The recent literature on “convergence” of cross-country per capita incomes has been dominated by the two hypotheses of “global convergence” and “club-convergence,” pertaining to limits of estimated income distribution dynamics. Utilizing a new measure of “stochastic stability,” we establish two stylized facts regarding short- and medium-term distribution dynamics. The first is non-stationarity of transition dynamics, in the sense of changing transition kernels, and the second is emergence, disappearance, and re-emergence of a “stochastically stable” middle income group. This middle income group emerges as the gap between rich and poor clubs gets larger, and it changes the dynamics of transition to and from the rich and poor clubs, eventually narrowing the gap between the poor and rich as the middle club vanishes. Analyzing the stochastic stability of middle-income groups is thus a first step toward understanding higher-order dynamics of narrowing or widening of the gap between rich and poor countries.