Peart and Levy are perhaps a little optimistic about the capacity of the doctrine of equal competence to survive without modification in the twenty-first century. Previous attacks on consumer sovereignty were fought off handily enough -- Maurice Dobb by Abba Lerner, John Kenneth Galbraith by Hayek. But much more powerful forces are being marshaled for an assault from within technical economics -- the analysis of the effects of monopolistic competition, the insights of behavioral economics and evolutionary psychology, not to mention the discoveries of molecular biology. Cavalier assumptions about race and hierarchy are being supplanted by more narrowly-framed hypotheses about cognitive capacity.
The argument is terrifically important and it may be that there will eventually be a time when the evidence for difference is unequivocal and carefully framed. For now, as Warsh rightly points out, Peart and Levy rely on Robbins' 1938 as if equality position:
I have always felt that, as a first approximation in handling questions relating to the lives and actions of larges masses of people, the approach which counts one man as one, and, on that assumption, asks which way lies the greatest happiness, is less likely to lead one astray than any of the absolute systems. I do not believe, and I never have believed, that in fact men are necessarily equal or should always be judged as such. But I do believe that, in most cases, political calculations which do not treat them as if they were equal are morally revolting.