Merger arbitrage is not a bond substitute, it is selling insurance on a deal closing. The spread is the market’s live odds, and antitrust is now the risk that prices it.
Key takeaways
- The spread between a target’s price and the offer price is the market’s live estimate of how likely the deal is to close. Buying it is a bet that the deal completes on time.
- A raw spread is meaningless without the clock. Annualise it: gross spread scaled by 365 ÷ days to close. Anything that delays the close quietly cuts the return.
- The payoff is asymmetric. In the worked example you risk about 37% to make about 5.3%, so you only do it when you judge completion to be very likely, and you size so no single break sinks the year.
- Antitrust is now the dominant risk. A wide spread on a deal under competition review is the market pricing the regulator’s odds, not free money, as JetBlue-Spirit and Tapestry-Capri both showed.
When a public company agrees to be bought, its shares jump towards the offer price the same day. They almost never get all the way there. A company agreed to be bought for £57 a share will sit at £54, or £52, or £49, right up until the cheque clears. That leftover gap is the whole game.
Merger arbitrage, also called risk arbitrage, is the business of buying that gap. You buy the target after a deal is announced, you wait for it to close, and you collect the difference between what you paid and what the buyer pays you. Here are the mechanics: what the spread is, how a fund turns it into an annualised return, how the trade changes between cash and stock deals, and why the thing that wipes out a year of these small wins is a regulator saying no.
The reader who gets this wrong treats merger arb as a safe bond substitute. It is closer to writing insurance. You collect a premium for taking on a specific risk the market wants off its hands, and once in a while you pay a claim. Understand the premium and the claim, and the strategy makes sense. Ignore the claim and it will find you.
The spread is the market’s odds on the deal closing
Start with the number itself. A deal is announced: Buyer will pay 55 a share for Target. Target closed the day before at 40, and the morning after the announcement it opens at 51. It does not open at 55.
That four-point gap between 51 and 55 is the spread. It exists because the deal is not certain. Regulators can block it. Financing can fall through. Target’s shareholders can vote it down. The buyer can find something in the books it does not like and walk. Until the deal legally completes, none of the offer price is guaranteed, so the market discounts it.
Read the spread as a probability. If the market were certain the deal would close tomorrow, Target would trade at 55 and there would be nothing to buy. The fact that it trades at 51 tells you the market is pricing in some chance of failure and some cost of waiting. A wide spread means the market is nervous; a tight one means it thinks the deal is close to done. The size of the spread correlates with the perceived risk that the deal will not complete at its original terms. When you buy the spread, you are betting the deal is more likely to close than the price implies.
So the arbitrageur is not making a call on whether the business is good or the sector is cheap. That call has been outsourced to the acquirer, who already agreed a price. The arbitrageur makes one narrower call: will this deal, on these terms, actually complete, and when.
The annualised return is the only honest way to read a spread
A raw spread means little on its own, because time is doing half the work. A 4% spread that closes in 60 days is a completely different trade from a 4% spread that takes 240 days, even though the headline number is identical.
The fix is to annualise. Take the gross spread as a percentage of what you pay, then scale it by how much of a year your capital is tied up:
Annualised return = (offer price − current price) ÷ current price × 365 ÷ days to close.
Run the numbers on the example above. You buy Target at 51. The offer is 55. That is a gross spread of 4 ÷ 51, or 7.8%. If the deal is expected to close in 90 days, you annualise it: 7.8% × 365 ÷ 90 = about 31.8% annualised. If instead the deal is expected to drag on for 300 days, the same 7.8% gross becomes 7.8% × 365 ÷ 300 = about 9.5% annualised. Same spread, wildly different trade. Annualising the spread is not a trick; it is the only way to compare one deal with another.
This is also why anything that pushes back the closing date quietly destroys return. A regulator opening a longer review, a “second request” for information, a court date being set: none break the deal, but each extends the days-to-close, and an unexpected extension lowers the expected annualised return, which pushes the stock down to compensate even when the probability of eventual completion has not changed. The clock is part of the risk.
Across a diversified book of deals closing in an average of three to four months, the strategy has historically thrown off net annualised returns of roughly 7% to 12% in normal conditions. That is the premium for the insurance you are writing.
A worked spread: what you make and what you lose
Numbers make this concrete. Take a cash deal, the simplest kind, and put £100,000 of capital into it.
- Offer price: £50.00 a share, all cash.
- Current price after announcement: £47.50.
- Gross spread: £2.50, or 5.26% of what you pay.
- Expected time to close: 120 days.
You buy 2,105 shares at £47.50, committing £99,987. If the deal closes on schedule at £50.00, you receive £105,250. Your profit is £5,263, a 5.26% gross return over 120 days. Annualise it and that 5.26% becomes 5.26% × 365 ÷ 120 = about 16% a year. Against cash yielding, say, 4%, that is a real premium for four months of completion risk.
Now the claim side. Say the deal breaks. Announced targets that lose their deal do not fall back to yesterday’s price; they usually fall well below where they traded before the bid, because the bid itself flattered the shares and its collapse signals trouble. Assume Target drops to £30 on the break, a level below the pre-announcement price. Your 2,105 shares are now worth £63,150. The loss is £36,837, or roughly 37% of the position.
That is the shape of the whole strategy in one trade. You risked about 37% to make about 5.3%, and you did it because you judged the odds of closing to be very high. Do that across forty deals a year and the maths only works if you are right on completion the overwhelming majority of the time and you size each position so a single break cannot take out a year of spread capture. A few broken deals can erase a year of small wins, which is why position sizing, not spread-picking, is where good funds earn their fee.
Cash deals versus stock deals, and shorting the acquirer
Not every deal pays cash. The mechanics change depending on the currency of the offer, and the change matters.
In a cash deal, the trade is clean: buy the target, wait, get paid a fixed amount of money. The only real variable is whether and when the deal closes. Capri’s takeover, below, was one of these.
In a stock deal, the buyer pays in its own shares, at a fixed exchange ratio, say 0.5 acquirer shares for each target share. Now you have a problem. The value of what you will be paid moves with the acquirer’s share price between now and closing. If the acquirer’s stock falls 20% while you wait, the value of your payout falls with it, and the spread you thought you locked in evaporates. You have taken on market risk you never wanted.
The standard fix is to hedge it out. You buy the target and simultaneously short the acquirer in the exact proportion of the exchange ratio. If the ratio is 0.5, you short half a share of the acquirer for every target share you own. Whatever the acquirer’s stock does, your long target position and your short acquirer position move against each other and cancel out, so your profit and loss tracks the spread and the deal outcome rather than the broad market. When the deal closes, your target shares convert into acquirer shares at the ratio, and you deliver those straight into your short to close it out. The hedge is what lets a stock-deal arbitrageur isolate the one bet they want: completion.
This is the line between the amateur and the professional version of the trade. Buying a target in a stock deal without shorting the acquirer is not merger arbitrage; it is a leveraged bet on two companies at once.
Antitrust is the real risk now
Every risk in this strategy comes down to the deal not closing on its terms: financing failing, shareholders voting no, a material adverse change clause being triggered. The one that has done the most damage in recent years is regulatory, and specifically antitrust. Being cleared by one regulator does not guarantee completion, and a competition authority that decides to fight can hold a deal for a year or kill it outright. Two recent breaks show what that looks like on the tape.
JetBlue and Spirit. JetBlue agreed to buy Spirit Airlines in a deal valued at roughly $3.8 billion. The US Department of Justice challenged it in March 2023 on the grounds that removing a low-cost carrier would raise fares. A federal judge blocked the deal in January 2024 and JetBlue abandoned it. Anyone holding the spread on the theory that the case was weak lost when the court sided with the DOJ.
Tapestry and Capri. This is the cleanest illustration of the whole strategy, break included. In August 2023, Tapestry agreed to buy Capri Holdings, owner of Michael Kors, for $57.00 a share in cash, about $8.5 billion, a premium of roughly 59% to Capri’s prior 30-day average price. Capri shares jumped above $50 on the news but never reached the $57 offer, because the market priced in antitrust risk from the start; that gap between the mid-$50s and $57 was the spread arbitrageurs were buying. In October 2024 a federal judge, siding with the Federal Trade Commission, blocked the deal on the finding that the two firms were close competitors in “accessible luxury” handbags. Capri fell around 45% in a single session to roughly $21, below the pre-deal price. Every arbitrageur long the spread took a loss that dwarfed the few points on offer. That is the worked example above, playing out with real money.
The lesson is not that antitrust makes the strategy uninvestable. It is that antitrust is now the dominant variable in the spread, so a wide spread on a deal under competition review is not free money. It is the market’s estimate of how likely the regulator is to win. When a deal faces a second request or a court fight, spreads widen and deal certainty falls, particularly on megadeals above $10 billion. The arbitrageur’s edge is being better than the market at reading which of those fights the regulator loses.
The regime is not static, which cuts both ways. Where regulators pull back, reviews get faster and fewer deals break, so spreads compress and the premium shrinks. Where they lean in, spreads widen and the trade pays more, but only to those who read the outcome correctly. That is why merger arb pays a knowable premium in some years and springs on you in others, and why the honest way to hold it is as insurance you have priced, not as a bond.
Our guide to hedge funds covers how event-driven strategies like this one fit alongside long/short and macro. The same reading of probability against price drives distressed debt investing, and the wider family of event-driven investing covers the spin-offs and special situations that use the same toolkit.
FAQs
Is merger arbitrage low risk?
It has low volatility in normal conditions because most announced deals close, so returns look steady and bond-like. The risk is in the tail, not the everyday volatility. When a deal breaks, the loss on that position is far larger than the spread you were earning, so the honest description is low-frequency, high-severity risk, not low risk.
What return does merger arbitrage make?
Across a diversified book of deals closing in three to four months, the strategy has historically produced net annualised returns of roughly 7% to 12% in normal conditions. Individual deals annualise anywhere from mid-single digits to well over 20% depending on the spread and the timeline. None of it is guaranteed, and a cluster of breaks can turn a positive year negative.
Why doesn’t the target just trade at the offer price?
Because the offer is not certain until the deal legally completes. The gap prices in the chance the deal fails, from antitrust, financing or a shareholder vote, plus the time value of the money you tie up waiting. The tighter the spread, the more confident the market is that the deal closes.
How do funds hedge a stock-for-stock deal?
They short the acquirer in the proportion of the exchange ratio. If the deal pays 0.5 acquirer shares per target share, they short half an acquirer share for every target share they own. That cancels out moves in the acquirer’s stock, leaving the position exposed to the spread and the deal outcome rather than the market.