Amazing social science studies show that America is still a nation of income mobility.

* 12% of all Americans will find themselves in the top 1% of income distribution at least one year of their lives

* 73% of all Americans will find themselves in the top 20% of income distribution at least one year of their lives

* 54% of Americans will experience a year in poverty or near-poverty at least once between ages 25 - 60.

Although I strongly suspect a lot of this variation has to do with unfortunate one-time payments (insurance payout, selling a house, lawsuit) or long-term investment (school, starting a business), these numbers are still startling.

Why don't more Americans have empathy for people in poverty?

Ultimately, this information casts serious doubt on the notion of a rigid class structure in the United States based upon income. It suggests that the United States is indeed a land of opportunity, that the American dream is still possible — but that it is also a land of widespread poverty. And rather than being a place of static, income-based social tiers, America is a place where a large majority of people will experience either wealth or poverty — or both — during their lifetimes.

A further example of such fluidity can be found in an analysis by the tax-policy expert Robert Carroll:

Using data from the Internal Revenue Service, Mr. Carroll showed that between 1999 and 2007, half of those who earned over $1 million a year did so just once during this period, while only 6 percent reported millionaire status across all nine years.