Robert Putnam is professor of public policy at Harvard University's John F. Kennedy School of Government and also the author of Bowling Alone.

ROBERT PUTNAM: People of different income levels, educational backgrounds, are less likely to marry people of other, outside their own social class. Less intermarriage across class lines is declining, not rising. Racial, I'm sorry residential segregation in class terms is increasing not decreasing. Integration in schools - that is kids going to school with people whose dads do something or mums do something other than them, that's class segregation in schools - is increasing not decreasing.

MARK COLVIN: What's causing it?

ROBERT PUTNAM: A number of things actually. It's not a single thing. I think part of it, the largest single explanation is the growth of the economic gap in America and frankly not only in America. I think that's true in most other countries. I don't know the details of the data here in Australia but I do know in most parts of the world there is a growing gap between rich and poor. And that means that kids coming from upper middle class backgrounds are living in a different world now from kids coming from working class or less well off backgrounds. This is sometimes in America mistaken as a racial problem because people think, well you know if there are class differences you've got racial differences. But actually it's clear it's not racial problem. It's a class problem.

http://www.abc.net.au/pm/content/2011/s3181969.htm