Arbitrageurs: The Traders Who Hunt Risk-Free Profits (Chapter 17, Part 1)
Book: Trading and Exchanges: Market Microstructure for Practitioners Author: Larry Harris Publisher: Oxford University Press, 2003 ISBN: 0-19-514470-8
Arbitrage sounds like free money. Buy something cheap here, sell it expensive there, pocket the difference. No risk, pure profit. The reality is a lot messier than that. Chapter 17 of Harris’s book explains what arbitrage actually looks like, why most of it involves real risk, and how arbitrageurs serve a critical function in keeping markets honest.
What Arbitrageurs Do
Arbitrageurs are speculators who trade on relative values. They buy instruments that seem cheap compared to related instruments and sell those that seem expensive. They profit when prices converge, meaning the cheap thing gets more expensive and the expensive thing gets cheaper.
The key word is “relative.” Arbitrageurs are not betting on whether something goes up or down in absolute terms. They are betting that the relationship between two or more things will return to normal.
When arbitrageurs take their positions, they “put on” the arbitrage. When they close out, they “unwind” or “take off” the arbitrage. The portfolio they construct is called the hedge portfolio, and the individual positions in it are called legs.
Most hedge portfolios consist of long positions in some instruments and short positions in others. The beauty of this structure is that common risk factors cancel out. If both legs move up or down together due to some market-wide event, the gains in one offset the losses in the other. What remains is basis risk, the risk that the specific relationship between the instruments changes in unexpected ways.
The Language of Arbitrage
The difference in prices between instruments in the hedge portfolio is called the basis. The fair value of the basis is what it should be if everything is correctly priced. The arbitrage spread is the difference between the actual basis and the fair value of the basis. Arbitrageurs trade when the arbitrage spread is wide enough to justify the costs and risks.
The values of the basis at which arbitrageurs are just willing to trade are called arbitrage bounds. They sit on either side of fair value. Inside the bounds, the opportunity is not worth the trouble. Outside the bounds, arbitrageurs start putting on positions.
Carrying costs also matter. These include interest on borrowed money, dividends paid on short positions, storage fees for physical commodities, and depreciation. Carrying costs can make some arbitrages very expensive to hold, even if the spread looks attractive.
A Simple Way to Think About Arbitrage
Harris offers a clean mental model. Think of the hedge portfolio as its own instrument that you can buy or sell. When you “buy” the hedge portfolio, you go long on one leg and short on the other. When you “sell” it, you reverse both.
Arbitrageurs who trade the hedge portfolio frequently, like dealers, are essentially market makers in the spread. They buy and sell quickly, profiting from short-term reversals. These dealer-type arbitrageurs do not care much about fair value. They care about short-term price patterns. They offer immediacy but usually will not take big positions.
Arbitrageurs who take large positions in the hedge portfolio are essentially value traders in the spread. They need to know fair value precisely. They supply depth. When dealer-type arbitrageurs want to offload positions, they often trade with these value-type arbitrageurs.
Pure Arbitrages
A pure arbitrage involves instruments where the value of the hedge portfolio is strictly mean-reverting. That means it always tends to return to some average value. If the basis gets too wide, it will come back. If it gets too narrow, it will widen again. The key feature is that some mechanism guarantees eventual convergence.
Pure arbitrages come in three flavors.
Shipping arbitrages involve identical instruments trading in different markets at different prices. The arbitrageur buys where it is cheap and sells where it is expensive. If necessary, they physically ship the instrument between markets to settle. The cost of shipping sets the arbitrage bounds.
Harris gives the example of crude oil trading in New York and London. When oil is significantly more expensive in New York, shippers reroute their tankers from Europe to the U.S. They simultaneously sell futures in New York and buy futures in London. The arbitrage spread can not be wider than the cost of shipping oil across the Atlantic, but in practice it is usually narrower because oil companies never actually ship between the two markets. They ship from source (the Persian Gulf or the Gulf of Mexico) and just reroute where appropriate.
Virtual shippers are arbitrageurs who put on the trade but never actually ship anything. They hope prices converge on their own. Actual shippers are the backstop. If virtual shippers get squeezed, they lay off their positions (usually at a loss) to actual shippers who can physically deliver.
Delivery arbitrages involve contracts for future delivery of a commodity. The delivery mechanism forces the basis to close on the maturity date. Before maturity, the basis depends on carrying costs. If cash wheat is cheap relative to wheat futures, you buy the wheat, sell the futures, and deliver when the contract matures.
Conversion arbitrages involve buying and selling instruments that embody the same risk in different forms. When you buy one and sell the other, you are essentially converting risk from one form to another. You are a manufacturer of financial instruments.
Stock options are a great example. Public investors generally want to buy more option contracts than they want to sell. Arbitrageurs fill the excess demand by manufacturing options. They sell the option and hedge with the underlying stock. As prices change, they constantly rebalance their hedge.
This is risky. If prices move too fast, the arbitrageur can not rebalance quickly enough and their hedge portfolio gets out of balance. Price changes in either direction can cause losses when you are imperfectly hedged. When arbitrageurs expect high volatility, they demand bigger premiums. This is actually one way to explain why option values depend on volatility.
The soybean crush is maybe the most entertaining example in the chapter. At the Chicago Board of Trade, futures trade for soybeans, soybean oil, and soybean meal. Millers crush soybeans into oil and meal. When soybeans are cheap relative to oil and meal, floor traders buy beans and sell oil and meal. They are crushing soybeans on the trading floor. And sometimes they reverse it, effectively reconstituting soybeans from oil and meal. Try doing that in a real factory.
Speculative Arbitrages
Most real-world arbitrages are not pure. They involve hedge portfolios whose values are nonstationary, meaning they can wander permanently in one direction rather than reverting to a mean. But they have a strong tendency toward short-term mean reversion. Traders take the bet that the mean reversion will dominate before the nonstationary component has time to hurt them.
Maturity spreads involve contracts that are identical except for their expiration dates. The December oat contract and the March oat contract share almost all the same risk factors. Their prices are very highly correlated. When the spread between them looks unusual, traders put on the spread and wait for it to normalize.
But not all maturity spreads are equally safe. The spread between a May oat contract and a July oat contract can be wildly unpredictable because it depends on uncertainty about the upcoming harvest.
Pairs trading involves two different instruments that a trader believes are mispriced relative to each other. Harris gives a great example. Washington Mutual rises on good interest rate news. FirstFed Financial, a similar but smaller savings and loan, does not move. A pairs trader buys FirstFed and sells Washington Mutual short. The next day, FirstFed catches up. The trader makes 2.8% net profit in one day.
But pairs trading can fail. If you buy GM and sell Ford because you think the spread has widened for no reason, and then Ford announces massive SUV sales at GM’s expense, the spread never closes. You lose your transaction costs plus financing.
Statistical arbitrage generalizes pairs trading using factor models. Statistical arbitrageurs identify instruments that appear mispriced relative to common factors like interest rates, inflation, or market volatility. They buy the underpriced ones and sell the overpriced ones, carefully constructing portfolios to minimize risk while maximizing expected returns.
Risk arbitrage (in the narrow sense) involves merger situations. When a company announces it will acquire another, the target’s stock usually trades at a discount to the announced deal price. The discount reflects the risk that the deal might fall through. Risk arbitrageurs buy the target and sell the acquirer, betting the merger will close. This is really more of a risky speculation than a traditional arbitrage.
What Makes Arbitrage Interesting
The deeper insight in this chapter is that arbitrageurs do more than just make money. They supply liquidity, move liquidity between markets, and produce financial products. When a shipping arbitrageur connects excess demand in one market to excess supply in another, they are essentially making both markets more liquid. When an options arbitrageur manufactures options by hedging with stock, they are producing a product that investors want.
Restrictions on arbitrage therefore have real costs. If you ban index arbitrage because people blame it for market crashes, you break the connection between the futures market and the stock market. Prices diverge more. Liquidity suffers. The complaints about arbitrage often ignore the benefits it provides.
Understanding whether a particular spread is truly mean-reverting or just looks that way is the central challenge of all arbitrage. Get it right and you make money. Get it wrong and you are just making a bet with expensive carrying costs.
This is part of a series on Larry Harris’s “Trading and Exchanges: Market Microstructure for Practitioners.” The book is dense and technical, but it explains the real mechanics behind how markets work. If you trade anything, it is worth your time.