[*] We should emphasize that the Piketty and Saez data only allow us to say how well different income groups are doing, not how well individual households fare over time—an issue with traditional surveys of income as well. We can say the rich of today are richer than in the past, not how much change there is from year to year in who is rich and who is not. But, as we shall see, taking into account the upward or downward income mobility of households does not change this basic picture. It may even strengthen it, since long-term income mobility is much more limited than Americans believe, and may have declined since the 1960s. In any case, income groups are not statistical fictions. If the rich grow much richer, while the poor and middle class do not, the structure of society will look very different.