What cannot be in a postnuptial agreement comes down to a handful of categories courts will not touch: anything that decides child custody or support, anything that encourages divorce or requires illegal conduct, waivers of federal retirement rights that skip the ERISA process, attempts to override IRS rules, clauses regulating personal behavior, and terms so one-sided they qualify as unconscionable. A judge will strike these provisions no matter how carefully they are drafted or how willingly both spouses signed.
The rest of this article walks through each category, explains why courts refuse to enforce it, and ends with what happens to the rest of your agreement when a prohibited clause slips in.
Child Custody, Visitation, and Child Support
You cannot lock in custody arrangements, visitation schedules, or child support amounts through a postnuptial agreement. Courts treat decisions about children as their own responsibility, governed by the child’s best interests at the time those decisions actually need to be made. An arrangement that looked sensible when the agreement was signed may be badly wrong for a child’s situation years later, and judges refuse to be bound by what two parents negotiated in advance.
Child support gets even stricter treatment because it is considered the child’s right, not a bargaining chip between spouses. Even when both parents agree to waive or reduce support, a court will override that provision whenever the children’s needs aren’t being met. Parents can make financial deals with each other; they cannot contract away obligations owed to their children. Any clause attempting to cap, eliminate, or predetermine child support is almost certain to be disregarded.
Provisions That Encourage Divorce or Require Illegal Acts
Clauses creating a financial incentive to end the marriage are unenforceable as a matter of public policy. A provision that triggers a large payment only when one spouse files for divorce, or penalizes a spouse for staying married, will not survive judicial review. Courts view these terms as undermining the institution of marriage itself.
Any provision requiring or promoting illegal conduct is equally void. A clause directing a spouse to underreport income on tax returns, conceal assets from creditors, or engage in any other unlawful act cannot be included. Beyond refusing to enforce the clause, courts may treat its presence as a reason to question the credibility of the whole agreement.
Clauses Controlling Personal Behavior
Courts consistently refuse to enforce provisions that try to regulate non-financial, personal aspects of a spouse’s life. Weight requirements, appearance standards, religious practices, household chore assignments — these are treated as unenforceable invasions of personal autonomy. Even when both spouses agreed willingly, judges have no practical way to monitor compliance and no interest in getting the legal system involved at that level of marital detail.
Infidelity Penalty Clauses
Infidelity clauses that impose a financial penalty for cheating sit in a gray area, and their enforceability depends heavily on where you live. Courts in California, Iowa, and Hawaii have struck them down on public policy grounds, reasoning that enforcing them would reintroduce fault into a no-fault divorce system and encourage the kind of adversarial evidence-gathering that no-fault was meant to prevent.
Maryland has gone the other way, upholding infidelity clauses and enforcing significant financial penalties triggered by adultery. The split means you cannot assume an infidelity clause will hold up without checking your state’s position. Even where allowed, these clauses create practical headaches: defining what counts as infidelity, proving it, and litigating the factual disputes that inevitably follow.
Waivers of ERISA Retirement Benefits
This is one of the most commonly misunderstood limitations, and one of the most expensive to get wrong. A postnuptial agreement by itself is generally not enough to waive a spouse’s rights to retirement benefits governed by the Employee Retirement Income Security Act. Under federal law, a spouse has a right to survivor benefits from qualified plans like 401(k)s and pensions, and waiving those rights requires a specific process that goes beyond what a postnup provides.
For a waiver of survivor benefits to be valid, federal law requires that the spouse consent in writing, that the waiver name an alternate beneficiary or payment form that cannot be changed without the spouse’s further consent, and that the signature be witnessed by a plan representative or notary public. The waiver must also be submitted to the plan within a specific election window.1Office of the Law Revision Counsel. United States Code Title 29 – Section 1055 A general statement in a postnuptial agreement waiving “all retirement benefits” will not satisfy these requirements.
When a divorce occurs and retirement assets need to be divided, the proper tool is a Qualified Domestic Relations Order. A QDRO is a court order recognized by the retirement plan that directs the plan administrator to pay a portion of benefits to the other spouse. It can assign some or all of a participant’s retirement benefits to a former spouse, but it cannot require the plan to pay more than the plan provides or offer a benefit type the plan doesn’t otherwise allow.2U.S. Department of Labor. QDROs: The Division of Retirement Benefits Through Qualified Domestic Relations Orders Including a retirement waiver in your postnup without following the separate ERISA process can leave that waiver worth nothing when it actually matters.
Tax Provisions That Conflict With Federal Law
A postnuptial agreement can address how spouses divide property, but it cannot override federal tax rules. Transfers between spouses during marriage are generally tax-free under the Internal Revenue Code, and the receiving spouse takes the transferring spouse’s original tax basis.3GovInfo. United States Code Title 26 – Section 1041 A private agreement cannot change that treatment.
Trouble arises when postnups try to allocate tax obligations in ways that conflict with IRS rules. A clause assigning all tax liability to one spouse does not bind the IRS. If you filed jointly, both spouses remain jointly and severally liable for the full tax bill regardless of what the agreement says. The agreement might give one spouse a breach-of-contract claim against the other, but it will not stop the IRS from collecting from whichever spouse it can reach.
A postnup also cannot reclassify the tax character of income or deductions. You cannot agree that one spouse’s business income will be treated as the other spouse’s for tax purposes, or that a non-deductible payment will somehow become deductible because the contract says so. The IRS follows its own rules.
Unconscionable or Grossly Unfair Terms
Even when a postnup covers permissible subjects, it can still be struck down if the terms are unconscionable. Courts look at this from two angles, and a serious problem on either side can sink the agreement. Postnuptial agreements face sharper scrutiny than prenuptial ones because spouses already owe each other fiduciary-like duties during marriage, so the fairness bar is higher.
Procedural Unconscionability
Procedural unconscionability focuses on how the agreement came together. The most common red flag is incomplete financial disclosure. Both spouses must give a full and honest accounting of their income, assets, debts, and expected inheritances before signing. When one spouse hides a brokerage account or understates business income, the other cannot meaningfully consent, and courts treat that concealment as grounds to void the agreement.
Other procedural problems include signing under duress or coercion, fraud about what the agreement contains, and severe imbalance in bargaining power. If one spouse had an attorney and the other didn’t, or the document was presented on a take-it-or-leave-it basis with no time to review, those facts weigh heavily toward a finding that the process was unfair. The absence of independent legal counsel for both parties is one of the fastest ways to undermine enforceability.
Substantive Unconscionability
Substantive unconscionability looks at the terms themselves. An agreement that gives one spouse every major asset while leaving the other destitute is the textbook example. Courts do not require perfect equality, but they draw the line at terms so one-sided that no reasonable person with a choice would have agreed to them.
Fundamental Rights You Cannot Waive
Certain rights cannot be signed away regardless of how the agreement is drafted. Neither spouse can waive the right to file for divorce, and any clause requiring a spouse to stay married or penalizing a divorce filing is unenforceable. No clause can waive the right to legal representation; every person keeps the right to hire an attorney regardless of what a contract says.
Government benefits that are non-waivable by law also fall outside the scope of a postnup. A spouse cannot agree to give up eligibility for programs like Medicaid or Social Security through a private contract. These are statutory entitlements that exist independently of any agreement between the parties.
What Happens When a Prohibited Clause Slips In
Including a forbidden provision does not necessarily destroy the entire agreement. Most well-drafted postnups include a severability clause, which tells the court to strike the offending provision and enforce the rest. If an agreement waives property rights, waives spousal support, and tries to decide custody, a court would likely throw out the custody clause and leave the property and support terms intact, assuming those terms are otherwise fair.
Without a severability clause, the risk grows that one bad provision takes the whole agreement down. Even with one, a court has discretion to reject the entire document if the unenforceable clause was so central that removing it fundamentally changes what the parties agreed to. The safest approach is to keep prohibited topics out of the agreement in the first place, rather than counting on severability to save what’s left.