The Difference Principle

Rawls does not require strict equality of outcome — only that inequalities be justifiable to the least advantaged. If allowing doctors to earn more than nurses produces medical innovations that improve the health of the worst-off, that inequality satisfies the difference principle. Rawls's insight is that the real baseline against which inequalities should be measured is not perfect equality but what the least advantaged could have under any feasible alternative arrangement.

The identification of the "least advantaged" as the benchmark for justice is a deliberate political choice. It forces institutional design to focus on the bottom rather than the average or the top. A society that raises the Gross Domestic Product by enriching the already wealthy while leaving the worst-off no better off fails the test of justice, however impressive its aggregate performance.

Rawls distinguishes the difference principle from two rival interpretations of the second principle: the system of natural liberty (which merely requires formal opportunity) and liberal equality (which requires fair equality of opportunity but does not constrain outcomes). Only democratic equality — fair opportunity plus the difference principle — constitutes his full theory. It is the interpretation that rational persons behind the veil of ignorance would choose.

The difference principle is developed in sections 12–13 of A Theory of Justice. Rawls's account of the "least advantaged" appears in section 16.