AMERICANS are much more willing to move around within their great continental expanse than residents of other countries are to migrate. As the chart at right indicates, migration across state lines is far more common than is movement across Australian territories, Canadian provinces, or regions within the typical euro-area economy—and vastly more so than across national borderns within the EU.

What's most striking about Americans' greater propensity to move, however, is the fact that Americans are far more rooted than they used to be. Since 1990, the percentage of Americans moving from one state to another in a given year has fallen by about half. Americans rely on the natural flexibility of their labour market to do a lot of the shock-cushioning work that falls to strong safety nets or other labour-market interventions in other countries. If Americans begin to move at European rates, demands for European-style labour-market rules may rise.

It's critical, then, to understand exactly why mobility is falling. This week's Free exchange column investigates:

Some reckon the decline is caused by demography. Young workers, at the start of their working life, have most to gain from moving. An ageing population may therefore be less mobile. Growth in two-earner households could also play a role. When both partners work, it is harder to move if one of them loses his job or is offered a post somewhere else. New research by Greg Kaplan of the University of Pennsylvania and Sam Schulhofer-Wohl of the Federal Reserve Bank of Minneapolis casts doubt on these explanations. The authors analyse census data gathered monthly between 1991 and 2011, and find that the pattern of falling mobility persists across all parts of the workforce. Mobility is down across “all education levels, for people of all marital statuses, and for both single-earner and multiple-earner households.” Ageing is a red herring: mobility rates have dropped most for young workers. It isn’t so much the American worker that is changing, they argue, but the American economy. Reduced mobility largely reflects two shifts in the nature of economic activity. The first is that the mix of jobs offered in different parts of America has become more uniform. The authors compute an index of occupational segregation, which compares the composition of employment in individual places with the national profile. Over time, their figures show, employment in individual markets has come to resemble more closely that in the nation as a whole. This homogenisation reflects the rising importance of “non-tradable” work. As the name suggests, non-tradable goods and services are not traded across long distances. Californian dentists tend not to clean Floridian teeth; every city has its own dentists. Cars, by contrast, are tradable, so not every state has its own car plant. Recent research by Michael Spence and Sandile Hlatshwayo of New York University’s Stern School of Business found that 98% of employment growth between 1990 and 2008 occurred in non-tradable industries. Education and health-care jobs now account for 15% of employment, up from less than 10% in 1990. With more of the country’s employment mix present in each state, it is less necessary to move to find work. Yet a more uniform job distribution alone cannot account for falling mobility. As Messrs Kaplan and Schulhofer-Wohl point out, mobility has fallen for manufacturers, where jobs are more dispersed, as well as for service-sector workers. What is more, if workers know that they can find jobs they want in different places, they may become more willing to move for other reasons—to be by the coast, for example, or to savour a particular music scene. Yet survey data reveal that moves for these other reasons have not risen. The authors suggest another force is also reducing migration: the plummeting cost of information... Deregulated airlines and innovative online-travel services have slashed travel costs, allowing people to visit and assess different markets without moving. The web makes it vastly easier to study every aspect of a potential new home, from the quality of its apartment stock to the surliness of its baristas, all without leaving home. Falling mobility isn’t simply caused by labour-market homogenisation, the authors argue, but also by greater efficiency. People are able to find the right job in the ideal city in fewer hops than before.

Not too much cause for concern then. But the piece goes on to note that while fewer Americans are moving each year, the pace of net shifts in population—as from the Northeast to the Sunbelt, for instance—has held steady. And what's a least a little worrying is that those moves appear to be primarily motivated by cheap housing, rather than rising Sunbelt productivity. This was true in the decades before the crisis. And as Trulia's Jed Kolko explains here, it also seems to be the case in the wake of the crisis; households searching across markets for new homes are focusing on places with cheap homes rather than low unemployment.

As Mr Kolko notes, people probably aren't moving to high-unemployment areas unless they already have a secure job in hand. The trend nonetheless suggests that cheap housing continues to trump other economic signals (like low unemployment rates) in generating net migration trends. That's a little worrying. It is disconcerting to observe households passing up on high-productivity, low-unemployment markets because the high cost of housing in such places makes life there a bad deal.