I'm coming back to the same point because it's an important one: Absolute mobility matters more than relative mobility.
*Absolute mobility means that living standards are increasing in absolute terms: You are better off than your parents, and your children will be better off than you.
*Relative mobility means that if your family is poor, you have a decent chance of moving up the relative income ladder. That is, the rank order of people in society is malleable.
The two have very little to do with each. You can have an economy with a lot of absolute mobility, and little relative mobility. Or an economy with a lot of relative mobility, and little absolute mobility.
Most writers blur the distinction. Brad DeLong does a better job of laying out the issues:
To the extent that goods are valued not for the services they provide by themselves but as indices of exclusivity, it is pointless to produce them for more people because then they become less exclusive and so less valuable. Paul Krugman, for example, has placed himself on Orwell’s side: he would rather be middle-class in the fifties than working poor in the nineties-—even though the material standard of living of America's working poor in 1990 is higher than that of America's middle class in 1950. He:know[s] quite a few academics who have nice houses, two cars, and enviable working conditions, yet are disappointed and bitter because they have never received a [job] offer from Harvard and will probably not get a Nobel Prize. The live very well... but they judge themselves relative to their reference group, and so they feel deprived. And on the other hand, it is an open secret that the chief payoff from being really rich is, as Tom Wolfe once put it, the pleasure of "seeing ‘em jump." Privilege is not merely a means to other ends, it is an end in itself.It may be a very big mistake to think that human happiness is necessarily and significantly increased by piling up larger and larger heaps of material goods. Richard Easterlin in his Growth Triumphant points to evidence from public-opinion surveys that suggests that money does not buy happiness over time or across countries, and believes (though I think he is wrong) that people are no happier in the U.S. today than they are in India today, or were in the U.S. a century ago. Happiness is attained when you achieve your dreams and solve your problems. Material abundance helps you do so, but it also teaches you to dream bigger dreams and pose yourself more complicated problems. Easterlin thus concludes that modern economic growth is a "hollow victory": the "triumph of economic growth is not a triumph of humanity over material wants; rather, it is the triumph of material wants over humanity."
On the other hand, it may not be a very big mistake to think that human happiness consists in expanding our powers and capabilities to accomplish things (not the least of which are maintaining our comfort and satisfying our curiosity), and that wealth is a powerful tool to those ends.
Personally, I vote with Brad's last thought, and against Krugman and Easterlin. Over the long run, dreaming bigger dreams and posing more complicated problems is the right thing to do.