What is the relation between this principle and another that seems ethically appealing, namely, equality of treatment? In part, the two principles are not contradictory. Payment in accordance with product may be necessary to achieve true equality of treatment. Given individuals whom we are prepared to regard as alike in ability and initial resources, if some have a greater taste for leisure and other for marketable goods, inequality of return through the market is necessary to achieve equality of total return or equality of treatment. One man may prefer a routine job with much time off for basking in the sun to a more exacting one paying a higher salary; another man may prefer the opposite. If both were paid equally in money, their incomes in a more fundamental sense would be unequal. Similarly, equal treatment requires that an individual be paid more for a dirty, unattractive job than for a pleasant rewarding one. Much observed inequality is of this kind. Differences of money income offset differences in other characteristics of the occupation or trade. In the jargon of economists, they are “equalizing differences” required to make the whole of the “net advantages,” pecuniary and non-pecuniary, the same.
Milton Friedman – Capitalism & Freedom p.162