It is a truth grudgingly acknowledged that mixed ethnic communities are not as mixed as they appear. In the school playground I find myself talking to the other white middle-class mums and dads, in spite of the fact that in a Hackney school there are plenty of parents who are neither. We know the white couple at number four but have had little contact with the African family at number two. It's not something I am proud of, but there it is.

Many people fear that ethnic diversity can bring out the worst in us. Most famous is the American political scientist Robert Putnam, author of Bowling Alone, who in 2006 published a study of ethnic diversity and community trust in the US. He told the Financial Times that “the effect of diversity is worse than had been imagined. And it's not just that we don't trust people who are not like us. In diverse communities, we don't trust people who do look like us.”

But while many people will agree with Putnam, few of us ask precisely why ethnicity and co-operation might be connected. Macartan Humphreys, an Irish political scientist at Columbia University, offered me a list of possibilities as to why we seem to get things done more easily in homogenous communities.