Having undertaken, for the Glory of God, and advancements of the Christian faith and honor of our King and Country, a voyage to plant the first colony in the Northern parts of Virginia, do by these presents, solemnly and mutually, in the presence of God, and one another, covenant and combine ourselves together into a civil body politic; for our better ordering, and preservation and furtherance of the ends aforesaid; and by virtue hereof to enact, constitute, and frame, such just and equal laws, ordinances, acts, constitutions, and offices, from time to time, as shall be thought most meet and convenient for the general good of the colony; unto which we promise all due submission and obedience.

So reads the heart of the Mayflower Compact, adopted as the ship of that name lay at anchor off the coast of Cape Cod in November 1620. It was the first governance document adopted by America's European settlers, who a year later would celebrate the first Thanksgiving. Notice this line:

[A]s shall be thought most meet and convenient for the general good....

They hadn't even stepped ashore and already they were pledging themselves to the common good. Odd how, 391 years later, there no longer is universal agreement on these shores that the common good should be a goal.

We celebrate this Thanksgiving in times of near-record income inequality and near-record levels of poverty. Many Americans rationalize this with the comfortable myth that in America, anyone with enough grit and gumption can climb society's ladder.

But for decades, it has been abundantly obvious that economic mobility in the United States isn't what it used to be. Put simply, despite what many American believe, their chances of moving up the wealth and income ladder are far worse than they used to be.

Study after study has shown this to be true. Continuing high unemployment and revelations about record income inequality — 90 percent of all forms of wealth belonging to 20 percent of the population — have begun to cast doubt on these assumptions.

Here's another ominous sign: Even some thoughtful conservatives have begun to acknowledge that a lack of opportunity has become a problem. In February, Indiana's Republican Gov. Mitch Daniels had the nerve to go before the Ronald Reagan Centennial Dinner of the Conservative Political Action Committee and say:

"We must display a heart for every American, and a special passion for those still on the first rung of life's ladder. Upward mobility from the bottom is the crux of the American promise, and the stagnation of the middle class is in fact becoming a problem, on any fair reading of the facts. Our main task is not to see that people of great wealth add to it, but that those without much money have a greater chance to earn some."

Most discussions of sociality mobility proceed from the Census Bureau's division of the population into fifths according to family income. In 2009, the bottom quintile earned $20,454 or less; the top point for the second quintile was $38,530; for the third, the top point was $61,800; the fourth topped out at $100,000 and the fifth quintile earned more than $100,000.

In the cover story for the Nov. 14 edition of The National Review, the Bible of mainstream conservative thought, Scott Winship of the Brookings Institution says income mobility can be viewed two ways: in absolute terms, meaning whether people are better or worse off than their parents were at the same age, or in relative terms, meaning whether you are better or worse off than your parents were compared with everyone else in society.

In absolute terms, Mr. Winship suggests, two-thirds of American 40-year-olds are better off than their parents were at the same age. But in relative terms, most are not gaining ground.

He notes that according to the Pew Foundation's study of Economic Mobility in America, 40 percent of Americans who were raised in the bottom quintile still are there as adults. That's twice the percentage that a random distribution of socioeconomic indicators would suggest.

And of the 60 percent who manage to get out of the bottom quintile, most don't get very far out. Only one in three makes it to the top three quintiles. At the other end of the scale, a kid who chooses the right parents and is raised in the top two quintiles has a 60 percent chance of being there as an adult.

A few weeks back, we noted that a new Brookings report had found that the number of Americans living in extreme poverty — defined as neighborhoods where at least 40 percent of residents live below the poverty line, making it harder to break free — rose by a third between 2005 and 2009. In the St. Louis metro area, the number rose 37 percent, from 65,428 people in 2005 to 89,917 in 2009.

Concentrated poverty has all sorts of pernicious effects. Schools tend to be bad. Teachers tend to be less experienced. Students tend to have greater social welfare needs. Violent crime rates tend to be higher. People have less access to health care. Housing stock becomes dilapidated. Employers avoid these neighborhoods, limiting opportunity and raising costs.

These handicaps make it extraordinarily difficult for young people to escape this environment.

Some Republicans haven't gotten the word that there's a problem. Addressing the Heritage Foundation last month, House Budget Committee Chairman Paul Ryan, R-Wis., said, "Class is not a fixed designation in this country. We are an upwardly mobile society with a lot of movement between income groups."

For conservatives like Mr. Daniels and Mr. Winship, restoring the chance for social mobility — particularly for children growing up in the bottom quintile — is part of a pro-growth, pro-competitiveness agenda that is vital to America. If nothing else, not addressing the issue is a waste of money.

"The mass of people stuck at the bottom is likely to represent an incredibly costly misallocation of human resources," Mr. Winship writes. "Of course, one-fifth of the population has to be in the bottom fifth, but that quintile does not have to be filled so disproportionately with the children of disadvantaged parents."

In 2011 as in 1620, no one can guarantee equality of outcomes. But America, in the name of the common good, can and must ensure equality of opportunity.