Early in my career, I negotiated the purchase of an apartment building for a client. The seller was an elderly man who had owned the property for 30 years. My client assumed the seller wanted maximum price. That was the obvious assumption. Who does not want maximum price?

We spent three weeks hammering on price. The seller rejected every offer. We raised our number twice. He rejected both. My client was ready to walk away. Then, during a coffee break in the third meeting, the seller said something that changed everything: “I just want to make sure the families who live there are taken care of.”

It was never about the price. The seller was willing to accept a lower number if the buyer committed to maintaining the building and keeping current tenants in place. We had been fighting the wrong battle for three weeks because of an assumption we never tested.

This story has repeated itself, in different forms, in hundreds of negotiations I have been involved in since. The assumption trap is the single most common source of failed deals, wasted time, and money left on the table. And yet, almost nobody prepares for it.

What the assumption trap actually is

The assumption trap occurs when you treat your beliefs about the other side as facts without ever verifying them. You assume you know their budget. You assume you know their deadline. You assume you know their priorities. You assume you know what “success” means to them. And because these assumptions feel obvious, you never question them.

The problem is that the most dangerous assumptions are the ones that feel most obvious. “Of course they want the lowest price.” “Of course they are under time pressure.” “Of course they have other options.” Each of these statements might be true. But each might also be completely wrong. And the cost of being wrong is enormous.

The most expensive sentence in negotiation is: “I already know what they want.” The moment you stop being curious about the other side is the moment you start negotiating with a ghost instead of a person.

Daniel Kahneman’s research on cognitive biases gives us the psychological framework for understanding why this happens. Humans are pattern-completion machines. We take incomplete information and fill in the gaps with what we expect to see. This is efficient in most of life. It is catastrophic in negotiation, where the gaps contain the information that actually determines the outcome.

The five most dangerous assumptions in negotiation

Assumption 1: “They want the lowest price”

This is the assumption that kills more value than any other. Price is almost never the only variable, and it is frequently not the most important one. Buyers care about risk, speed, reliability, support, reputation, and a dozen other factors that get ignored when both sides fixate on the number.

I worked with a software company that was losing bids to competitors who undercut them by 15 to 20 percent. They assumed they needed to lower their price. When I interviewed three prospects who had chosen competitors, the reason was not price in any of the cases. It was implementation timeline. The company took 12 weeks to deploy. Their competitor took 4. The prospects were willing to pay more for faster deployment, but nobody had asked.

Assumption 2: “They are under time pressure”

You assume the other side needs to close quickly, so you push hard on timing. But what if they are not in a hurry? What if their deadline is self-imposed and flexible? What if they have told you a deadline that is actually a negotiation tactic?

Conversely, you assume they have plenty of time, so you take a relaxed approach. Meanwhile, they are desperate to close by Friday because of a board meeting you know nothing about. By the time you figure it out, you have missed the window.

Time pressure is one of the most commonly fabricated and most commonly misjudged variables in negotiation. Never assume you know the other side’s timeline without testing it.

Assumption 3: “They have other options”

You assume you are competing against three other vendors, so you preemptively lower your price to be “competitive.” In reality, you are the only serious contender, and the other two bids were collected to satisfy an internal procurement policy. You just gave away margin to beat competitors who were never in the race.

I see this constantly. Sales teams discount reflexively because they assume fierce competition. The number of times I have asked “How do you know they have other options?” and received the answer “Well, they must have” is staggering. “They must have” is not research. It is fear dressed up as analysis.

Assumption 4: “The decision-maker is at the table”

You spend two hours negotiating a complex deal with someone you believe has authority to commit. You reach agreement. Then you hear: “This looks great. Let me take it to my boss for final approval.” Everything you agreed to is now a starting point for a second negotiation with someone you have never met.

This assumption is so common that professional negotiators deliberately exploit it. The “absent authority” tactic, where the person at the table claims they need approval from someone else, is one of the oldest moves in negotiation. But it works because people assume the person across from them has the power to decide.

Assumption 5: “Their opening position reflects their real position”

Someone offers you $200,000 for your business. You assume that is roughly what they value it at, so you counter at $280,000, expecting to land somewhere around $240,000. What you do not know is that their internal valuation was $350,000, and they opened low as a negotiation tactic. You just anchored yourself $110,000 below what they were prepared to pay.

Opening positions are strategic, not informational. They tell you what the other side wants you to think, not what they actually think. Treating an opening offer as a signal of true value is like treating a poker player’s bet as a statement of their hand. It might be honest. It probably is not.

How to test your assumptions before they cost you

The cure for the assumption trap is not to stop making assumptions. That is impossible. Your brain will do it automatically. The cure is to treat every assumption as a hypothesis that needs testing. Here is the framework I use.

Step 1: Write down your assumptions. Before any significant negotiation, take five minutes and list everything you believe about the other side. Their budget. Their timeline. Their priorities. Their alternatives. Their decision process. Write it all down. Most people are shocked by how many unverified beliefs they carry into a negotiation.

Step 2: Rate each assumption. For each item, ask: how confident am I in this, on a scale of 1 to 10? Anything below 8 is a hypothesis, not a fact. Mark it as such.

Step 3: Design questions to test the uncertain ones. For each assumption rated below 8, create a question that would help you verify it. Not a leading question. Not a question designed to confirm your belief. An open question designed to discover the truth, even if it contradicts your expectation.

Step 4: Ask early. Do not wait until the bargaining phase to test your assumptions. Ask in the opening and exploration phases, when the conversation is more relaxed and information flows more freely. People are more candid before positions harden.

Step 5: Listen for disconfirming evidence. This is the hardest part. When the other side says something that contradicts your assumption, your brain wants to dismiss it as an anomaly or a tactic. Resist that impulse. If you assumed their budget was tight and they mention a recent capital raise, update your model. The ability to revise your assumptions mid-negotiation is the mark of an expert.

I prepare for every negotiation with what I call an “assumption audit.” A page with two columns. Left column: what I believe about the other side. Right column: how I know this. If the right column says “I assume” or “it seems logical,” then I have work to do before I sit down.

When your assumptions are confirmed, be skeptical anyway

Here is a subtlety that even experienced negotiators miss. When the other side confirms one of your assumptions, that does not necessarily make it true. They may be telling you what they want you to believe.

If a buyer says “We have three other proposals on the table,” that confirms your assumption of competition. But is it true? It might be. It might also be a tactic to pressure you into discounting. The confirmation itself is data, but it is not proof.

The practice I recommend is triangulation. Verify important claims through multiple sources. If the buyer says they have other proposals, ask about timeline (“When do you need to decide?”), ask about evaluation criteria (“What are the most important factors in your decision?”), and ask about process (“Where are you in the evaluation?”). The consistency or inconsistency of their answers tells you more than any single statement.

A real-world case study: assumptions that almost killed a deal

A client of mine was negotiating a joint venture with a German manufacturing company. My client assumed the Germans prioritized precision and process over speed. Classic cultural assumption. So he prepared detailed documentation, formal presentations, and a meticulous timeline.

The German side came to the first meeting with a one-page term sheet and wanted to sign within two weeks. They were under pressure from their board to enter the Polish market before a regulatory change, and speed was their top priority. My client’s elaborate preparation actually frustrated them because it signaled a long, drawn-out process.

We caught this in the first meeting only because I asked an open question: “What does an ideal timeline look like for you?” The answer restructured our entire approach. We stripped down the documentation, accelerated the schedule, and closed in 11 days. If we had operated on the assumption that Germans always want elaborate process, we would have lost the deal to a competitor who moved faster.

Cultural assumptions are a particularly dangerous subset of the assumption trap, because they feel informed rather than lazy. You are not just guessing. You are applying knowledge. But knowledge about cultures is about tendencies, not guarantees. Every individual may deviate from the tendency, and the deviation is where the deal lives.

The bottom line

Every negotiation begins with a set of assumptions. That is unavoidable. What separates successful negotiators from everyone else is what they do with those assumptions. Amateurs treat assumptions as facts. Professionals treat them as hypotheses. Amateurs defend their beliefs. Professionals test them.

Before your next negotiation, run the assumption audit. Write down what you believe about the other side. Identify what you actually know versus what you are guessing. Design questions to test the guesses. And when the evidence contradicts your assumption, be grateful, not defensive. That contradiction just saved you from making a decision based on fiction.

The negotiator who asks the best questions almost always gets the best deal. And the best questions are the ones that challenge what you think you already know.