What criteria should one rely on to assess whether a policy is moral and what system we should live under? Rawls introduced in his Theory of Justice* the notion of the veil of ignorance. The idea is quite simple and starts from the premise that a group of people must agree on a set of policies that would constitute the rights of people and the rules under which the society is structured. In other words, they must establish the principles of justice of the society they will live in.
The trick is that, when making these decisions, do not know what positions they would have once the rules are agreed: they are under a veil of ignorance concerning their positions in the system. In this thought experiment, the ignorance is assumed to be total, you do not ex ante know what your abilities, assets or income will be. One of his main conclusion lies with his second principle of justice which consists of accepting inequalities only to the exten that they benefit the worst off in society (also referred to the difference principle).
According to this second principle of justice, inequality can only be justified if it makes the people at the bottom of the income distribution better off, for instance through higher long term growth. When wages at the bottom of the income distribution are stagnating this then questions both the efficiency and the equity grounds on which inequality was acceptable.
*Rawls (1971) A Theory of justice.