Test the big assumption behind your immunity
Design a small experiment to check whether the assumption keeping you stuck is actually true.
Key takeaways
- What it is: Design a small experiment to check whether the assumption keeping you stuck is actually true.
- Why it works: Kegan and Lahey identify the "big assumption" as the core driver of immunity: an unquestioned belief ("If I ask for help, people will see me as incompetent") that makes the competing commitment feel necessary. Most big assumptions were formed in earlier developmental contexts and have never been tested as adults. Running a controlled, low-risk test treats the assumption as a hypothesis — a shift from subject (this is true) to object (I am assuming this is true).
- Evidence: Established clinical or therapeutic practice.
- Avoid: Designing a thought experiment rather than a behavioral one — the assumption can only be updated by actual evidence, not by thinking harder about it.
Why it works
Kegan and Lahey identify the "big assumption" as the core driver of immunity: an unquestioned belief ("If I ask for help, people will see me as incompetent") that makes the competing commitment feel necessary. Most big assumptions were formed in earlier developmental contexts and have never been tested as adults. Running a controlled, low-risk test treats the assumption as a hypothesis — a shift from subject (this is true) to object (I am assuming this is true).
How to do it
- 1Identify your big assumption from the immunity map (step 4 above).
- 2Rate your current confidence that it is true (0–100%).
- 3Design the smallest possible real-world test that would generate actual evidence about it. Not a thought experiment — actual behavior.
- 4Run it and observe outcomes, including outcomes that contradict the assumption.
- 5Update your confidence level and the behavior plan accordingly.
What the evidence says
Clinical practiceBehavioral experimentation to test beliefs is the core of cognitive behavioral therapy and behavioral experiments research, which has strong evidence. Kegan and Lahey’s immunity framework applies this to developmental assumptions.
Honest caveat: The clinical evidence is for CBT behavioral experiments generally; the specific Kegan framing of "big assumption testing" carries that evidence by analogy, not by direct trial.
- — Bennett-Levy et al. (2004), Oxford Guide to Behavioural Experiments in Cognitive Therapy
Common mistake
Designing a thought experiment rather than a behavioral one — the assumption can only be updated by actual evidence, not by thinking harder about it.
IX Coach tracks your behavior experiments, helps you design them to be genuinely informative, and reviews the results with you to update the belief systematically.
Practice this with IX Coach →