THE Callahans, Karen and Kevin, got married in 1973 when they were college students in Des Moines, Iowa. Forty years, two children and umpteen moves across the country later, they put down roots. Karen took a job in Durham, North Carolina, while Kevin stayed in Parkville, Missouri, where he could be close to relatives and prepare their home for retirement. They see each other at least once a month, for birthdays, holidays and mini-breaks. They look forward to their phone calls, almost every night at around nine.

About 3.9m married Americans aged 18 and over live apart from their spouses, up from around 2.7m in 2000. That number comes with some caveats, says Jonathan Vespa, a demographer with the US Census Bureau. Some of those people, like the Callahans, may be party to a commuter marriage. Others may be living apart, against their wishes, from spouses who are incarcerated, in nursing homes or serving in the armed forces. “We know it’s increasing,” says Danielle Lindemann, a professor of sociology at Lehigh University who surveyed a group of 97 commuter spouses. “But we can’t really tell who is living apart just because of their jobs.”

A higher share of men and women in their 30s and 40s live away from their partners than do those in younger and older working-age groups, according to census data. The number of separate spouses tapers off as people trade full-time employment for retirement. And the geographical patterns differ for men and women. Texas is home to the highest number of men who report an absent spouse, whereas Alaska takes the top spot for women. Nevada and New York, states with large tourism and manufacturing industries, are in the top five for both sexes.

Technological change has made living separate lives more bearable, and has thus probably contributed to long-distance marriage becoming more common. “With air travel and e-mail and FaceTime it’s a whole different ballgame,” Mr Callahan says. As communication and travel became easier and cheaper, the logistical challenges of keeping two homes and bringing up children together while physically apart dwindled. Ms Lindemann, who lived apart from her husband when she accepted a position in Nashville, is a case in point. She had no children and saw the separation as a temporary arrangement with a set end-date. Commuter couples in academia say the choice to live apart is a “professional necessity rather than financial necessity”, Ms Lindemann says.

It is no fluke that there has been a shift away from cohabitation within marriage since the financial crisis of 2008. Mark Penn, a political strategist, argues that only a minority of commuter spouses are highly educated careerists and academics like Ms Lindemann. Most have been “forced apart by economics”, he says. The timing is suggestive: as the economy went into recession many people faced a choice between a job far away and no job at all. Curiously, though, the number of long-distance marriages has not declined, even as America’s economy has recovered. An enduring shift in America’s familial norms may be under way.

Long-distance marriage is often unglamorous. Some lucky commuters are able to visit their partner every weekend. Others go months, or even years, without a reunion. Holidays can provide a brief respite. Among the usual throngs of travellers this December will be husbands and wives who are neither estranged nor living together.