In 2013, French economist Thomas Piketty postulated that in periods when the average annual rate on return on investment in capital (r) exceeds the average annual growth in economic output (g), the rate of inequality will increase.[104] According to Piketty, this is the case because wealth that is already held or inherited, which is expected to grow at the rate r, will grow at a rate faster than wealth accumulated through labor, which is more closely tied to g. An advocate of reducing inequality levels, Piketty suggests levying a global wealth tax in order to reduce the divergence in wealth caused by inequality.