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opportunity or an affordable home (Michael Bloomberg, New York’s mayor, calls his town a "luxury city").

Between 2000 and 2009 Maine surpassed Florida as having the highest concentration of those 50 and older, according to the Census Bureau's American Community Survey. Of the 20 oldest States in 2009, 14 were in the north-east and Midwest. The sunbelt, in contrast, was home to eight of the ten States with the highest concentration of youth. Austin's number of married couples with children rosę by 24%; Youngstown's shrank by the same.

America's young and old are differentiated not just by region but by race. Hispanic immigrants in particular are helping to swell the ranks of the young. The decade saw America’s foreign-born population grow by 24%, or about 7.4m. In 2009 Hispanics comprised 21% of those younger than 25; those 65 and older were 80% white and only 7% Hispanic.

But this divide—what William Frey of the Brookings Institution in Washington, DC, calls "the cultural generation gap"—is very much wider in some States and cities. In Arizona, for example, 83% of the elderly are white and 42% of those under 25 are Hispanic. This can lead to divergent priorities, such as the reluctance of the old to pay for education, or even a political eruption. This year Arizona's anti-immigration ordinance sparked protests far beyond the state's borders and a lawsuitfrom the federal government.

Such conflict may well be replicated as other places welcome (or fail to) new residents. Immigrants are increasingly dispersed, settling in areas unused to outsiders. South Carolina's Hispanic population expanded by 116% between 2000 and 2009. South Dakota, Tennessee and Alabama also saw big jumps.

In the long term, these immigrants or their children may become local economic stars. In the short term, tension is mounting. Mr Frey found that many of the new magnet States attract immigrants unlikely to speak English or to have completed school. Voters in such communities may view immigration rather differently than do those in San Francisco or Pittsburgh, hubs for skilled, foreign-born workers.

There is also a rangę of subtler shifts. In his 2008 book "The Big Sort", Bill Bishop argues that communities are differentiated not just by demography but by the way people live. In the 1960s and 1970s communities on the whole grew ever morę alike. But sińce the 1980s they have diverged on a rangę of indicators, from the age at which women have children to the age at which