As an ambition for society, social mobility is flawed

How much importance should be placed on income mobility— what happens to the average income of the bottom 50% — over and above social mobility, where the 21st-40th percentile (say) might aspire to transition to a higher class, perhaps to the 41st-60th percentile?

This picture gives us an answer. Based on logic alone, as an ambition for society, social mobility is deeply flawed. Upward social mobility only ever comes with downward social mobility. Social mobility is a zero-sum proposition: someone always has to lose for someone else to win.

In the Figure, each of the left and right panels—societies (a) and (b) respectively—shows a distribution of income y changing through time t. Each small circle represents 20% of the group whose income distribution is depicted. The left panel, that for society (a), sees an increase in average income, but leaves unrestricted who goes where in the income distribution. When (a) is confined to the bottom 50% in society, this change in average income is what the text refers to as income mobility. The right panel, that for society (b), shows the group originally in the 21st–40th percentile, (i.e., the second 20%) experiencing upward social mobility in transitioning to the third 20%: they rise in society. For this to happen, however, some group originally in the top 60% must fall into the bottom 40%, thereby experiencing downward social mobility. Put directly, if previously you had 60% of the population richer than you but now only 40%, then one-fifth of the population, somewhere, must have fallen in the process. Upward social mobility is impossible without, at the same time, equal and opposite downward social mobility. Social mobility is a zero-sum proposition.

http://www.DannyQuah.com/Quilled/Output/Quah-D-2019-Mobility-Political-Upheaval-Inequality.pdf

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