The Most Dangerous Part of Any Decision: The Assumptions You Forgot You Made

Every strategic plan rests on a quiet foundation: assumptions.

Market conditions will hold.
Customers will behave a certain way.
Competitors won’t react too quickly.
Resources will remain available.

We rarely say these out loud, but they are there, embedded in every decision.  And that’s the problem.

Decisions Don’t Fail.  Assumptions Do

When a strategy “fails,” it’s easy to blame execution. But more often, the real issue is that the underlying assumptions changed and no one noticed.

• The market shifted faster than expected
• A key customer segment evolved
• Internal capacity wasn’t as stable as believed
• A competitor made an unexpected move
• The decision itself may have been perfectly logical at the time. It just didn’t survive contact with a changing environment.

The Hidden Risk: Unexamined Assumptions

Most teams don’t explicitly identify their assumptions. They move straight from analysis to decision:  “Given what we know, this is the right move.”

But buried inside “what we know” are beliefs that haven’t been tested:

• “Demand will continue growing.”
• “This team can scale.”
• “This technology will remain viable for the next 2 years.”

When these go unspoken, they can’t be challenged, monitored, or updated.  So when the world shifts (as it always does), the team is caught off guard.

A Better Approach: Make Assumptions Visible

High-performing teams treat assumptions as first-class citizens in decision-making.  Before committing to a strategy, they ask:

• What must be true for this to work?
• What are we taking for granted?
• Where are we most likely to be wrong?

This simple shift does two powerful things:

1. It improves the quality of the initial decision
2. It creates a mechanism to adapt when conditions change

Because now, you know what to watch.

From Static Plans to Living Decisions

Once assumptions are visible, the next step is to actively monitor them.  Think of it like this: every assumption should have a “tripwire.”

• If customer acquisition cost rises above X…
• If delivery timelines exceed Y…
• If market growth drops below Z…

…then we revisit the decision.  This turns strategy from a one-time event into an ongoing process.  Instead of asking, “Was this the right decision?”  You ask, “Are the conditions that made this decision right still true?”

The Real Competitive Advantage

In a stable world, perfect planning wins.  In a changing world (and that’s the one we live in), adaptability wins.  Teams that succeed aren’t the ones who get every decision right upfront. They’re the ones who:

• Surface their assumptions early
• Revisit them often
• Adjust before small shifts become big problems

At the end of your next strategic discussion, add one question:  “What assumptions are we making that could invalidate this decision?”  Write them down. Revisit them regularly.

It’s a small discipline, but it fundamentally changes how your team navigates uncertainty.

Related Articles:

What If Your Customer Were in the Room?

Stop Choosing. Start Thinking: Escaping the Binary Trap in Team Decisions

From the Bookshelf: The Engagement Effect