Market Risk

Market risk is the risk that an investment loses value because of broad market conditions rather than anything specific to it. FINRA lists it as a named category, describing how "your investment value might rise or fall because of market conditions." It is the residue left over after diversification has done everything it can do: spreading money across many companies removes the risk attached to any one of them, and leaves untouched whatever moves all of them together.

The naming here is genuinely inconsistent across sources and worth stating once. In finance the same idea is usually called systematic risk, and its complement, the part specific to one holding, is called unsystematic or idiosyncratic risk. FINRA's investor page instead pairs "systemic risk (risk affecting the economy as a whole)" with "non-systemic risk." That is a third spelling of the idea, and it collides with the more common use of systemic risk to mean something else entirely: the risk that the financial system itself breaks down. A reader who meets all three phrases is not missing a distinction; they are meeting one concept with three labels and one of the labels doing double duty.

Why diversification cannot reach it, which is structural rather than a finding about how markets have behaved. Diversification works by combining holdings whose fortunes are not the same, so that one going badly is offset by others going well. That requires the reasons for their movements to be independent. When the reason is shared, meaning a recession, a broad reassessment of prices, a change in the level of interest rates, the offsetting never occurs, because every holding is responding to the same input. Adding a five-hundredth stock to a portfolio of four hundred and ninety-nine removes almost nothing, not because the count is high but because what is left after the first several dozen holdings is the common factor, and no quantity of the same market dilutes it.

It is the risk investors are paid for, and that framing changes what to do about it. A risk that can be removed for free earns no compensation, which is why concentration in a single company is described as uncompensated. Market risk cannot be removed by anyone, so bearing it is what the long-run return on owning assets is payment for. That makes it a thing to size rather than a thing to eliminate. The instruments for sizing it are the asset mix, meaning how much is in stocks at all, and the time horizon, meaning how long the money can stay invested through a decline. Adding holdings is not one of them, and a portfolio that keeps adding funds in the hope of reducing it is answering the wrong question.

Bonds have their own, and it is the version most often overlooked. A portfolio of forty different corporate bonds has diversified away a great deal of credit risk, and has done nothing at all about interest rates. When yields rise, all forty fall together, because the discount rate applied to future payments is exactly the kind of shared factor diversification cannot touch. In the same way, a portfolio of thirty municipal issuers still carries the risk of a broad change in the tax treatment of municipal interest. Each asset class has a common factor of its own, which is why holding more than one asset class, rather than more securities within one, is the only structural response available.

It is not the same as volatility, and not the same as sequence risk. Volatility measures how much a price moves in both directions, which is a statistic rather than a definition of harm. Market risk is the chance that returns are poor overall. Sequence of returns risk is the chance that adequate returns arrive in a damaging order for someone who is withdrawing money. A retiree can be hurt badly by the third while the first and second look ordinary, which is why the three are managed with different tools.

What is left when it cannot be diversified. Three responses exist and none of them is diversification. Reduce the exposure by holding less of the asset class, which lowers expected return in the same motion. Extend the horizon, so a decline has time to be recovered from, which is available only to money that is genuinely not needed soon. Or arrange spending so that a decline does not force a sale, which is what an emergency fund and a cash reserve inside a retirement portfolio are for.

Separate a holding's movement into the part explained by the market as a whole and the part that is specific to it. Diversification steadily removes the second, and the first is what remains no matter how many holdings are added. Sizing that remainder is a decision about the asset mix.

A hypothetical example, from the bond side, because that is where the point is easiest to see. Nadia holds $100,000 spread evenly across bonds from 20 different corporate issuers, so $5,000 each.

The diversifiable event. One issuer defaults, and bondholders eventually recover 40 cents on the dollar. Nadia loses 60 percent of one $5,000 position, which is $3,000, or 3 percent of the portfolio. Splitting the money across twenty issuers is precisely what kept a company failure from being a catastrophe.

The undiversifiable event. Yields across the bond market rise by 1 percentage point. If the portfolio's average modified duration is 6, all twenty positions fall together by about 6 percent, which is $6,000, or 6 percent of the portfolio.

The second loss is twice the first, and the twenty issuers did nothing about it. Holding forty issuers would not have helped either, because every bond in the market was responding to the same change. The lever that would have changed the second number is the duration Nadia chose to hold, which is a decision about the asset mix rather than about how many names are in the account.

Pros (of understanding it as the undiversifiable part)

• It stops the search for a portfolio that cannot fall, which is the search that leads to over-collecting funds and products.
• It identifies the levers that actually work, meaning the asset mix and the time horizon, and rules out the ones that do not.
• It explains why long-run returns exist at all, since bearing a risk nobody can remove is what the compensation is for.
• It separates cleanly from concentration risk, which can be removed and therefore earns no compensation.

Cons (and limits of the concept)

• Being paid to bear it is a statement about long periods, and it says nothing about whether any particular decade is rewarded.
• Reducing it always costs expected return, so the trade is real rather than an optimization.
• The vocabulary is inconsistent across sources, so systematic, systemic and non-systemic are used in ways that conflict.
• Correlations between asset classes are not fixed, and holdings that usually move differently can move together in a severe decline, which is when the separation was most wanted.