Glossary
The COM-B Model: A Practitioner's Guide to Behavioral Diagnosis
What is the COM-B model?
The COM-B model describes behavior through interactions among Capability, Opportunity, and Motivation. Susan Michie, Maartje van Stralen, and Robert West introduced it in 2011 as the behavioral system at the center of the Behaviour Change Wheel.
Its practical question is simple: Why isn't the behavior happening? The three components divide into six categories that can help organize an investigation. They are categories to explore, not six guaranteed answers.
The Three Components and Six Subcomponents
Capability: Can they do it?
- Physical capability includes the skills, strength, and stamina needed for an action. Opening a difficult bottle or climbing stairs may exceed someone's current physical capacity.
- Psychological capability includes knowledge and mental skills. Knowing how to use a device, remembering a procedure, and understanding instructions are examples of questions to investigate.
Opportunity: Does the environment allow it?
- Physical opportunity includes time, resources, access, and other conditions outside the person. A distant gym may make a gym-based program impractical. It does not rule out every other way to exercise.
- Social opportunity includes social and cultural influences. Do colleagues smoke together during breaks? Does the workplace make it difficult to take time away? What other people do and expect can affect the available choices.
Motivation: What moves them to act?
- Reflective motivation includes evaluations, intentions, and plans: deciding to exercise, weighing its benefits, or thinking of yourself as a runner.
- Automatic motivation includes emotions, impulses, and learned responses: a craving, anxiety about a dental visit, or reaching for a phone without much thought. Motivation in COM-B means more than a conscious desire or stated intention.
How the Components Interact
The model allows capability and opportunity to influence motivation, and behavior to change all three. Learning a skill may change willingness to try it. Performing an activity may alter what happens next. These are possible relationships to investigate.
A behavior does not necessarily require changes to all six components. Find the constraints that matter for this person, behavior, and setting.
How to Use COM-B in Practice
- Start with the outcome. List different behaviors that could help achieve it. Do not assume the first behavior proposed is the best fit.
- Choose and specify a behavior. Who would do what, where, when, and how often? Make the action clear enough to observe.
- Investigate the six components. Use observation, relevant data, and conversations with the people involved. Separate what you found from what you suspect.
- Consider intervention options. A missing skill suggests different possibilities from missing transport or a task people have little reason to value.
- Test the result. Measure the behavior and intended outcome, with a credible comparison where possible. Revise the explanation when the evidence calls for it.
The Theoretical Domains Framework can provide a more detailed vocabulary for the assessment. More categories are useful only if they help answer the question.
COM-B and the Behaviour Change Wheel
COM-B is the central model within the Behaviour Change Wheel. The full Behaviour Change Wheel adds nine intervention functions: education, training, persuasion, incentivisation, coercion, restriction, environmental restructuring, modelling, and enablement.
Its seven policy categories are communication/marketing, guidelines, fiscal measures, regulation, legislation, environmental/social planning, and service provision. These help consider how an intervention could be supported.
A missing skill might lead a team to consider demonstration of the behavior and behavioral practice/rehearsal. If access is the main obstacle, training alone may miss the problem.
The broader BCW process uses APEASE to compare options: affordability, practicability, effectiveness and cost-effectiveness, acceptability, side-effects and safety, and equity. These criteria support judgment; they do not calculate the right intervention automatically.
A Practical Example
Imagine a team investigating low use of a workplace service. Employees may not understand how to use it, may be unable to access it during their shifts, or may see little value in it. Those explanations suggest different work. An instructional message cannot create time in a schedule. Easier access cannot make an unwanted service worthwhile.
Before adding prompts or incentives, ask whether a different service or behavior would better meet the underlying need.
A Published COM-B Example: Hearing Care
Barker, Atkins, and de Lusignan interviewed ten audiologists from five English departments about collaborative planning for hearing-aid use. They used COM-B to identify possible influences on the professionals’ planning behavior and the Behaviour Change Wheel to develop an intervention. This was a study of intervention design, with effectiveness still to be tested. It illustrates how COM-B can guide questions about professional behavior rather than simply assign a deficit to patients. See their 2016 hearing-care study.
What Does the Evidence Show?
The 2011 paper reviewed 19 frameworks, developed the wheel, and assessed whether it could reliably classify interventions in selected policy areas. This supports its use as an organizing framework. It does not establish that COM-B-based interventions outperform other approaches.
Limitations and Criticisms
A complete diagram can still contain the wrong explanation. Broad categories make it possible to describe many influences, but naming a category does not identify a cause or predict an effect size.
Self-reports can help, yet people may not be able to explain every automatic response or recognize every environmental constraint. Check what they say against behavior and circumstances where possible.
The full BCW process also requires choices about evidence, delivery, and priorities. Its level of detail should serve the work, rather than become a reason to postpone testing.
COM-B vs. Other Frameworks
The Fogg Behavior Model gives a prompt a separate place alongside motivation and ability. COM-B explicitly separates capability from opportunity and reflective from automatic motivation. Similar words do not make the models interchangeable.
EAST offers four design prompts: Easy, Attractive, Social, and Timely. The Theory of Planned Behavior focuses on attitudes, subjective norms, perceived control, and intentions. The BCT Taxonomy describes intervention techniques. These tools ask partly different questions.
The Behavioral State Model works at a finer level, separating Personality, Perception, Emotions, Abilities, Social Status/Situation, Motivations, and the Social and Physical Environments. BSM is a developing framework, not an independently validated replacement for COM-B. COM-B offers broader categories and a published development process; BSM offers more specific questions whose practical value still needs direct testing.
Frequently Asked Questions
What does COM-B stand for?
Capability, Opportunity, Motivation, and Behavior.
Who created the COM-B model?
Susan Michie, Maartje van Stralen, and Robert West presented it in their 2011 Behaviour Change Wheel paper.
What is the difference between COM-B and the Behaviour Change Wheel?
COM-B is the behavior model at the center. The wheel adds intervention functions and policy categories for considering possible responses.
Does COM-B tell me which intervention will work?
No. It can help organize the investigation and possible choices. The chosen intervention still needs evidence about its outcomes.
Why it matters
COM-B can help a team move beyond blaming low motivation for every problem. Its categories are useful when they lead to better investigation and better choices. They cannot replace either.