A armadilha de liquidez é uma situação em que as taxas de juro já estão em 0% e, por isso, operações de open market do Banco Central deixam de ter efeito prático. Nestas circunstâncias, a política monetária perde a tracção sobre a economia real e a política orçamental torna-se o derradeiro instrumento de estabilização macroeconómica.
Ou talvez não seja bem assim. Em Methods of policy accomodation at the interest rate lower bound, Michael Woodford – provavelmente o maior teórico da actualidade no que diz respeito a política monetária – explora duas vias alternativas que a autoridade monetária pode seguir depois de os juros atingirem o zero: o quantitative easing e o forward guidance.
Woodford conclui, como Milton Friedman havia sugerido há duas décadas, que um banco central nunca está sem munições. Mas mostra, de uma forma elegante, como é difícil conseguir usar com eficácia estas munições. Um paper essencial para se perceber os dilemas de Ben Bernanke e companhia.
Central bankers confronting the problem of the interest-rate lower bound have tended to be especially attracted to proposals that oﬀer the prospect of additional monetary stimulus while (i) not requiring the central bank to commit itself with regard to future policy decisions, and (ii) purporting to alter general ﬁnancial conditions in a way that should aﬀect all parts of the economy relatively uniformly, so that the central bank can avoid involving itself in decisions about the allocation of credit. Unfortunately, the belief that methods exist that can be eﬀective while satisfying these two desiderata seems to depend to a great extent on wishful thinking.
On the one hand, a number of central banks have sought to give signals about the likely future evolution of their policy rates, in order to inﬂuence current longer-term interest rates and hence, hopefully, current spending decisions; but this has typically been done in a way that avoids actually making any promises about how future policy decisions will be made, and in particular without giving listeners any reason to suppose that future decisions will be made on anything but a purely forward-looking basis. It is unclear why announcements of this form should have the desired eﬀect, and on at least some occasions they seem to have little eﬀect. Moreover, simply presenting a forecast that the policy rate will remain lower for longer than had previously been expected, in the absence of any reason to believe that future policy decisions will be made in a diﬀerent way, runs the risk of being interpreted as simply an announcement that the future is likely to involve lower real income growth and/or lower inﬂation than had previously been anticipated — information that, if believed, should have a contractionary rather than an expansionary eﬀect (though it might lead long-term interest rates and indicators of interest-rate expectations to decline).