Rawls's difference principle in A Theory of Justice states,

Social and economic inequalities are to be arranged so that they are to the greatest benefit of the least advantaged members of society, consistent with the just savings principle.

This is a justification for affirmative action on behalf of marginalized members of society. Rawls believes that one should not be disadvantaged by arbitrary factors such as the wealth of one's family. A college admissions policy which judges all candidates according to their examination results or test scores might appear to be fair. However, Rawls argues, not all candidates were equally well prepared, so a truly fair policy would give preference to a student from a poor family over one who was carefully coached for success at an elite preparatory school.

The "just savings" principle is not very clearly explained, but it is essentially the idea that decisions made today should not harm future generations. For instance, a particular group might be disadvantaged now but cease to be disadvantaged in the future. An admissions policy which automatically gave an advantage to this group would be correcting an imbalance today, but at some point in the future, this would cease to be the case. The advantage would then be contributing to injustice. The principal point, therefore, is that policies of affirmative action need to be reviewed regularly to ensure that they are correcting injustice rather than increasing it.