You're in the final stretch of a deal that's been in negotiation for weeks. Your team has done the work—or so you think. You've defined your targets, mapped out your concessions, and identified your walk-away point. But there's a quiet problem lurking underneath your preparation that you haven't examined: assumptions.
Assumptions are the mental shortcuts that let you make decisions when you don't have complete information. Without them, you'd be paralyzed. With the wrong ones, you end up leaving money on the table or accepting terms that don't serve you. The negotiators who consistently get better outcomes aren't the ones with the most information—they're the ones who've learned to distinguish between useful assumptions and dangerous ones.
This post walks through how to identify the assumptions you're bringing to your next negotiation, stress-test them before they cost you, and build a preparation process that catches errors before the deal starts.
Why assumptions are a necessary but risky tool
Every negotiation involves uncertainty. You don't know exactly what your counterpart's priorities are, what their internal constraints look like, or how they'll respond to your proposals. Assumptions let you fill those gaps and move forward with preparation.
The risk is that assumptions feel like knowledge. Once you've formed one, it takes on the weight of fact—especially when you're the one who formed it. Research on confirmation bias shows that we naturally seek information that supports what we already believe and discount evidence that challenges it. This means the assumptions you form earliest in a deal tend to be the ones you defend longest.
The practical consequence: Negotiators who don't stress-test their assumptions often discover the gap only when the deal is done—and the numbers don't add up. By then, it's too late to adjust.
Common assumption categories that tend to cause problems:
- Market assumptions: "This is what the market will bear" based on a single data point or outdated benchmark
- Counterpart assumptions: "They need this deal as much as I do" without verification
- Relationship assumptions: "We have a good relationship, so they'll give me X" when the two domains don't overlap
- Leverage assumptions: "We have the upper hand" when the balance of power has shifted without your noticing
A framework for identifying and stress-testing your assumptions
The goal isn't to eliminate assumptions—you can't. The goal is to catch the ones that are guesses in disguise before they drive your strategy. Here's a practical approach:
Step 1: Capture every assumption you've made
Write down every "I assume..." statement in your preparation. Don't filter yet. Include assumptions about the counterparty's priorities, the market, your leverage, and what a fair outcome looks like. Getting them on paper makes them manageable.
Step 2: Sort by evidence level
For each assumption, ask: What's the evidence? Rate each one:
- Evidence-based: Grounded in recent data, documented patterns, or direct experience
- Inferred: A reasonable extrapolation from available information, but not confirmed
- Guess: Formed without verification—essentially an opinion you haven't tested
Assumptions in the first category are worth treating as working facts. Assumptions in the second deserve a verification step. Assumptions in the third need either validation or contingency planning.
Step 3: Design tests for your high-stakes assumptions
For assumptions that could significantly change your strategy if wrong, design a test. This might mean:
- Asking a direct question during the negotiation's discovery phase
- Checking a public source (news, filings, industry reports)
- Consulting someone with relevant experience who has no stake in the outcome
- Using a competitive intelligence scan to surface recent developments that might invalidate your belief
Step 4: Build contingencies, not certainties
The goal of testing isn't to eliminate uncertainty—it's to know how to respond when your assumptions turn out wrong. For each major assumption, identify: what would you do differently if it doesn't hold? Having a contingency keeps you from being caught flat-footed.
This process fits naturally into the structured preparation workflow. Rather than treating assumptions as background noise, making them explicit turns them into a deliberate part of your deal preparation.
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Common mistakes to avoid
Even experienced negotiators fall into these patterns. Here's what tends to go wrong:
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Treating assumptions as facts. The moment you stop questioning an assumption, you've locked in a belief that will shape every subsequent decision. Treat all assumptions as hypotheses until they're confirmed.
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Confirming the assumption you started with. If you only test assumptions by looking for evidence that supports them, you're not testing—you're confirming. Actively seek information that would prove you wrong.
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Failing to update when new information arrives. Initial assumptions are often formed with limited data. When you learn something new—whether from discovery questions or external sources—revisit your earlier beliefs. Static assumptions in a dynamic negotiation lead to stale strategy.
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Burying assumptions in the preparation document. Assumptions often hide in the footnotes of a negotiation prep document—mentioned in passing but not called out as something to be verified. Give them their own section. Give them weight.
Real-world example: The assumption that sank the deal
A manufacturing company was negotiating an annual supply agreement with a key vendor. Their team had done substantial work: targets defined, walk-away points set, concessions mapped. Their core assumption was that the vendor was under pressure to close the deal before quarter-end to hit their revenue targets.
This assumption shaped their entire approach—they took a firm stance on pricing, reasoning the vendor would eventually concede to protect their numbers.
The problem: the vendor's fiscal year ended two months earlier than the buyer's team assumed. When the vendor refused to move on price, citing that they had no quarter-end pressure, the buyer's team was caught unprepared. They'd anchored their strategy on a timing assumption that turned out to be wrong. The negotiation nearly collapsed.
What the buyer's team learned: their assumption about the vendor's internal calendar was a guess—and a consequential one. They had no verification for it. A quick check of the vendor's public filings would have revealed the mismatch. Instead, they spent the negotiation scrambling to adjust their strategy on the fly.
A structured preparation process that called out and tested this assumption before the negotiation started would have caught the error. They could have entered with a calibrated approach rather than an overconfident one.
Conclusion
Assumptions are a necessary part of negotiation preparation—they fill the gaps that would otherwise prevent you from making decisions. The problem isn't having assumptions. The problem is not knowing which ones are grounded in evidence and which ones are guesses wearing a confidence costume.
The negotiators who consistently get better results treat assumptions as first-class citizens in their preparation: named, tested, and contingency-planned. You can do the same. The next time you enter a negotiation, make your assumptions visible before they make your decisions for you.