reinvestment and disinvestment risk

Money inside an insurer or pension fund rarely sits still: bonds mature, coupons arrive, and that cash has to go somewhere — usually back into new investments. Reinvestment risk is the danger that when the time comes to put cash to work again, the going interest rate is lower than you had assumed, so the new investments earn less than the maturing ones did. Its mirror image, disinvestment risk, is the danger that you must sell assets earlier than planned — to pay an unexpected claim, say — at a moment when prices are low and you crystallize a loss.

Take reinvestment first. Suppose you bought a 10-year bond yielding 6 percent to back a liability you priced assuming 6 percent throughout. Each year you receive a coupon and must reinvest it. If rates have since dropped to 3 percent, those coupons now compound at 3 percent, not 6, and over a long horizon that shortfall quietly erodes the return you were counting on. The effect is largest for short-dated assets backing long-dated promises, because you have to reinvest again and again. Disinvestment risk is the flip side of holding assets shorter or longer than the liabilities: if a wave of claims forces an early sale of long bonds after rates have risen, you sell below cost and lock in the loss you hoped to avoid.

Why it matters: these two risks are the cash-flow face of interest-rate risk, and they are exactly what cash-flow matching and immunization are designed to tame. When asset and liability cash flows line up, there is little to reinvest and little to sell early, so both risks shrink toward zero. A common misconception is that a high headline yield on a bond locks in that return; it does not, because the coupons you reinvest along the way are exposed to whatever rates prevail when they arrive. The promised yield to maturity is only realized if every cash flow can be reinvested at that same rate — which the real world rarely permits.