When foreign visitors used to describe American culture, they generally settled on different versions of one trait: energy. Whether driven by crass motivations or spiritual ones, Americans, visitors agreed, worked more frantically, moved more and switched jobs more than just about anybody else on earth.

That’s changing. In the past 60 years, for example, Americans have become steadily less mobile. In 1950, 20 percent of Americans moved in a given year. Now, it’s around 12 percent. In the 1950s and 1960s, people lived in the same house for an average of five years; now people live in the same house for an average of 8.6 years. When it comes to geographic mobility, we are now at historic lows, no more mobile than people in Denmark or Finland.

Why is this happening? A few theories offer partial explanations, but only partial ones.

It is true that we are an aging nation and older people tend to move less. But today’s young people are much less mobile than young people from earlier generations. Between the 1980s and the 2000s alone, mobility among young adults dropped by 41 percent.

It’s also true that many people are locked into homes with underwater values. But as Timothy Noah pointed out in Washington Monthly, mobility among renters is down just as sharply as mobility among homeowners.