Three groups of people generally cannot legally enter into a contract: minors under 18, people who lack the mental capacity to understand what they are agreeing to, and people who are severely intoxicated at the moment of signing. In most cases the resulting agreement is not simply thrown out; it is “voidable,” meaning the protected person can walk away but the other side stays bound unless the protected person cancels. A related situation, where someone has capacity but was coerced into signing, produces a similar result.
Minors Under 18
In most states, anyone under 18 is a minor for contract purposes, and a contract with a minor is voidable at the minor’s choice. The minor can cancel and owe nothing, even after receiving the goods or services. The adult on the other side has no matching exit; they stay bound unless the minor pulls the plug. The law builds in this one-sided arrangement because it assumes minors lack the judgment and experience to fully appreciate what they are agreeing to.
Necessaries Are the Exception
Minors do face contract liability for “necessaries”: food, clothing, shelter, medical care, and in some jurisdictions education. Without this exception, few businesses would sell to minors, and minors living independently would struggle to meet basic needs.
Even here, the minor does not owe the full contract price. Courts limit liability to the reasonable value of what was actually received. A minor who agrees to pay $2,000 a month for an apartment worth $1,200 in the local market owes something closer to $1,200.
Ratification After Turning 18
Once a minor turns 18, they can “ratify” a contract made during their minority and make it fully binding going forward. Ratification can be explicit, like signing a confirmation, or implicit through conduct like continuing to make payments or keep using the item. In most jurisdictions, doing nothing within a reasonable time after turning 18 also counts as ratification. If you turned 18 three months ago and have kept driving the car you bought at 17 without a word, a court may treat the contract as ratified.
What a Minor Owes Back on Cancellation
When a minor cancels, the general rule is to return whatever they still have. Buy a laptop at 16, cancel at 17, hand back the laptop, get your money back. The harder question is what happens when the item has been used, damaged, or is gone. Many jurisdictions follow a “benefit rule” that lets the minor return the item in its current condition without paying for depreciation or wear. A few states take a harder line and reduce the minor’s refund by the value of the benefit received. The rules are genuinely inconsistent across states, so the specific jurisdiction matters.
Emancipated Minors
A minor who has been legally emancipated by court order is generally treated as an adult for contract purposes. They can enter binding agreements without parental consent and cannot later void them simply because of age. Courts may still scrutinize complex financial arrangements or unusual employment terms for fairness. Emancipation removes the shield of minority from contract law; it does not grant every adult right. An emancipated 16-year-old can sign a lease but still cannot buy alcohol.
People Who Lack Mental Capacity
A person who cannot understand what a contract means or what it requires of them lacks the mental capacity to be bound. Courts apply two tests. The traditional cognitive test asks whether the person could understand the nature and consequences of the transaction. The newer motivational test asks whether, even with some understanding, the person was unable to act reasonably in relation to the transaction; under this test, the other party must have had reason to know about the mental condition.
The distinction matters. Someone with a severe psychiatric condition might technically understand that they are signing a car loan, satisfying the cognitive test, but might be in a manic episode that leaves them incapable of exercising reasonable judgment about whether the loan is a good idea. The motivational test catches situations the older test misses.
With a Guardian vs. Without One
The legal effect depends heavily on whether a court has already declared the person incompetent and appointed a guardian. If a guardian is in place, any contract the ward signs on their own is generally void from the start. It has no legal effect, and no one has to take any action to cancel it.
If no court has weighed in, but the person lacked capacity when they signed, the contract is only voidable. The incapacitated person or their representative can cancel it, but until they do, it remains in effect.
There is a fairness limit. When the contract was made on fair terms and the other party had no way to know about the mental illness, a court may decline to void it entirely, particularly if it has already been performed in whole or in part. The court can then craft a remedy that is fair to both sides instead of simply unwinding the deal.
Diagnosis Alone Does Not Decide It
Mental incapacity is not determined by diagnosis. A person with schizophrenia may have full capacity on most days. A person with early-stage dementia may understand simple transactions but not complex ones. What matters is the person’s cognitive state at the specific moment the contract was formed. Witness testimony about behavior at the time, medical records from around that date, and the complexity of the transaction relative to the person’s abilities carry far more weight than a general medical history.
People Who Are Severely Intoxicated
Severe intoxication from alcohol or drugs can destroy a person’s capacity to contract, but the bar is high. A few drinks before signing a lease will not make the lease voidable. The impairment must be severe enough that the person could not understand what they were agreeing to or could not act reasonably in relation to the transaction. From a contract standpoint, someone that impaired is in the same position as someone whose mental illness prevents comprehension.1Legal Information Institute. Johnson v. Harmon
A critical extra requirement separates intoxication from other capacity defenses: the other party must have known, or had reason to know, about the severe impairment. If the person appeared sober enough and the terms were fair, most courts will enforce the agreement even if the signer was privately more impaired than they let on.
Intoxication is almost always temporary, which creates a tight window to act. Once the person sobers up, they must promptly disaffirm the contract or risk having it treated as ratified. Continuing to accept benefits after regaining sobriety, like making a payment on a loan signed while drunk, is particularly damaging to any later claim of incapacity. Courts are not sympathetic to people who sober up, realize they made a bad deal, and wait weeks to complain.
Duress and Undue Influence
Duress and undue influence are not strictly capacity problems. The person understood the contract; they simply had no real choice about signing. The practical result is often the same, which is why coercion belongs alongside the capacity rules.
Physical duress, where someone is literally forced to sign under threat of violence, makes a contract void outright. The signer was treated as nothing more than a “mechanical instrument,” and the law acts as if the agreement never existed. Duress by threat is more common and more nuanced. Someone signs because they believe they have no reasonable alternative, perhaps because the other party is threatening to destroy their business or reveal damaging information. Contracts signed under this kind of pressure are voidable at the victim’s option.
Undue influence involves a relationship where one person holds significant power or trust over another, such as a caregiver over an elderly person or an attorney over a client, and uses that position to push the weaker party into an agreement that benefits the stronger one. These contracts are also voidable. Courts look at the vulnerability of the influenced party, the nature of the relationship, and whether the stronger party cut the weaker one off from independent advice.
Who Can Sign for Someone Who Lacks Capacity
Lacking capacity does not mean no contract can ever be made on the person’s behalf. Two legal tools fill the gap.
Guardians and Conservators
When a court declares someone incompetent, it typically appoints a guardian for personal decisions and sometimes a separate conservator for financial matters, though terminology varies by state.2Elder Justice Initiative. Guardianship: Key Concepts and Resources A conservator or guardian with financial authority can enter contracts on behalf of their ward, but the power is not unlimited. Most jurisdictions require court approval for major transactions like selling real estate, taking on significant debt, or settling legal claims. The role is fiduciary: the guardian must put the ward’s interests ahead of their own and cannot use the position for personal benefit.
Durable Power of Attorney
A power of attorney lets a person (the “principal”) appoint an agent to handle legal and financial matters on their behalf, including signing contracts. The crucial word for incapacity planning is “durable.” A standard power of attorney expires the moment the principal loses mental capacity, which is exactly when it would be most needed. A durable power of attorney includes language keeping the agent’s authority alive after the principal’s incapacity, so it still works if the principal later develops dementia or suffers a cognitive decline.
The catch is timing. The principal must have capacity at the moment they sign. Someone who already lacks capacity cannot create one. A person in the early stages of a degenerative illness may still validly execute the document during a lucid period, as long as they understand what authority they are granting and to whom.
Void vs. Voidable and Why It Matters
Two terms run through all of this and sound alike but mean very different things. A “void” contract never had legal force. Neither side can enforce it, and no one has to do anything to cancel it. A “voidable” contract is valid and enforceable unless the protected party, or their representative, affirmatively chooses to cancel it. Until that happens, the contract stands.
The practical gap is wide. If a contract is only voidable, the other side can enforce it for as long as the protected party stays silent. For a family dealing with a relative who signed a bad contract during a period of mental decline but was never formally declared incompetent, the label can mean the difference between an easy rescission and a difficult court fight. It is one of the strongest reasons to pursue formal guardianship when someone’s capacity is deteriorating, because the protection shifts from voidable to void.