The poor in America are not a permanent class of people. Who's poor in any given year is different from who's poor a few years later.

Census data on who participates in assistance programs suggests as much. But Mark Rank, a sociologist at Washington University, has for several years been compiling far more comprehensive evidence of this pattern. He and colleagues have been studying the economic fortunes of several thousand families in the longest running longitudinal survey in America, going all the way back to 1968. Follow people over a really long period of time, they've found, and an incredible number of them experience economic insecurity at some point.

In fact, a vast majority do.

By the time they're 60 years old, Rank has found, nearly four in five people experience some kind of economic hardship: They've gone through a spell of unemployment, or spent time relying on a government program for the poor like food stamps, or lived at least one year in poverty or very close to it.

Because this long-running survey, the Panel Study of Income Dynamics, was designed to reflect the demographics of America, we can think of these figures as probabilities that we all face: By age 60, nearly 80 percent of us will have gone through a rough stretch.

"Rather than an uncommon event," Rank says, "poverty was much more common than many people had assumed once you looked over a long period of time."

Here is another way of slicing this data: The below chart, from data in a new paper Rank and Cornell's Thomas Hirschl have just published in this ongoing project, shows how likely people are over the course of their working lives to spend a year living either in the bottom 20 percent of the income distribution, or — worse — in the bottom 10 percent:

By the time we're 60, three in five Americans will have spent at least a year at the bottom. If that sounds like a surprisingly high number, it's worth noting that an awful lot of us will spend at least some time at the top, too. This next chart, using data from the same project, shows the share of Americans who will spend at least one year by age 60 in the top 20 percent — or even the infamous top 1 percent.

That's right: 11 percent of us will make an appearance in the top 1 percent by age 60. We just may not stay there that long.

"The story of the American life course is marked by a surprising degree of economic movement and volatility," Rank says.

That means that the poor (or even the wealthy) are not some abstract other. The poor are, well, us — or us 10 or 15 years from now. If more people recognized this, Rank suggests, it's reasonable to think there'd be greater public support for programs that aid the poor. If you don't like food stamps because you think you'll never need them, maybe these probabilities would change your mind.

"A lot of people tend to experience a year or two of economic insecurity, then get back on their feet, then maybe experience another year down the road," Rank says. "This movement in and out of economic insecurity is a more typical pattern than somebody who is there for 10 years in a row, although there is a part of the population that is."

His data can tell us a lot too about these long-time users. In his study, only 7 percent of people relied on government programs for the poor for 10 years in total between the ages of 25 and 60. Half as many — 3.8 percent — relied on them for 10 years straight. A lot of us, in short, need a little help; only some of us need a lot of help for long stretches of time.

This data, though, is based on what happened to Americans between 1968 and 2011. So the poverty figures may well be a conservative estimate for what someone who's 25 today could expect in the coming decades as incomes continue to stagnate and job security worsens.

It's also worth noting that these probabilities vary dramatically by demographics. Americans with at least some college education are more likely to experience affluence than those without. Similarly, whites are much more likely to experience affluence — and less likely to spend some time in poverty — than minorities.

This last graph based on Rank's work captures that reality. It shows that about 44 percent of whites are likely by age 60 to experience at least one year of affluence (or an income about nine times the poverty rate). The same is true for only 16 percent of non-whites. Non-whites, conversely, are much more likely by their late working years to have spent time poor (or below 150 percent of the poverty rate):