As proposed by Thomas Piketty in Capital in the 21st Century, when "r > g" (where r = return on capital and g = rate of economic growth) the rich (those with capital) get richer.
Apart from between 1930 and 1975 (which Piketty suggests were unique circumstances due to WW2) r has generally been greater than g.
Apart from between 1930 and 1975 (which Piketty suggests were unique circumstances due to WW2) r has generally been greater than g.