The zero lower bound: why interest rates hit a floor and what central banks use instead

The zero lower bound is the level below which a central bank cannot push its nominal policy rate without triggering a flight into cash. The floor exists not because any law forbids going lower, but because of arbitrage: a banknote is a non-interest-bearing liability of the central bank that can always be held instead of a deposit. As soon as the return on a deposit turns negative, a rational depositor withdraws the money and puts it in a safe, so further rate cuts stop feeding through to the economy. In practice the bound sits not exactly at zero but slightly below it: storing, transporting and insuring cash costs money, and it is precisely this cost that determines how far the rate can go into negative territory before cash becomes the better option. That is why economists speak not of a zero lower bound but of an effective lower bound.

Why the floor breaks the usual logic of easing

In normal times a central bank fights a downturn by cutting its rate: cheap credit revives demand. But what matters is not the nominal rate but the real rate, that is, the nominal rate minus expected inflation. At the floor a trap emerges: if the downturn also dampens inflation expectations, the real rate rises even while the nominal rate stays put. The economy gets tightening at the very moment it needs easing, and the main policy tool has already been used up. This is the "liquidity trap" in its modern formulation: additional reserves in the banking system stop turning into lending, because the rate cannot go any lower, and money and short-term debt become almost indistinguishable to the holder.

The second consequence is an asymmetry of risk for the regulator. The room for the rate to rise is unlimited, while the room for it to fall is finite. Hence the argument for not taking the rate to the floor unless necessary, and for keeping the inflation target higher than would seem ideal in a world without the bound: a higher target means a higher normal nominal rate, and therefore more room to cut.

What replaces the rate once it has hit the floor

The toolkit beyond the policy rate relies on other parts of the transmission mechanism.

• Quantitative easing. The central bank buys long-dated government bonds and compresses the term premium, lowering long-term yields when short-term rates are already stuck.
• Forward guidance. A promise to keep the rate low for longer than a policy rule would prescribe works through expectations: today's long-term rates are an average of expected short-term rates.
• Yield curve control. The regulator announces a target yield on a chosen segment of the curve and defends it with purchases of unlimited size.
• A moderately negative rate on reserves. What turns negative is not the rate on household deposits but the rate on banks' excess reserves, often with tiering so as not to destroy banks' margins. For a bank, moving and storing billions in cash is expensive, so its floor lies lower than a depositor's.
• The fiscal channel. At the bound the government spending multiplier is higher: rates do not rise in response to the stimulus, and there is almost no crowding out of private investment.

What this means for asset valuation

A low risk-free rate changes the arithmetic of discounting: the weight of distant cash flows increases, the duration of both equities and bonds lengthens, and the range of valuations becomes painfully sensitive to the interest-rate assumption. It is exactly where the risk-free rate is close to zero that the cost of capital is most often misstated: the required return is made up not only of the risk-free component but also of risk premiums, which do not disappear at the floor. This mistake is examined in the article on economic value added: profit can be positive and yet no value is created if the cost of capital is understated. For a portfolio framing of the problem, it is useful to recall how the efficient frontier shifts when the risk-free asset yields almost nothing: the curve presses against the risk axis, and return can only be "bought" with risk.

The Russian context and an honest caveat

The Bank of Russia has never been at the zero bound: in recent history its key rate has stayed well above zero, and the policy problem has been inflation rather than a lack of it. For a Russian investor, therefore, the subject is relevant mainly as an external factor: through the valuation of foreign-currency assets, through the behaviour of global long-term rates and through the way cheap global funding affects risk appetite. The easiest way to compare this with the domestic picture is via the government debt curve, which is collected in the OFZ section, while the broader debt market is covered in the bonds section; the regulator's decisions and their interpretation are tracked in the news feed, and access to foreign instruments in packaged form is available in the funds section. Definitions of related concepts can be found in the glossary.

What the site's data does not contain is a ready-made "rate at the zero bound" series for foreign central banks: the database covers the Russian market, and any international comparisons here have to be made using external sources. This limitation is worth keeping in mind before applying the logic of the zero floor directly to the valuation of Russian securities.