1 Answers
Heterodox
The shadow rate is an interest rate in some financial models. It is used to measure the economy when nominal interest rates come close to the zero lower bound. It was created by Fischer Black in his final paper, "Interest Rates as Options".
The shadow rate derives from Fischer Black's insight that currency is an option. If someone has money, they can either spend it today or not spend it and have money tomorrow. Thus, when loans would return less money than was initially loaned out, investors will choose to "exercise the option" and not loan their money. Thus, the nominal short-term interest rate is always greater than or equal to zero. In Black's model, the shadow nominal short-term rate is what the nominal short-term rate would be if it was allowed to go below the zero lower bound.
When the shadow nominal short-term rate is positive, the nominal short-term rate is equal to the shadow rate. But when the shadow short-term rate is negative - such as during deflation or a bad recession with low inflation - the nominal short-term rate will diverge and stay above zero. In Black's model, even when nominal short-term interest rates stay close to zero, the long-term nominal interest rates can be well above zero. This is because nominal interest rates behave like options and there is some chance that the shadow short-term rate becomes positive in the future.