Compounding Concepts5 分钟阅读

What Is Reinvestment Risk?

Is receiving bond interest faithfully an unconditional win? In fact, 'at what rate you reinvest that interest' greatly changes the final return. This is exactly reinvestment risk.

Reinvestment Risk, in a Nutshell?

Reinvestment risk refers to 'the risk of not being able to reinvest the cash flows from an investment at as good a rate as the original return.'

Let's take an example. Say you invested in a bond paying 5% interest per year now. Here, to get a compounding effect, you must reinvest the interest you receive each year rather than just tuck it into a drawer. But if market interest rates plunge to 2% a few years later, you can now only put the received interest to work at 2%.

In the end, your actual return falls below the total return you'd first expected. This 'risk that the rate will have fallen when you reinvest' is reinvestment risk. It's mainly discussed for fixed-income assets like bonds, but it also applies to stock investors who reinvest dividends.

The core direction: when rates fall, reinvestment risk works against the investor; when rates rise, it works in the investor's favor.

Two Faces: Interest Reinvestment Risk and Call Risk

Reinvestment risk splits broadly into two branches.

First, 'interest (coupon) reinvestment risk.' A coupon bond that pays interest periodically requires reinvesting that interest each time it's received, and if rates have fallen in the meantime, you have to put it to work at a lower rate.

Second, 'early-repayment (call) risk.' Some bonds give the issuer the right to repay the money early, before maturity (a call option). When rates fall sharply, the issuer calls and repays the existing bond on which it was paying expensive interest, and borrows anew at cheaper interest. From the investor's side, a nicely paying high-yield product suddenly disappears, and you have to reinvest the returned money at a lower rate.

Early repayment of a mortgage is similar. The lender (the investor) gets the principal back earlier than scheduled and has to reinvest at the lowered rate.

The U.S. financial authority FINRA also advises investors that 'callable bonds are called and repaid early in a falling-rate environment, imposing a reinvestment burden.'

Why You Shouldn't Trust Yield to Maturity (YTM) Alone

A metric commonly seen when buying a bond is the yield to maturity (YTM). It's the number for 'if you hold this bond to maturity, X% per year.'

Yet many textbooks explain that to actually enjoy this YTM in full, you must 'reinvest the received interest at the same rate as the YTM.' For example, if you can only reinvest the interest from a 10% YTM bond at 4% later, the annualized return you actually pocket falls below 10%.

There is, however, a counterargument in academia that 'the definition of YTM doesn't necessarily require a reinvestment assumption,' so this part is debated. One thing that's certain is this: the rate at which you reinvest interest greatly drives the long-term total return. So you shouldn't feel safe looking at only the single return figure printed in a table.

In other words, remember that YTM often rests on the idealized assumption that 'you keep reinvesting the interest on good terms.'

How to Reduce Reinvestment Risk

A prime example that makes reinvestment risk 0 is the 'zero-coupon bond.' Because it pays no interim interest and pays it all at once at maturity, there's no interim interest to reinvest at all. Since there's no need to reinvest interest, there's no reinvestment risk either.

Also, the shorter the maturity and the lower the interest rate, the smaller reinvestment risk tends to be. Conversely, the longer the maturity and the larger the interest, the more money there is to reinvest and the greater the risk.

In practice, strategies used include 'laddering' — splitting maturities into several tranches — and 'immunization,' which matches maturity/duration so that interest-rate risk and reinvestment risk offset each other. It uses the property that reinvestment risk (a loss when rates fall) and bond-price risk (a loss when rates rise) point in opposite directions.

There's no perfectly risk-free method. Reducing one risk usually increases another, so the key is deciding 'what you'll accept.'

What Long-Term Investors Should Remember

Reinvestment risk ultimately means 'you can't guarantee that compounding will keep running at the rate you want.' Whether interest or dividends, the terms when you reinvest depend on future interest rates.

So it's dangerous to think of a long-term outcome as settled by looking at only a single number for 'the current return is X%.' The actual result changes as rates rise and fall in the meantime.

'The Return of Almost Everything' is a site the author runs personally. Here, with the recurring and lump-sum simulators, you can check with your own eyes 'how much results diverge for the same asset depending on timing and conditions,' and what the drawdown and recovery period looked like in crisis scenarios. Building the sense that a return isn't a 'fixed promise' but 'a range that shifts with conditions' is the shortcut to understanding reinvestment risk.

常见问题

Q. Is reinvestment risk bad when rates rise too?

No, the direction is reversed. When rates fall, you have to reinvest interest at a lower rate, which is unfavorable to the investor; but when rates rise, you can put it to work at a higher rate, which is favorable. Reinvestment risk is a problem especially in a 'falling-rate environment.'

Q. Do stock investors have reinvestment risk too?

Broadly speaking, yes. When you reinvest received dividends to aim for compounding, the result changes depending on the terms at that time. But the term 'reinvestment risk' itself is used mainly for fixed-income assets like bonds that deal with interest.

Q. Can reinvestment risk be eliminated completely?

You can avoid this risk with a zero-coupon bond that has no interim interest, or if you plan to use the cash flows immediately rather than reinvest them. But eliminating one risk usually increases another (e.g., bond-price risk), so it's right to regard 'perfect risk-freeness' as nonexistent.