Also known as: mental capacities · mental competence · mental competency
Written by attorneys · grounded in primary & secondary sources — see below
The cognitive ability required to understand the nature and consequences of a legal act or transaction.
Michael Miller, recovering from surgery and on strong pain medication, attempted to assign his entire investment portfolio to a speculative fund. Family members later challenged the transfer. The assignment is invalid because Michael lacked the mental capacity to make the donative transfer at the time of execution.
Meredith Maxwell consumed hard cider and marijuana throughout the afternoon before participating in a marriage ceremony. She required repeated prompting to recite her lines and laughed inappropriately during the vows. The marriage is invalid because Meredith lacked mental capacity to understand and agree to the marriage at the time of the ceremony.
Select any source to read its text and confirm it supports the definition.
Miles Montgomery executed an irrevocable deed transferring his primary income-producing building while living in assisted living and relying on rental income. He occasionally confused family members and tenants during the transaction. The deed is invalid because Miles lacked the mental capacity to understand the gift's effect on his future financial security and that of his dependents.
Mason McCarthy, adjudicated incompetent with the mental capacity of an 18-month-old, executed a deed conveying real property while under guardianship. The guardian later sought to set aside the deed. The conveyance is void because Mason lacked the mental capacity to understand the nature and consequences of the transaction at the time of execution.
Megan Moore, while experiencing an acute psychotic episode, signed a written confession during police interrogation. Defense counsel later moved to suppress the statement. The confession is inadmissible because Megan lacked the mental capacity to understand the nature and consequences of the waiver at the time of signing.
Matthew Martinez, operating a group home, signed a long-term commercial lease while suffering from documented cognitive impairment. The landlord later sought enforcement. The lease is voidable because Matthew lacked the mental capacity to understand the nature and consequences of the contractual obligation at the time of execution.
A person lacks capacity to contract when mental illness or intellectual disability prevents reasonable understanding of the transaction's nature and consequences. The resulting contract is voidable at the option of the impaired party if the other side had reason to know of the condition. Courts assess capacity at the time of contracting and focus on functional ability rather than formal adjudication alone.
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Capacity is determined at the precise moment of the marriage ceremony. Intoxication from alcohol or drugs that prevents understanding the actions and agreement renders the marriage invalid. Visible signs of impairment such as disorientation or inability to follow instructions establish lack of capacity.
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The donor must possess the mental capacity necessary to make or revoke a will and must also understand the gift's effect on the donor's future financial security and that of any dependents. Medication, recent surgery, or reliance on the transferred assets for support can demonstrate failure to meet this standard.
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Yes. Once a court adjudicates a person incompetent and places property under guardianship, the ward lacks capacity to incur contractual duties regarding that property. Any later agreement is void rather than merely voidable.
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…in a special class is not presumptively irrational. The differences between mentally retarded persons and those with greater mental capacity are obviously relevant to certain legislative decisions. An impartial lawmaker — indeed, even a member of a class of persons defined as mentally retarded — could rationally vote in favor of…