Exchange rate
The next section lists some of the alternative proposals to an explanation of the Penn effect, but there are significant econometric problems with testing the BS-hypothesis, and the lack of strong evidence for it between modern economies may not refute it, or imply that it produces a small effect. For instance, other effects of exchange rate movements might mask the long-term BS-hypothesis mechanism (making it harder to detect if it exists). Exchange rate movements are believed by some to affect productivity; if this is true then regressing RER movements on differential productivity growth will be 'polluted' by a totally different relationship between the variables1.