Define: Capacity Defence

Capacity Defence
Capacity Defence
Quick Summary of Capacity Defence

A capacity defence is a defence strategy employed by a defendant in a legal case based on their lack of accountability for an illegal act or the plaintiff’s inability to pursue a lawsuit. For instance, if a corporation loses its corporate charter, it may lack the capacity to file a lawsuit. Similarly, if the defendant is a minor, they may lack the capacity to be held responsible for certain actions. It is important to note that a capacity defence differs from an affirmative defence, which involves the defendant presenting facts and arguments that, if proven true, would undermine the plaintiff’s or prosecution’s claim, even if all the allegations in the complaint are accurate. Examples of affirmative defences include duress (in civil cases) and insanity and self-defence (in criminal cases). For example, if a defendant is accused of assault but suffers from a mental illness that impairs their understanding of the consequences of their actions, they may employ an affirmative defence of insanity.

What is the dictionary definition of Capacity Defence?
Dictionary Definition of Capacity Defence

Capacity defence is a defence strategy employed by a defendant in a legal case that relies on the defendant’s lack of accountability for an unlawful act or the plaintiff’s inability to pursue a lawsuit. For instance, if the plaintiff is a corporation that has forfeited its corporate charter, it may lack the ability to initiate legal proceedings. Similarly, if the defendant lacks the mental capacity to comprehend their actions, they cannot be held liable for them. Capacity defence is just one of several defence tactics that can be utilised in a legal case.

Full Definition Of Capacity Defence

The concept of “capacity” in legal terms refers to an individual’s ability to understand and participate in legal proceedings or transactions. Capacity defences are crucial in various areas of law, including contract law, criminal law, and medical law. This overview will delve into the legal principles surrounding capacity defences, exploring their applications and implications in different contexts within the British legal system.

Capacity in Contract Law

In contract law, the capacity to contract is a fundamental requirement. A contract is only enforceable if the parties involved possess the legal capacity to enter into it. Under British law, certain groups are deemed to lack full contractual capacity, including minors, individuals with mental impairments, and intoxicated persons.


Under the Family Law Reform Act 1969, the age of majority in the UK is 18. Individuals under this age are generally considered minors and lack the full capacity to contract. However, there are exceptions:

  • Contracts for necessaries: Minors can enter into contracts for essential goods and services (e.g., food, clothing, education).
  • Beneficial contracts of service: Contracts that are deemed beneficial to the minor, such as apprenticeship or employment agreements, are also enforceable.

Mental Incapacity

A person with a mental disorder may lack the capacity to contract if they cannot understand the nature and consequences of the transaction. The Mental Capacity Act 2005 provides a framework for assessing mental capacity. According to the Act, a person lacks capacity if, at the time of making the decision, they are unable to understand, retain, use or weigh relevant information, or communicate their decision.


Contracts entered into while one party is intoxicated may be voidable if the intoxicated person is unable to understand the nature and consequences of the contract and the other party is aware of their intoxication. The intoxicated party must act promptly to repudiate the contract upon becoming sober.

Capacity in Criminal Law

In criminal law, capacity defences are essential in determining whether an individual can be held responsible for their actions. Key capacity defences include insanity, automatism, and intoxication.


The insanity defence is rooted in the M’Naghten Rules (1843), which state that a defendant may be considered legally insane if, at the time of the offence, they were suffering from a “defect of reason” due to a “disease of the mind” and either did not understand the nature of their act or did not know that it was wrong.

  • Defect of reason: This implies a cognitive impairment that affects the individual’s ability to reason.
  • Disease of the mind: This can include both mental illnesses and physical conditions that affect the brain, such as epilepsy or diabetes.

If successful, the insanity defence can lead to a special verdict of “not guilty by reason of insanity,” resulting in the defendant being subject to hospital orders, supervision orders, or absolute discharge under the Criminal Procedure (Insanity) Act 1964.


Automatism refers to involuntary actions performed without conscious control. For the defence of automatism to succeed, the involuntary act must be caused by an external factor, such as a blow to the head or the effects of medication. The distinction between sane and insane automatism is crucial:

  • Sane automatism results from external factors and can lead to complete acquittal.
  • Insane automatism results from an internal factor (a disease of the mind) and leads to the same consequences as the insanity defence.


Intoxication can be a defence to criminal charges if it negates the necessary mens rea (guilty mind) for the offence. The application of this defence depends on whether the intoxication was voluntary or involuntary:

  • Voluntary intoxication: Generally, it is not a defence to crimes of basic intent (where recklessness suffices as mens rea) but may negate specific intent required for certain crimes (e.g., murder to manslaughter).
  • Involuntary intoxication: Can be a defence to both specific and basic intent crimes if it renders the defendant incapable of forming the necessary mens rea.

Capacity in Medical Law

Medical law also incorporates the concept of capacity, particularly concerning consent to treatment. The Mental Capacity Act 2005 outlines the criteria for assessing capacity and provides guidelines for making decisions on behalf of those who lack capacity.

Assessing Capacity

Under the Mental Capacity Act 2005, a person is presumed to have capacity unless proven otherwise. The assessment involves determining whether the individual can:

  • Understand the information relevant to the decision.
  • Retain that information long enough to make the decision.
  • Use or weigh that information as part of the decision-making process.
  • Communicate their decision.

Best Interests

When a person is found to lack capacity, any decision made on their behalf must be in their best interests. The Act provides a checklist for determining best interests, including considering the person’s past and present wishes, feelings, beliefs, and values, and consulting with family members or carers.

Advance Decisions

The Mental Capacity Act 2005 allows individuals to make advance decisions to refuse specific medical treatments in the future, should they lack capacity at that time. These decisions are legally binding if they meet certain criteria, providing a means for individuals to exercise autonomy over their medical care.

Capacity in Family Law

In family law, capacity issues often arise in relation to marriage, divorce, and child custody.


To enter into a valid marriage, both parties must have the capacity to consent. This includes understanding the nature of the marriage contract and the responsibilities it entails. A marriage may be declared void if either party lacked the mental capacity to consent at the time of the marriage.


Capacity issues can also impact divorce proceedings. If one party lacks capacity, a litigation friend (a person appointed to conduct legal proceedings on behalf of someone who lacks capacity) may be required to represent them. The court must ensure that the interests of the incapacitated party are adequately protected throughout the process.

Child Custody

In child custody cases, the capacity of a parent to care for a child may be scrutinised. Courts consider the best interests of the child, which may involve assessing the parent’s mental and physical capacity to provide proper care. If a parent is deemed to lack capacity, alternative arrangements, such as appointing a guardian, may be necessary.

Capacity in Civil Litigation

Capacity to litigate is a crucial aspect of civil litigation. Under the Civil Procedure Rules (CPR), a party to legal proceedings must have the capacity to conduct the litigation. If a party lacks capacity, a litigation friend must be appointed to represent their interests.

Appointment of a Litigation Friend

A litigation friend can be a family member, friend, or professional, such as a solicitor. Their role is to make decisions in the best interests of the person lacking capacity. The court must approve the appointment, ensuring that the litigation friend can fairly and competently conduct proceedings on behalf of the incapacitated party.

Mental Health and Litigation Capacity

The presence of a mental disorder does not automatically imply a lack of capacity to litigate. Capacity is decision-specific, meaning an individual may lack capacity for some decisions but not others. A comprehensive assessment by a medical professional is often required to determine litigation capacity.


The concept of capacity defences is crucial in different areas of British law. It aims to protect individuals who may not have the ability to understand or participate in legal processes. These principles are essential in contract law, criminal law, medical law, family law, and civil litigation. They are meant to balance the autonomy of individuals with the need for protection and fairness in legal proceedings. Understanding these principles is vital for legal practitioners as they navigate complex capacity issues to uphold the rights and interests of all parties involved.

Capacity Defence FAQ'S

Capacity defence refers to a legal argument used to challenge a person’s ability to understand and make informed decisions regarding their actions or contracts due to mental or cognitive impairments.

Capacity defence can be used in various legal situations, such as contract disputes, criminal cases, and estate planning matters, where the individual’s mental capacity is in question.

Mental capacity is typically determined by assessing an individual’s ability to understand the nature and consequences of their actions, comprehend relevant information, and make rational decisions based on that information.

The burden of proof usually lies with the party asserting the capacity defence. They must provide sufficient evidence to demonstrate that the individual lacked the mental capacity required for the specific legal matter.

Yes, if it can be proven that one party lacked the mental capacity to understand the terms and consequences of the contract at the time of its formation, the contract may be deemed void or voidable.

Yes, the capacity defence can be used in criminal cases to argue that the accused lacked the mental capacity to understand the nature and consequences of their actions, thus negating the intent required for certain criminal offences.

If someone is found to lack mental capacity, the court may appoint a guardian or conservator to make decisions on their behalf, ensuring their best interests are protected.

Yes, capacity defence can be used to challenge the validity of a will if it can be proven that the testator lacked the mental capacity to understand the nature and extent of their assets or the individuals they intended to include or exclude from their will.

In some cases, if it can be proven that the accused lacked the mental capacity required to form the necessary intent for a specific crime, it may result in a reduced charge or a finding of not guilty by reason of insanity.

Yes, capacity defence can be used in cases involving minors to argue that they lacked the mental capacity to understand the consequences of their actions, potentially leading to reduced liability or different legal consequences.

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This site contains general legal information but does not constitute professional legal advice for your particular situation. Persuing this glossary does not create an attorney-client or legal adviser relationship. If you have specific questions, please consult a qualified attorney licensed in your jurisdiction.

This glossary post was last updated: 8th June 2024.

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