Legal capacity to contract is the legal ability to understand an agreement and be bound by it, and in the United States any person 18 or older with a sound mind generally has it. Four groups face limits: minors, adults whose cognitive function is impaired, people who are severely intoxicated at signing, and anyone under a court-ordered guardianship. For companies, the parallel question is whether the individual who signed had authority to bind the organization. When capacity is missing, the contract is either voidable at the protected party’s option or void from the start.
Minors and the Right to Walk Away
In most states, 18 is the dividing line. A contract signed by someone younger is voidable at the minor’s option: the minor can cancel, the adult on the other side cannot. A 16-year-old who joins a gym or finances a car can disaffirm and leave. The business has no equivalent exit.
When a minor cancels, they generally return whatever they still have. A 17-year-old who bought a laptop and still has it hands it back. If they’ve used it up, broken it, or lost it, most courts don’t require them to pay the seller for the lost value. The rule tilts hard in the minor’s favor, sometimes uncomfortably so for adults who dealt in good faith.
The major exception is necessaries: food, shelter, clothing, medical care, and similar essentials. When a minor contracts for these, courts hold them liable for the reasonable value of what they received, which may not match the contract price. A landlord who rents to a 17-year-old living independently can recover fair-market rent even if the minor tries to disaffirm. Without this carve-out, merchants and providers would simply refuse to deal with young people lacking parental support.
What Happens at 18
The right to disaffirm doesn’t last forever. Once a former minor turns 18, the clock starts. Continuing to use the goods or services without taking steps to cancel is treated as implied ratification. Keep driving the financed car, keep paying the dues, keep living in the apartment, and the contract becomes fully binding. Ratification can also be express, through a statement that the former minor intends to honor the deal. Either way, once it happens, it’s permanent. Doing nothing after an 18th birthday can lock a person into a contract they could have escaped with a simple written disaffirmance.
Emancipated Minors
Court-ordered emancipation generally grants most adult legal rights, including the ability to sign binding contracts. It isn’t universal. Some states keep restrictions on certain agreements, particularly labor contracts, and emancipation can be revoked if the minor violates its conditions. The specific rights depend on the state where emancipation was granted.
Mental Incapacity
For adults, the question isn’t age but cognitive ability at the moment of signing. Courts generally apply two tests from the Restatement (Second) of Contracts § 15. The cognitive test asks whether the person could understand the nature and consequences of the transaction. The affective test asks whether, even if they technically grasped the deal, they were unable to act reasonably about it, and whether the other party had reason to know something was wrong.
Timing matters more than the diagnosis. Someone with early-stage dementia might have full capacity on a Tuesday and lack it on a Thursday. A person in a severe manic episode might sign contracts they’d never consider when stable. Courts evaluate the snapshot in time when the signature went on the page, not the person’s overall medical history.
The burden sits on the person claiming incapacity. Medical records, treating-physician testimony, and forensic psychological evaluations all come into play. If the challenge succeeds, the contract is voidable, not automatically void: the incapacitated person or their legal representative chooses whether to rescind.
Lucid Intervals
A person who generally lacks capacity may still create an enforceable agreement during a period of clarity. If genuine understanding existed at the moment of signing, courts may uphold the deal. This respects the autonomy of people whose capacity fluctuates, and it also means a diagnosis alone won’t automatically void every contract the person signed.
Returning What You Received
Voiding a contract for mental incapacity doesn’t mean keeping everything you got. Courts routinely require the person who lacked capacity to return the benefits, or pay their reasonable value, before the deal is set aside. This keeps the capacity defense from becoming a tool for unjust enrichment.
The counterparty’s good faith matters. When the contract was made on fair terms and the other side had no reason to suspect impairment, some courts refuse rescission entirely if unwinding the deal would be unjust, particularly where the contract has already been substantially performed. Even when rescission is granted, the incapacitated person remains liable for the reasonable value of necessaries like housing, food, and medical care.
Intoxication
Alcohol, prescription medications, and other substances can theoretically invalidate a contract, but this is one of the hardest capacity defenses to win. Under the Restatement (Second) of Contracts § 16 framework, two things must be true: the person was so impaired they couldn’t understand the nature and consequences of the transaction, and the other party had reason to know about that level of impairment.
Being tipsy doesn’t cut it. Being drunk doesn’t cut it. You essentially need to have been incapable of understanding what you were agreeing to, and the sober party needs to have seen, or should have seen, that something was seriously wrong. If the other side genuinely didn’t notice and the terms were fair, most courts will enforce the deal.
Even a successful intoxication defense carries conditions. You’re expected to return what you received and to disaffirm promptly once sober. Waiting weeks or months undercuts the claim that you didn’t know what you were doing. And the evidentiary hurdle is steep. Proving an exact mental state at a past moment, without contemporaneous blood tests or medical records, is uphill, and most claimants lose.
Court-Ordered Guardianship
When a court formally declares someone incapacitated and appoints a guardian, the consequences are far more severe than in the other scenarios. A guardianship hearing involves testimony and evidence, often including professional capacity evaluations, after which a judge makes a legal finding about the person’s ability to manage their own affairs.1U.S. Department of Justice. Guardianship: Key Concepts and Resources Once the order is entered, the ward loses the right to enter contracts. Any agreement they sign is generally treated as void from the start, not merely voidable, but a legal nullity.
The guardian steps into the ward’s shoes for legal and financial decisions, and the court order specifies the scope.1U.S. Department of Justice. Guardianship: Key Concepts and Resources Third parties are effectively on notice that the ward cannot contract, so there’s no good-faith defense for someone who enters a deal with them.
Limited Guardianship
Not every guardianship strips away all rights. Courts increasingly favor limited guardianships that restrict only the specific areas where the person needs help. The ward retains capacity in any area the court’s order doesn’t address. If the order gives the guardian control over major financial transactions but says nothing about routine purchases or healthcare decisions, the ward can still handle those independently. Anyone dealing with a person under guardianship should ask to see the court order to understand exactly which rights have been removed.
When a Business Is the Party
Capacity questions aren’t limited to individuals. When a company enters a contract, the threshold issue is whether the human being who signed had authority to bind the organization. A corporation or LLC can’t physically agree to anything. Only its officers, employees, or designated agents can. If the signer lacked authority, the business may not be bound.
Corporate officers like CEOs and presidents typically have implied authority to enter contracts tied to ordinary business operations. Major transactions, such as acquiring another business, taking on substantial debt, or selling core assets, usually require board approval or a specific resolution. When an officer exceeds their authority, the contract may be unenforceable against the corporation.
Apparent authority softens this risk for outsiders. If a company puts someone in a position that would lead a reasonable person to believe they have signing authority, the company can be held to the deal even if that individual was privately told they couldn’t sign. The logic is straightforward: outsiders shouldn’t bear the cost of a company’s failure to communicate its own internal limits. A title like “Vice President of Procurement” alone can create apparent authority to sign a purchase agreement.
How someone signs matters too. If the signature clearly shows representative capacity, listing both the company name and the signer’s title, the representative typically isn’t personally liable. An ambiguous signature that doesn’t clearly show representative capacity can leave the signer personally on the hook.2Legal Information Institute. UCC 3-402 – Signature by Representative
Historically, corporations could escape contracts by arguing a deal fell outside their stated corporate purposes under the ultra vires doctrine. Modern corporate law has largely closed this door. Under the Revised Model Business Corporation Act, adopted in most states, a corporation’s actions generally cannot be challenged on the ground that it lacked power to act, with narrow exceptions for certain shareholder suits, corporate claims against officers or directors, and actions by the state attorney general.3LexisNexis. Model Business Corporation Act – Section 3.04 Ultra Vires
Delegating Your Capacity Through a Power of Attorney
A power of attorney lets you delegate your contractual capacity to an agent who can sign on your behalf. The critical distinction is between a standard power of attorney, which ends if you become incapacitated, and a durable power of attorney, which specifically survives your incapacity.
Under the Uniform Power of Attorney Act, adopted in some form in most states, powers of attorney are presumed durable unless the document explicitly says otherwise. Unless you include language terminating the agent’s authority on your incapacity, the agent retains power to act after you can no longer make decisions yourself. A non-durable power is suspended the moment the principal becomes incapacitated.4Uniform Law Commission. Uniform Power of Attorney Act – Section 110
Here’s the catch that trips up countless families. You must have capacity at the moment you sign the power of attorney. If you’ve already lost the ability to understand what you’re authorizing, the document is invalid from the start. By the time relatives realize a loved one needs someone managing their affairs, the window for executing a valid power of attorney may have closed. The only remaining option is a court-appointed guardianship, which is slower, more expensive, and more restrictive. Execute a durable power of attorney while capacity is clear, not when the need becomes urgent.
The agent’s authority extends only to what the document grants. A power covering financial matters doesn’t authorize healthcare decisions, which require a separate advance directive. Transactions beyond the granted scope don’t bind the principal, leaving the third party who relied on the agent in a difficult position.