Understanding the "warp" in long-term Treasuries

A common point of confusion for many investors is the distinction between average maturity and effective duration, two metrics that are often used interchangeably but serve very different roles in a portfolio. Average maturity represents the weighted average of the time remaining until the bonds in a fund reach their final payment date. Effective duration, however, measures the fund's actual price sensitivity to interest rate changes. For a fund like the Vanguard Long-Term Treasury ETF (VGLT), this gap is significant. As of early 2026, VGLT carries a weighted average maturity of approximately 21.90 years, yet its effective duration sits lower at roughly 14.10 years. (See figure below. Data source: Vanguard.) This occurs because the semi-annual coupon payments "shorten" the economic life of the investment, meaning you recover your capital faster than the final maturity date suggests.

The primary reason to understand the difference between these measures is to predict how your principal will react to a changing rate environment. The mathematical relationship is inverse: when interest rates go down, the principal value of the bond fund goes up. Using VGLT’s current effective duration of 14.10 as a guide, we can quantify this "warp" in value. If the 10-year and 30-year Treasury yields were to drop by 1% (100 basis points), the share price of VGLT would be expected to rise by approximately 14.10%. This leveraged-like sensitivity is exactly why long-term Treasuries are favored by those looking to hedge against economic slowdowns, as the price appreciation can be substantial during a "flight to safety."

This distinction is the cornerstone of sophisticated bond investing and the reason why this post is important. If an investor looks only at the 21.9-year maturity of VGLT, they might overestimate the time their capital is locked away or underestimate the immediate volatility of the fund. Conversely, failing to account for the 14.10 duration means ignoring the precise tool used to calculate risk. By understanding that duration is the "speedometer" of one’s bond portfolio’s price movement, one can better position their assets to benefit from shifting yields rather than being caught off guard by them. (Disclosure: the author owns VGLT shares at the time of this writing.)