What is Delusional Thinking – and how to spot it.

The ruling of Banks V Goodfellow tells us that in order to have testamentary capacity the testator must not be suffering from any insane delusion or disorder of the mind – but what does this actually mean and how do we spot it? 

What is Delusional Thinking? 

According to the ICD-11 (World Health Organization, 2022), a delusion is “a firmly held, false belief that is not amenable to change in light of conflicting evidence, and that is not ordinarily accepted by other members of the person’s culture or subculture. Such beliefs therefore often present as fixed (not easily changed), false or implausible and not shared by others in the person’s culture or subculture. 

However, it is also important to understand what they are not. Just because someone’s opinion may seem weird (e.g., flat Earth theories) or based on strong cultural or religious beliefs that are different to ours or even based on misinformation (e.g., believing something untrue because of poor sources) does not make them delusional. 

As practitioners we must be really careful that we do not allow our own ideas to lead us into believing a person is delusional. Years ago, I was caring for an adult with mental health problems that lived in sheltered housing. They were getting very upset that they had not received an invite to a royal wedding that was due to take place. In my ignorance I believed this to be part of her delusional belief system. I was humbled when the invite arrived a few days later – it had got lost in the post!  

Common Types of Delusions  

Delusional thinking are generally broken down into different types. Arguably the most common types are: 

• Persecutory – “They’re out to get me.”    The most common kind. The person believes they’re being followed, watched, poisoned or harassed. 

• Grandiose – “I have a divine mission.”    Belief that they have special powers, fame, or insight that others do not recognise. 

• Somatic – “There’s a parasite inside me.”    The person is convinced there’s something physically wrong, despite all medical reassurance. 

• Jealous – “My partner is cheating — I just know it.”    Fixed belief in infidelity without evidence. 

• Erotomanic – “They’re secretly in love with me.”    Often (but not always) about someone famous or of high status. 

How to Spot Delusions - Some Practical Signs to Look For 

• Certainty – the person has absolute confidence in the belief. No doubt.    “There’s no point checking the CCTV — they’ve already wiped it.” 

  
  

• Immunity to evidence – you show them proof, but it makes no difference.    “Yes, the test says negative, but that’s because the doctors are in on it.” 

• 
  

• Disruption – the belief interferes with their ability to function — at work, in relationships, or with decision-making. 

  
  

• Lack of shared reality – no one else in their community or culture holds this belief. It feels off-grid, not just unusual.  

  
  

Is there a causative nexus? 

When considering delusional thinking in relation to mental capacity, the key thing we are looking for is whether there is a causative nexus between the delusional thinking and the decision outcome.  

For example, I once assessed a paranoid schizophrenic who believed his neighbours were plotting to kill him. However, he could still understand the nature of the act of making a will and its effect, the extent of his property (£50k), the moral claims to which he ought to give effect (he had two brothers) and how he wished to divide up his estate (equally between his two brothers). The delusional beliefs about his neighbours did not impact upon his ability to meet the limbs of the Banks v Goodfellow and therefore, there was no casual nexus. 

Contrast this with a gentleman I assessed who was refusing surgery because he believed the surgeon was an assassin sent to kill him, and not actually a qualified doctor. There was a clear link between the gentleman’s delusional thinking and his decision making. 

Understanding delusional ideation is key for assessors and professionals dealing with Mental Capacity.  

Delusional ideation is not about holding quirky opinions or being misinformed — it’s about a fixed and false belief that persists even when all the evidence says otherwise.  

It’s also not enough to just identify the existence of a delusion — we must identify whether a causative nexus exists.  

We must be also mindful that our own personal views and assumptions do not lead us to misinterpret unusual situations, eccentricity or different cultural norms -because in doing so, we may reach the wrong conclusions about capacity and treatment.