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Legal Mental Capacity in California Probate Law

When someone’s ability to make decisions is called into question in estate planning, contracts, marriage, or medical care, California law provides detailed rules to assess their mental capacity. Part 17 of the California Probate Code lays out a clear framework for determining when a person legally has (or lacks) the capacity to act.

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810: Presumption of Capacity & the Role of Mental Deficits

Section 810 establishes the foundational presumption that every person has the capacity to make decisions and take responsibility for those decisions—unless proven otherwise. It emphasizes that a diagnosis of a mental or physical disorder alone is not enough to assume someone lacks legal capacity.

Key principles include:

• Presumption of capacity: Courts start with the assumption that individuals are capable unless there’s convincing evidence showing otherwise.
• Disorders ≠ incapacity: Having a mental illness or cognitive condition doesn’t automatically strip someone of their decision-making power.
• Focus on function, not labels: A person may only be found legally incapable based on specific functional impairments that affect their ability to understand or act appropriately—not just based on a diagnosis like dementia or schizophrenia.

811: Functional Criteria for Determining Incapacity

This section provides a detailed checklist of the mental functions that courts must evaluate when determining incapacity. A finding of incapacity must be backed by evidence of a deficit in at least one of these areas, plus a clear link between that deficit and the person’s inability to carry out the specific act in question.

The four mental function categories are:

1. Alertness & attention – Is the person aware of who they are, where they are, and what’s going on around them?
2. Information processing – Can they remember things, understand conversations, recognize people, and reason through problems?
3. Thought processes – Are their thoughts organized? Do they experience hallucinations, delusions, or repetitive/intrusive thoughts?
4. Mood and affect control – Are emotional reactions appropriate, or are they persistently exaggerated or detached from the situation?

Importantly, deficits must significantly impair decision-making. A minor memory issue doesn’t mean a person can’t understand the impact of signing a will or entering a contract.

812: Capacity to Make a Decision

This section defines what it means to have capacity to make a particular decision—outside of medical consent. The standard applies to most types of legal actions, including contracts, trusts, and property decisions.

To have capacity, a person must be able to:

• Communicate their decision in some form (not necessarily verbally).
• Understand and appreciate:
  • The legal consequences of their decision,
  • How it will affect themselves and others,
  • The major risks, benefits, and alternatives.

This is a functional, act-specific test. For example, someone may lack capacity to create a complex trust but still be able to make simple financial choices.

813: Capacity to Consent to Medical Treatment

Medical decisions require their own standard of capacity, detailed in Section 813. Courts ask whether the person can understand what their doctor is telling them, weigh the options, and respond with a rational choice.

A person has medical decision-making capacity if they can:

1. Knowingly respond to questions about the treatment.
2. Think rationally about the decision.
3. Understand:
   • Their medical condition,
   • The nature and risks of the recommended treatment,
   • Possible consequences of not getting treated,
   • Any alternative treatments.

And importantly, a person who has capacity to consent to a treatment also has the right to refuse it—even if that decision seems unwise to others.

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California’s laws on legal mental capacity don’t aim to disqualify people with mental health or cognitive challenges—they’re designed to protect individuals while respecting autonomy. By grounding capacity in specific mental functions and clear evidence, Sections 810–813 provide a thoughtful and balanced framework for answering one of the most delicate legal questions: “Can this person understand and decide for themselves?”

Whether you’re creating an estate plan, helping a loved one manage care, or navigating a legal dispute, understanding how the law defines capacity is essential to protecting rights and making sound decisions.