The latest round of research done by sociologist Robert Putnam has been spreading around the world in dribs and drabs for most of this decade. Mr Putnam, who teaches at both Harvard and the University of Manchester, is known for his work on social capital, which he defines as "social networks and the associated norms of reciprocity and trustworthiness".

Since social capital is linked to better health, wealth and education, longevity and a stronger democratic life, it is something worth guarding.Mr Putnam lamented its decline in a bestseller called Bowling Alone. At the start of this decade, Mr Putnam undertook a vast study that led him to a troubling conclusion: one of the big causes of the decline of social capital is racial diversity.