All communities are built on trust. It is, therefore, a worry when research by Robert Putnam, a professor at Harvard University, suggests that higher levels of ethnic diversity in a community lead to lower levels of trust between its members. Such research should not be abused by opponents of immigration and a multicultural society. But it does show the need for debate about how to make diverse societies succeed.

The forces that make areas diverse tend to lower trust anyway: cheap housing, for example, attracts immigrants but is also correlated with higher crime and lower social cohesion. It is also hard to generalise across ethnic groups: in all multicultural societies, some migrants have succeeded, and moved into business and the professions, while others have become trapped in unemployment and deprivation. But in the light of "home grown" Islamic terrorism in Europe, and evidence that some immigrants feel alienated by western society, academics and politicians are rightly thinking how to adjust the compromise on which a multicultural society is based.