Zero Lower Bound

The zero lower bound (ZLB) is the practical floor on the central bank's policy rate. While the theoretical lower bound is slightly below zero (the European Central Bank and Bank of Japan have experimented with negative rates), the Federal Reserve has treated zero as the effective lower bound for the federal funds rate.

The ZLB becomes a binding constraint during severe economic downturns when the central bank has already cut rates to near zero but the economy needs further stimulus. This was the case during:

• 2008-2015: the Fed held rates at 0-0.25% for seven years after the financial crisis
• 2020-2022: the Fed returned to 0-0.25% in response to the COVID-19 pandemic

When constrained by the ZLB, the Fed turns to unconventional tools:

• Quantitative easing: large-scale asset purchases to lower long-term rates
• Forward guidance: explicit communication about the expected path of future rates to influence market expectations
• Operation Twist: selling short-term securities and buying long-term securities to flatten the curve without expanding the balance sheet

The ZLB shapes the yield curve because it limits how far short-term rates can fall, effectively putting a floor under the front end. This explains why the curve tends to steepen dramatically during ZLB periods — long rates can still move freely while the front end is pinned at zero.

The blog post "Ten Treasury Curve Snapshots That Tell the Story of a Generation" documents curve shapes during both ZLB episodes.