Pension Funding Ratios and Surplus Volatility

A defined benefit pension fund's health is usually summarized by its funding ratio: the market value of its assets divided by the present value of its liabilities (what it owes retirees, discounted back to today). A ratio above 100% means the fund is in surplus — more assets than liabilities — and below 100% means it's in deficit. A fund manager or CFO watching this number quickly learns an uncomfortable fact: it can move sharply even on a day when the fund didn't trade a single share, because the liability side of the ratio is just as alive as the asset side.

Why the liability moves too

Pension liabilities are valued as the present value of decades of future promised payments, discounted at a rate tied to long-term interest rates (often high-quality corporate bond yields). When interest rates fall, that discount rate falls, and the present value of a fixed stream of future payments rises — exactly the same mechanic that makes a bond's price rise when yields fall. So a fund can watch its liabilities balloon in value purely because rates dropped, even while its assets stayed flat or even grew a little, and its funding ratio deteriorates anyway. The opposite happens when rates rise: liabilities shrink in present-value terms, often improving the funding ratio even in a year the assets did nothing special.

This is why surplus volatility — the swings in (assets minus liabilities) — is driven at least as much by interest-rate moves as by what the fund's investments actually did. A fund holding mostly equities against interest-rate-sensitive liabilities is effectively running a mismatched bet: its assets don't move with rates the way its liabilities do, so a rate move can hurt the funding ratio from the liability side, regardless of what equities are doing.

For example, a pension fund with $900 million in assets against $1 billion in liabilities starts at a 90% funding ratio. If long-term rates fall by one percentage point, and the fund's liabilities have an effective duration of 18 years, the present value of those liabilities rises by roughly 18%, to about $1.18 billion — while equity-heavy assets, largely insensitive to that particular rate move, might sit close to unchanged. The funding ratio falls to roughly 76%, a large deterioration driven almost entirely by a discount-rate move, not by any investment loss.