The dismal growth prospects of many European countries has raised an increasing number of questions about whether large parts of the continent will emulate Japan of the 1990s and endure a decade-long economic stagnation. On the face of it, a long-lasting Japan-style post-bubble slump with deflation seems a plausible outcome for a large part of the continent.

But then not many people who understand European economies also understand Japans. One person who does is Adam Posen, member of the Monetary Policy Committee of the Bank...