To what extent can mental illness create a defence or mitigation against liability in a civil case?

Question

I'm thinking here of both UK and US law, which I assume will differ.

Suppose a defendant in a civil case (type of case unspecified) would ordinarily have no defence against the plaintiff/claimant's claim, and would be ruled against in damages for a large sum. The defendant had a confirmed mental health condition (also of unspecified type) which is agreed overwhelmingly likely to have been a (or the) cause of whatever action(s) gave rise to the case.

In what circumstances and to what extent will this change the ruling, or the amount ruled against them, in US or UK law?

Some examples, although I'm really after general principles, these just illustrate the situation. (In each case assume the incident was unexpected and either no other person helps/cares for them, or any such people could not reasonably have foreseen/prevented it and would not be liable). I haven't numbered these because I'm really looking for principles and generalities rather than answers to these hypothetical examples:

• Person has "walking insomnia". While sleepwalking one night they commit an act against a person or property which would usually result in a valid civil claim for damages.
• Person has trauma and cannot always control their reactions when it is triggered; they run out of a shop when something triggers it and knock over numerous people and displayed products/shop fittings.
• Person has cancer in part of their brain that controls self regulation. Nothing happens until one day they attack somebody, an act for which they have no prior history, and agreed to be due to their medical condition.
• Ditto but the issue is a congenital or regenerative neurological condition, and their family do not knew or cannot react in time to prevent an actionable incident.
• Person has ADHD (ADD) or OCD, and is functional in society but medically it is agreed they will never have good or even usual control of attention wandering or forgetfulness or compulsive behaviours, there simply is no way to gain this. Damage results.
• A reluctant parent with a bad parent-child relationship hears a rumour on the internet that mental health is no defence against civil damage claims, and decides to sue their offspring for massive damage caused to their house on the night of their 18th birthday. Because of the stress they had also not recognised their parents who arrived, and believing themselves attacked by strangers, slashed at them with a knife in panic, causing life-changing injury. Their condition had never remotely been like this before and there was no realistic way to anticipate it might happen, nor any incident previously requiring precautions. Conveniently on that day the youngster also inherited the kind of sum that would be needed to pay any damages if the claim were successful.

Answer 1

The person with a mental health condition is as liable as the person without all else being equal.

The standard in common law (and civil law) systems is the reasonable person test - what would a reasonably prudent person in the same (external) circumstances do.

The standard can be increased by replacing the noun: for example, a person who is or purports to be a doctor will be held to the standard of a "reasonable doctor" if the damage flows from acts or omissions as a doctor. Similarly, a "reasonable oncologist" is held to a higher standard than a "reasonable doctor" if the damage stems from acts or omissions related to cancer.

Further a person who purports to be an "expert" in the field is held to the standard of a "reasonable expert".

However, the standard is not lowered to account for things that make a person less prudent than a reasonable person: like mental illness for example.

Answer 2

The answer really calls for a comparison of the level of mens rea required for various torts and the condition in question. Lots of conditions that constitute a "mental disorder" for DSM-V purposes would not cause someone to lack the proper mens rea and hence would not constitute a defense.

Also, the case law is often quite wedded to fact patterns and is not always reliable far afield of those fact patterns.

Person has "walking insomnia". While sleepwalking one night they commit an act against a person or property which would usually result in a valid civil claim for damages.

There is a split of authority over whether trespass is a strict liability offense or requires knowledge that one is trespassing, with the majority rule probably being strict liability. There is a similar split of authority over what are often considered intentional torts such as "conversion" and "battery". If there is strict liability, the person would have civil liability. If knowledge of the act is required, then there might not be liability in this case, although there might also be no remedy if someone used force against the person in response.

Negligence claims ordinarily hold a person to the standard of a reasonable person, but it isn't obvious from the case law whether one could evaluate it in terms of negligently failing to take precautions against sleep walking v. failing to act as a reasonable person while sleep walking, in a case like this one. There simply aren't many cases like this that come up. There is also reason to doubt whether this kind of sleep walking really happens and if so what it involves.

Person has trauma and cannot always control their reactions when it is triggered; they run out of a shop when something triggers it and knock over numerous people and displayed products/shop fittings.

Lack of self-control would almost never be a defense to any kind of civil action. The person knows what they are doing and engaged in an action which is controlled by the brain and not merely an instinctive muscle contraction.

Person has cancer in part of their brain that controls self regulation. Nothing happens until one day they attack somebody, an act for which they have no prior history, and agreed to be due to their medical condition. Ditto but the issue is a congenital or regenerative neurological condition, and their family do not knew or cannot react in time to prevent an actionable incident.

Again, this is almost surely not a defense since it goes to self-control rather than knowing what they are doing.

Person has ADHD (ADD) or OCD, and is functional in society but medically it is agreed they will never have good or even usual control of attention wandering or forgetfulness or compulsive behaviours, there simply is no way to gain this. Damage results.

A person with these conditions is still held to the standard of a reasonable person for negligence purposes, and still is responsible for their knowing actions. This is not a defense since it goes to self-control rather than knowledge that one is taking an action.

A reluctant parent with a bad parent-child relationship hears a rumour on the internet that mental health is no defence against civil damage claims, and decides to sue their offspring for massive damage caused to their house on the night of their 18th birthday. Because of the stress they had also not recognised their parents who arrived, and believing themselves attacked by strangers, slashed at them with a knife in panic, causing life-changing injury. Their condition had never remotely been like this before and there was no realistic way to anticipate it might happen, nor any incident previously requiring precautions. Conveniently on that day the youngster also inherited the kind of sum that would be needed to pay any damages if the claim were successful.

The damage to the house is almost surely their responsibility legally with no defense. This is a question of negligence or negligent entrustment and failing to maintain order at a party you throw is something you are responsible for, in general.

There could be a defense if a genuine hallucination that sincerely caused someone to believe that they were acting in self-defense to an attack, when, in fact, they were not. Usually, a self-defense defense to an action for civil liability is allowed even if battery would otherwise be a strict liability offense.

(Realistically, hallucinations like this would not be caused by mere stress, only by schizophrenia, but a first really definitive episode of schizophrenia at age 18 wouldn't be unusual, that is a typical age of first onset.)

If self-defense is allowed as to a battery claim, it is still conceivably possible that a negligence action would be allowed in the circumstances because a reasonable person would have known that they were not being attacked even though this person did not, but that is unlikely. Normally, good faith belief in self-defense in sufficient.