But while it is likely that the ‘neutral’ rate is subject to low-frequency fluctuations, I think it is reasonable to assume that it is sufficiently slow moving that it is still a useful concept, provided we don’t adopt the strong assumption that it never moves.
8  Indeed, Wicksell expected that a stable equilibrium where the market and natural rates were equal would rarely, if ever, occur for any length of time. 9  In a world in which capital flowed quickly to arbitrage away any differences in rates of return on capital across countries, there would presumably not be noticeable differences between the ‘neutral’ rate between countries.