Seriously, what does it mean and why is it good?
It means the probability that members of one class move to another. Say from blue collar to white collar, or from white collar to rich, or vice versa of course. Since class-structure is weak and poorly defined in the US (working- and middle-class overlap), they frequently use motility between the income quintiles as a surrogate.
And it is good because it means the poor can become rich without too much trouble if class mobility is easy in a certain society. It's also generally only worth discussing upward mobility, since downward mobility is always easy.
how is that different from any measure of just general equality of wealth?
I would add that in the USA upward class mobility has stagnated in the last 15 years.
IE those who are born in the lower class, tend to stay in the lower class.
You didn't say what class, but it generally isn't good. Class I mobility is a subjective rating of slightly more tooth movement than normal. Class II is tooth movement up to 1 mm. Class III is tooth movement of more than 1 mm and it generally means you're going to lose the tooth.
Or are you talking about economic class mobility? I'm not sure why it's good, unless it's based on the assumption that more people move up in economic class than down in economic class.
That assumption is somewhat valid, since individuals can be expected to be more likely to move up as they gain experience and get promoted during their career than to get laid off and have to retrain into a less well paying job. It is a good thing, since the promise of moving up in economic class can attract more people to buying into economic policies benefiting the middle and upper class than if people based their decision solely on their current economic class.
Ooooh. that makes sense. The rest of these replies were so off-topic, no one takes brushing seriously anymore these days.
A society could have perfectly equal distribution of wealth, but a society like that would by definition have no class mobility, since it would be classless.
To have class mobility, you first must have social/economic classes (rich, middle-class, poor, etc.). All it means when you say that a country/society has class mobility is that there are few or no boundaries for those who want to try to move up the ladder. In a society with class mobility, the poor can become rich through working hard, and the rich can equally become poor by not working hard.
If all people were equal, there'd be no mobility.
Downward mobility may be easy in the sense that yes, anyone can move to a lower class if they decide to, but that doesn't mean that it isn't a factor. If you have a society where estate tax is low, old money will tend to stick around, and no matter how poorly a person might do financially, they'll always have their parents' money to bail them out. Ask the President about that.
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