COM-B model
Behavior happens when Capability, Opportunity, and Motivation align.
What it means
The COM-B model places behavior (B) at the center of a system of three necessary, interacting conditions: Capability, Opportunity, and Motivation. Capability is the individual's physical and psychological capacity to perform the behavior, including the requisite knowledge and skills; Opportunity comprises the physical and social factors outside the person that make the behavior possible or prompt it; and Motivation encompasses the brain processes that energize and direct behavior, both reflective (plans, beliefs, evaluations) and automatic (habits, impulses, emotional responses). The mechanism that makes it useful is interaction and sufficiency: the three components influence one another and a behavior occurs only when all are adequately present, so any one of them can be the missing ingredient. This turns COM-B into a diagnostic tool — first identify which component is deficient for the target behavior, then intervene precisely there rather than defaulting to information campaigns. It sits at the hub of the broader Behaviour Change Wheel, linking diagnosis to matched intervention functions and policies, and it matters across public health, organizational, and policy contexts as a structured alternative to guessing why a behavior is or is not happening.
Where it came from
COM-B was not derived from a single theory but assembled to be usable. Michie, van Stralen and West set it out in 2011 as the hub of the Behaviour Change Wheel: their review of nineteen existing intervention frameworks produced the Wheel's outer rings of intervention functions and policies, while COM-B itself was a compact synthesis of the conditions any behaviour requires. The aim was broad coverage rather than parsimony: a checklist wide enough that no major class of influence is forgotten. It sits at the hub of the Behaviour Change Wheel, whose outer rings link each diagnosed deficit to intervention functions -- education, restriction, environmental restructuring, incentivisation -- and policy categories. The finer-grained Theoretical Domains Framework, with fourteen domains, maps onto the three components, so teams often use COM-B for the first cut and the TDF to specify what to change.
What the evidence shows
Most published support for COM-B is organisational rather than predictive: systematic reviews across vaccination, chlamydia testing, healthy eating and stroke rehabilitation use it to sort barriers and enablers into tidy categories, which shows the boxes are usable, not that they forecast behaviour. Direct predictive tests are rarer. Willmott and colleagues (2021) surveyed young adults and found the model explained roughly 31% of variance in physical activity and 23% in eating -- comparable to the Theory of Planned Behaviour's typical 20 to 30%, though most variance still sits outside the model. Keyworth and colleagues (2020) built a generic six-item self-report measure with acceptable reliability, filling a long-standing gap: for years COM-B was applied with no standard instrument. These effect sizes are moderate and come mostly from convenience samples skewing female, educated and Western.
Using it in practice
The model earns its keep as a diagnostic sequence, and the first step is the one teams skip. Specify the target behaviour precisely -- who does what, when and where -- because "eat better" cannot be diagnosed but "swap a sugary drink at lunch" can. Then interrogate all six sub-components: physical and psychological capability, physical and social opportunity, reflective and automatic motivation. The point is to find which is actually deficient rather than reaching for the default lever of information. If opportunity is missing, another leaflet cannot supply it; if a behaviour fails on automatic motivation, a persuasive argument aimed at reflective beliefs slides off. Only once the deficit is located does the Behaviour Change Wheel matter, matching it to intervention functions and policies instead of to the intervention someone already had in mind.
Limits and caveats
The model's breadth is also its weakness. Because any behaviour that occurs must, by definition, have had sufficient capability, opportunity and motivation, the framework is hard to falsify: it explains outcomes after the fact more comfortably than it forecasts them. The three components overlap -- self-efficacy can be read as capability or as motivation -- so independent coders do not always agree, and a "COM-B analysis" can amount to relabelling. Critics note that it takes a static snapshot, treating behaviour as a moment rather than a dynamic that shifts as circumstances change, and that keeping the individual at the centre can underweight culture, power and economic structure. It tells you where a behaviour breaks, not how large the gap is or which fix will move it most; that still requires measurement.
Examples
Hand-washing fails if soap is absent (opportunity), the technique is unknown (capability), or it isn't a priority (motivation).
Office recycling stalls not because staff don't care but because the bins are three floors down. The missing component is opportunity, and another poster won't supply it.
Vaccine uptake can fail three ways: no clinic within reach, no idea how to book, or no felt urgency — and a leaflet through the door only fixes the middle one.
A new expense-reporting tool goes unused: staff can log in (capability) and have the time (opportunity), but the old spreadsheet still works and nothing rewards the switch. The deficit is motivation, not training.
Farmers know a drought-resistant seed exists and want the higher yield, yet cannot buy it locally. Capability and motivation are present; the missing component is opportunity, so agronomy classes change nothing.
First described in Michie, van Stralen & West (2011).
Key references
- Sari, T. B., Ningsih, A. P., Makkau, B. A., & Sudirham. (2023). Capabilities, opportunities and motivations (COM) model for understanding changes in behavior: a critical examination. Journal of Public Health, 46(2), e336-e337. doi.org/10.1093/pubmed/fdad252
- Willmott, T. J., Pang, B., & Rundle-Thiele, S. (2021). Capability, opportunity, and motivation: an across contexts empirical examination of the COM-B model. BMC Public Health, 21(1), 1014. doi.org/10.1186/s12889-021-11019-w
- West, R., & Michie, S. (2020). A brief introduction to the COM-B Model of behaviour and the PRIME Theory of motivation. Qeios. doi.org/10.32388/WW04E6
- Keyworth, C., Epton, T., Goldthorpe, J., Calam, R., & Armitage, C. J. (2020). Acceptability, reliability, and validity of a brief measure of capabilities, opportunities, and motivations ("COM-B"). British Journal of Health Psychology, 25(3), 474-501. doi.org/10.1111/bjhp.12417
- Michie, S., van Stralen, M. M., & West, R. (2011). The behaviour change wheel: A new method for characterising and designing behaviour change interventions. Implementation Science, 6, 42. doi.org/10.1186/1748-5908-6-42