The rich are getting richer – but does this curtail social mobility? Not in America it seems – “where people at the bottom are just as likely to move up the income ladder today as they were 50 years ago.”

This is but one nugget in a treasure trove of an article by Matthew O’Brien for the Atlantic. He draws on a major academic study – which we’ve covered before on the Deep End – which looks at local variations in social mobility across the United States.

The differences between the most and least socially mobile areas are extreme:

“Kids born into the bottom 20 percent of households, for example, have a 12.9 percent chance of reaching the top 20 percent if they live in San Jose. That’s about as high as it is in the highest mobility countries. But kids born in Charlotte only have a 4.4 percent chance of moving from the bottom to the top 20 percent. That’s worse than any developed country we have numbers for.”

America is a big place, of course, but in terms of lifetime opportunity some parts look like Scandinavia while others (notably the South and Ohio) drag down America’s average to the bottom of the league.

O’Brien considers a number of possible explanations, starting with race:

“The researchers found that the larger the black population, the lower the upward mobility. But this isn’t actually a black-white issue. It’s a rich-poor one. Low-income whites who live in areas with more black people also have a harder time moving up the income ladder. In other words, it’s something about the places that black people live that hurts mobility.”

So what is it about these places that holds back the people who live there? O’Brien’s argument focuses on three words – isolation, segregation and sprawl:

“Something like the poor being isolated—isolated from good jobs and good schools… the more sprawl there is, the less higher-income people are willing to invest in things like public transit.

“That leaves the poor in the ghetto, with no way out for their American Dreams. They’re stuck with bad schools, bad jobs, and bad commutes if they do manage to find better work. So it should be no surprise that the researchers found that racial segregation, income segregation, and sprawl are all strongly negatively correlated with upward mobility.”

In particular there is evidence that communities are more conducive to social mobility when the poor live side by side with those in the middle of the income range (i.e. the people that the Americans accurately call the middle class, but which we have no proper name for).

Back in Britain, there’s a broad coalition in favour building more homes. Certainly, if we want a socially mobile, aspirational society then we need to build enough homes and in the right places. However, if we do this the cheap-and-nasty way – by weakening key planning protections like the green belt and letting the big developers do their worst, then the big push could backfire horribly.

Sprawling, poorly-planned, ‘spread city’ development is more likely to create the sort of segregated communities in which the poor are isolated from the mainstream and denied access to social capital.

Finally, it should be said that there’s one factor that’s more important than any other in explaining why some American cities do so much better than others:

“Nothing matters more for moving up than who raises you. Or, in econospeak, nothing correlates with upward mobility more than the number of single parents, divorcees, and married couples. The cliché is true: Kids do best in stable, two-parent homes.”

As an added bonus, people who are happy living together also require fewer homes.