For me, like many Americans, that means "middle class".

But the truth is that the social classes in the United States are rapidly pulling away from one another.

The emerging class system looks different, with its casual dress codes and somewhat greater variety of skin tones and ethnicities.

But we are the accomplices in a process that is strangling the economy, destabilizing our politics, and eroding democracy.

There is a familiar story about rising inequality in the United States.

The villains are the plutocrats, the fat cats, the tech bros and the rest of the so-called 1%.

The good guys are the 99 percent, also known as "the people".

In fact, it is the top 0.1% who have been the big winners in wealth over the past 50 years.

Their share of the pie climbed from 10 percent in 1963 to more than 22 percent today.

But not everybody below them had to give up something.

Only the bottom 90% did.

In between, there is a third group that has held on to its share of the wealth year after year.

Let's call it the 9.9%.

The 9.9% is rich in more than mere money.

It has substantially lower rates of heart disease, diabetes, and obesity.

It marries later and has more stable family structures.

It lives in gilded neighborhoods with richer social networks and vastly better educational opportunities.

And it is able to pass all of this along to its offspring, leaving the bottom 90% in the dust.

We like to pretend that none of this matters because in the land of opportunity, everybody has a chance to make it.

But in fact, social mobility is lower here than in many other developed countries, and it's been going down.

Aristocrats take wealth out of productive activities and invest it in walls.

They lock themselves and their offspring in place at the expense of other people's children.

The escalating inequality of our time appears new.

But if you take in a broad sweep of history, starting, say, with the Great Pyramids of Egypt, inequality looks like the norm of human experience.

History tells us that aristocrats come to believe their own propaganda, that their superiority is an artifact of nature.

Today's 9.9 percent has convinced themselves they don't have any privileges, and the delusions are understandable.

A strange truth about rising inequality is that even as it locks our privileges in place, it doesn't seem to make things that much easier.

That's because the greater the inequality, the less your money buys.

Our insecurity grows as the chasm beneath us expands.

Rising inequality leads to political instability and typically ends in catastrophic violence.

Still, there have been exceptions.

America's founders were mostly 9.9 percenters.

But they turned their backs on the man at the top in order to create a government of, by, and for the people.

Reversing the calcifying effects of rising inequality isn't just a matter of helping out the less fortunate.

The challenge we face now is to renew the promise of American democracy.