Unequal societies generally turn a corner once they have passed through a long spell of political instability. Governing elites tire of incessant violence and disorder. They realize that they need to suppress their internal rivalries, and switch to a more co-operative way of governing, if they are to have any hope of preserving the social order. We see this shift in the social mood repeatedly throughout history — towards the end of the Roman civil wars (first century BC), following the English Wars of the Roses (1455-85), and after the Fronded (1648-53), the final great outbreak of violence that had been convulsing France since the Wars of Religion began in the late 16th century. Put simply, it is fear of revolution that restores equality.
A driver of equality?
Peter Turchin, Professor of ecology and mathematics at the University of Connecticut and Vice-President of the Evolution Institute, quoted by Mathew Taylor (http://www.matthewtaylorsblog.com/).