Setting the threshold of understanding - the CMSL principle

A common problem

Setting the threshold of understanding is a problem that everyone who conducts a mental capacity assessment faces. Occasionally, we are lucky and the courts have already set that for us (see IM v LM & Others (2014) for the threshold in relation to sexual relationships)

In my book ‘Grandpa on a Skateboard’ I talk briefly about the need to ensure it is not set too high or too low (PH and a Local Authority v Z Limited & R [2011] EWHC 1704 (Fam)) and using the ‘man on the Clapham Omnibus’ principle i.e. what would you expect the average man on the street to know. Whilst this is undoubtedly useful, it can still leave people struggling, which is why I have developed the CMSL principle.

The CMSL Principle

CMSL stands for Concept, Mechanics, Short term and Long term. It is my experience that when setting the threshold of understanding, every decision can be broken down into the four areas. This then makes it much easier to identify what the person should be expected to understand for each element and how you would expect them to demonstrate it. You will find that every decision will have a different weighting on each of the elements and so therefore still requires some subjectivity from the assessor. You must still be mindful not to set the bar too high or low but by using the CMSL principle it will ensure that you have considered all the aspects of the decision to be made before setting the threshold.

Case Study

Let’s consider the decision of whether an individual has the capacity to make decisions about their property and financial affairs. Using the CMSL principle we can break it down into the following questions/ areas of understanding (please note, the questions noted below are not intended to be definitive);

Concept: What is money? Where does it come from? The fact that it is finite in nature

Mechanics: How do you practically get your money? How do you invest? What are the practicalities of managing money?

Short term: This would cover day to day budgeting. What does the person need to know or demonstrate in relation to this?

Long term: This would cover the longer term investment of larger sums of money, for example a compensation payment.


Setting the threshold of understanding is always going to contain an element of subjectivity. The CMSL Principle will help you to break down the threshold into its constituent parts and enable you to demonstrate how and why you have set the threshold at the point that you have done.