John Rawls, answered from the texts and cited to the page.
The difference principle holds that social and economic inequalities in the basic structure are just if and only if they work to improve the expectations of the least advantaged members of society.1 The intuitive idea is straightforward: the social order is not to establish and secure the more attractive prospects of those better off unless doing so is to the advantage of those less fortunate.2
To make this concrete, consider the distribution of income between, say, entrepreneurs and unskilled workers in a property-owning democracy. Those starting out as members of the entrepreneurial class have better life prospects than those who begin as unskilled laborers — and this will likely remain true even when existing social injustices are removed.
What justifies that initial inequality? Only this: that the difference in expectation works to the advantage of the representative unskilled worker. If lowering the expectations of the better-off would make the working class worse off still, then the inequality is permissible; if lowering those expectations would improve the situation of the least favored, the arrangement is unjust.3
It is worth being precise about the two cases that arise in applying the principle. A perfectly just scheme is one in which the expectations of the least advantaged are maximized — no change in the expectations of those better off could further improve the situation of those worst off. A scheme is just throughout, though not the best just arrangement, when the higher expectations of each more favored group at least contribute to the welfare of the less fortunate, even if the maximum has not yet been reached.
A scheme is unjust when the higher expectations are excessive — when decreasing them would actually improve the situation of the least favored.4 The principle emerges from a specific starting point. Among free and equal moral persons, the obvious benchmark is equal division: everyone should have an equal share. But organizational requirements and economic efficiency make it unreasonable to stop there.
The basic structure should permit inequalities so long as they improve everyone's situation, including that of the least advantaged, consistent with equal liberty and fair equality of opportunity. Because we begin from equal shares, those who benefit least have, so to speak, a veto — and it is from this that the difference principle follows.5
One further thing to keep clearly in mind: the difference principle applies to the basic structure and its system for acquiring entitlements, not to individual transactions within a just structure. Within appropriate limits, whatever distributive shares result from a fair social process over time are just. There is no answer to the abstract question of whether one distribution of a given stock of things to definite individuals is more just than another, considered in isolation.6 The principle governs the institutional design that makes the process fair — not each outcome the process produces.
the higher expectations of those better situated are just if and only if they work as part of a scheme which improves the expectations of the least advantaged members of society.A Theory of Justice, p. 65
The intuitive idea is that the social order is not to establish and secure the more attractive prospects of those better off unless doing so is to the advantage of those less fortunate.A Theory of Justice, p. 65
According to the difference principle, it is justifiable only if the difference in expectation is to the advantage of the representative man who is worse off, in this case the representative unskilled worker. The inequality in expectation is permissible only if lowering it would make the working class even more worse off.A Theory of Justice, pp. 67–68
The first case is that in which the expectations of the least advantaged are indeed maximized... A scheme is unjust when the higher expectations, one or more of them, are excessive. If these expectations were decreased, the situation of the least favored would be improved.A Theory of Justice, pp. 67–68
The basic structure should allow organizational and economic inequalities so long as these improve everyone's situation, including that of the least advantaged, provided these inequalities are consistent with equal liberty and fair equality of opportunity. Because they start from equal shares, those who benefit least (taking equal division as the benchmark) have, so to speak, a veto.Political Liberalism, pp. 262–263
Therefore, if it is asked in the abstract whether one distribution of a given stock of things to definite individuals with known desires and preferences is more just than another, then there is simply no answer to the question.Political Liberalism, pp. 262–263