The ‘COMA-B’ System for Behaviour Change: Reset of the COM-B

The COM-B System

In 2011, three psychologists, Susan Michie, Maartje M van Stralen and Robert West (MSW, 2011), proposed “a ‘behaviour system’ involving three essential conditions: capability, opportunity, and motivation…This forms the hub of a ‘behaviour change wheel’ (BCW).”

MSW mention two sources for the idea of the COM-B:

“a US consensus meeting of behavioural theorists in 1991 [see this], and a principle of US criminal law dating back many centuries…Under US criminal law, in order to prove that someone is guilty of a crime one has to show three things: means or capability, opportunity, and motive.”

They continue:

“This suggested a potentially elegant way of representing the necessary conditions for a volitional behaviour to occur…We have built on this to add nonvolitional mechanisms involved in motivation (e.g., habits) and to conceptualise causal associations between the components in an interacting system.”

A conceptual framework used by the courts to prove the innocence or guilt of accused felons may not be the most appropriate model to apply to the behavioural choices of non-criminals. The COM-B has been influential and highly cited (4220 times by 08/10/20).  However, the hub of the COM-B is incomplete and manifestly it does not work when applied to the most basic of choices in health behaviours such as smoking, drinking and the wearing of masks in a pandemic.  Here I explain why. Screen Shot 2020-04-03 at 10.34.04.png


In the COM-B system, Capability, Opportunity, and Motivation are said to ‘interact’ to generate Behaviour. C, O and M are claimed to be sufficient and necessary conditions for B. Before discussing the model, we need some definitions:


MSW give the following definitions:

‘Capability’ is defined as the individual’s psychological and physical capacity to engage in the activity concerned. This includes having the necessary knowledge and skills. In plain language, ‘capability’ means ‘fit to’.

‘Motivation’ is defined as all those brain processes that energize and direct behaviour, not just goals and conscious decision-making, e.g. habitual processes, emotional responding, as well as analytical decision-making.  Thus motivation equates with ‘need to’.

‘Opportunity’ is defined as all of the factors that ‘lie outside the individual that make the behaviour possible or prompt it’.  Thus opportunity means ‘can do’.

The model suggests that if one is fit to, needs to and can do a particular behaviour, then one will do it. Unfortunately, there is a key process missing from this scheme, as I will demonstrate below. Taking into account the original context for the COM-B model – proving guilt or innocence in the courtroom – I will use an example from the same domain.

Robbing a Bank

According to the COM-B, capability and opportunity cause changes in motivation and changes in behaviour.  I refer to that fictitious character Joe Blow (pronouns: him/her/their). 

According to the COM-B:

Joe Blow (JB) would (X rob a bank, Y kiss the queen, Z fly to the moon, whatever) if it can be shown that JB is fit to, needs to, and can do X, Y or Z. Yet this account is plainly incomplete. A key element that is missing from the COM-B is wanting: JB must want to carry out X, Y or Z if he is to actually to do it.  If JB doesn’t want to, he/she/they simply won’t do it, no matter what.

Imagine the following:

1) JB is fit to rob a bank because he/she/they is physically strong and has a jemmy and a set of tools for breaking open doors and safes – (fit to).

2)  JB is hugely in debt (to a bank, as it happens) so he/she/they need(s) money very badly, and so has a strong motive to rob a bank – (needs to).

3) JB knows there is a back alley and a door with an alarm that a friend who works in the bank will leave switched off on any night of their choosing – (can do).

Thus, JB ticks all three boxes of the COM-B but JB still chooses not to rob a bank. Why so? It could be for a million and one  possible reasons.  JB does not rob the bank because:

robbing the bank would be wrong,

it would be risky – i.e if he is found out he would go to prison,

if found out, it would look bad in front of the neighbours,

it would upset the bank manager who he knows well drinks beers with in the local pub,

it would be an unreasonable and unfair


For a host of different reasons, JB may desperately need money but does not want to rob the bank to get it.

In spite of JB ticking all three of COM-B boxes, the COM-B fails to correctly predict JB’s behaviour. There is a hidden barrier. In multiple situations people do not choose to do something, something for which they could be handsomely rewarded, because they simply do not want to do it.

Another similar individual who ticks all three COM-B boxes might actually proceed to commit the bank robbery. Imagine that Joe Blow has a twin, Les Blow (LB/him/her/their) who lives on the other side of town. JB tells LB about the bank, the back alley and the dodgy security guard.  LB now meets all three criteria – LB is fit to, needs to, and can do the bank robbery. However, LB has none of the moral and social scruples held by JB and proceeds to rob the bank.

The twins act differently under essentially similar circumstances, revealing a crucial difference between the twins.

The COMA-B Reformulation

The COM-B requires reformulation because a crucial process is missing. However, the COM-B does not work because there are other problems that prohibit a good fit to real world data. The diagram of the model shows five arrows representing so-called ‘interactions’, three of which point in both directions. Four of the ‘interactions’ do not exist and none works in both directions. 

Only one ‘interaction’ in the COM-B diagram is anywhere near causal.  In order to do something, a person also has to want to do it more than they want not to do it. This is a delicate balancing act that takes place when we make decisions in our everyday lives. The COM-B is reformulated as the ‘COMA-B’ in the diagram below.

(A = ‘Agrees to’, which makes a better acronym than using W for Wants : COMW-B).

amended COM-B model.png

Adding a box for wants to and placing arrows differently converts the COM-B into the COMA-B.  In removing arrows, it is necessary to distinguish enablers from causes. Fit-to capability and can-do opportunity are both enablers of need-to motivation; wants-to is actually causal.


There are four necessary and sufficient conditions for any action X:

  1. needing to do X
  2. having the capability to do X
  3. having the opportunity to do X
  4. wanting to do X

The original COM-B framework is incapable of predicting behaviour and behaviour change. Recently, the authors of the COM-B produced a reformulation, which will be discussed in another post.

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