Finally, a review I can understand. Here’s one simple but good bit:
The rate of return on capital tracks the level of capital income, and not its growth. If you have a million dollars in wealth, and the rate of return on capital is 5%, then your capital income is $50,000. Level, not growth. On the other hand, g tracks the growth of average income, not its level. For instance, if average income is $100,000 and the growth rate is 3%, then the increase in your income is $3000. Saying that r > g implies that capital income will grow faster than labor income is a bit like comparing apples and oranges.
To make the point clear, I’m going to expand upon this argument in two ways. First, let us look at a situation in which the argument apparently holds. Suppose that capital holders save all their income. Then r not only tracks the level of capital income, it truly tracks the rate of growth of that income as well, and then it is indeed the case that capital income will come to dominate overall income, whenever r > g. But the source of that domination isn’t r > g. It is the assumption that capital income owners save a higher fraction of their income!
Debraj adds his own sobering thoughts on technological progress and inequality at the end of his review.