ethnic backgrounds; they distrust people from the same background as themselves.

Mr Putnam’s research has prompted the view that diversity’s advocates have oversold its advantages. Mr Putnam is more optimistic. We have been here before, he says. A century ago, the Italian and Polish Catholics and Russian Jews who flooded into the US kept their distance from each other and from white Protestants. Today, those religious differences barely matter. He is right: an interesting aspect of this summer’s hit movie Knocked Up, in which a television presenter finds herself pregnant by a failed internet entrepreneur, is how their different religions are the least of their problems.

Mr Putnam writes: “The central challenge for modern, diversifying societies is to create a new, broader sense of ‘we’.” For those who think armed border guards are a better idea, it is too late. For better or worse (better, in my view, despite everything), diversity has already happened. According to United Nations statistics, the number of people living outside their countries of origin rose from 96m to 174m between 1980 and 2000. Many will go home, but most will not – so we might as well start looking for the answers.

While the school gate remains a good place for parents to chat, they rarely do so across barriers of race or class. In most places, affluent white parents have become adept at ensuring their children are educated among their own.

For those without dogs, that leaves work. In his search for reasons to be hopeful, Mr Putnam mentions two institutions that have forged a new “we”: the US army and evangelical churches. American soldiers have many more interracial friendships than their civilian counterparts and the evangelical churches are striking in their racial integration.

Here are some other institutions that have created a new “we”: McKinsey, PwC, Goldman Sachs. Run your eye down the list of Goldman Sachs managing directors: Peter Tomozawa, Eiji Ueda, Lucas van Praag, Ashok Varadhan. It is no surprise that these blue-chip organisations are so diverse. Not only do they have offices around the world but in their principal cities, restricting themselves to white Anglos would substantially narrow their choices. In New York, 43 per cent of employees are immigrants. In London, the figure is 33 per cent. At work, people are thrown together in a way they no longer are in their ethnically divided neighbourhoods.

Do companies benefit from a diverse workforce? In his book The Difference, Scott Page, a University of Michigan professor, analyses a wide range of studies and concludes that, on balance, they do. The benefits come not from having people of different races and religions (“identity diversity”) but from having employees with different outlooks and training (“cognitive diversity”). Diversity means more than recording employees’ ethnic background.

There is substantial evidence that cognitively diverse groups are more creative. Nobel Prizes increasingly go to teams rather than individuals and a striking number of the US winners are foreign-born. But Mr Page warns that there is a potential drawback to diversity: people from different backgrounds may not get on.

We need a mature discussion, one that recognises that diversity’s costs need to be managed if the benefits are to emerge. But we also need to cheer up. I would bet most of us see more interracial amity at work than the pessimists believe possible.

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