The Rule Against Perpetuities is a property law doctrine that puts a time limit on how long a future interest in property can remain uncertain. In plain terms: if someone creates a future interest in a will or trust, that interest has to become fixed and certain within the lifetime of a person alive when the interest was created, plus 21 years. If there is any theoretical chance it could stay unresolved longer than that, the interest is void from the start.
The rule has a reputation as one of the most confusing concepts in American law. The purpose behind it is not confusing at all.
Why the Rule Exists
The rule is aimed at what lawyers call “dead hand” control, where a deceased person’s instructions dictate who owns property generations into the future. Without a limit, someone could write a trust saying “this land goes to the first of my descendants to become governor,” and the property could sit tied up for centuries while everyone waits to see if the condition is ever met. Nobody could sell it, develop it, or put it to productive use in the meantime.
The compromise built into the rule is that you can plan for the next generation or two, but not forever. At a predictable point, ownership has to become absolute so that living people can make real decisions about real property.
How the Rule Works
The rule sets a deadline for when a future interest must “vest.” An interest vests when the person who will receive the property is identified and their right to it is no longer conditional. A gift “to my daughter Sarah” is already vested, because Sarah is known and her right is unconditional. A gift “to my first grandchild who graduates from college” is not vested, because it is uncertain which grandchild, if any, will meet the condition.
Three pieces work together to measure the deadline.
The Measuring Life
The clock starts with a “life in being,” sometimes called the measuring life. This is a person who is alive, or at least conceived, when the interest is created. The measuring life does not have to be someone who will receive the property. It can be anyone identifiable at the time the will takes effect or the trust is created. The practical limit is that the measuring lives cannot be so numerous or obscure that proving when the last one died becomes unreasonable.
The 21-Year Add-On
After the last measuring life dies, the rule allows an additional 21 years for the interest to vest. The grace period was historically meant to cover a minor who was young when the measuring life died, giving them time to reach adulthood. Put the pieces together and the full test is this: the interest must vest, or fail, within the lifetime of someone alive when it was created, plus 21 years after that person’s death.
Which Interests the Rule Covers
The rule applies only to future interests that have not yet vested. The two most common types are contingent remainders and executory interests. A contingent remainder is a future right to property that depends on a condition being met. An executory interest is one that, when triggered, cuts short someone else’s ownership.
Several categories sit outside the rule. Any interest that is already vested is exempt because ownership is already certain. A grantor’s own retained interests, like the right to get property back at the end of a life estate, are exempt. Charitable trusts get a blanket exception and can last indefinitely because they serve a public purpose.
What Happens When an Interest Violates the Rule
If a future interest fails the test, it is treated as void from the start. The Latin term is void ab initio: the law acts as if the offending clause was never written. The rest of the will or trust stays valid, and only the specific provision that violates the rule gets struck.
Once the invalid interest is removed, the property passes under whatever valid terms remain. If the document provides no alternative, the property falls to the state’s intestacy laws, which distribute assets based on family relationships. That outcome is almost never what the property owner intended, which is why careful drafting matters.
The Traps That Made the Rule Famous
The rule’s notorious difficulty comes from how it was traditionally applied. Courts did not ask whether an interest would likely vest in time. They asked whether there was any conceivable scenario, however absurd, in which it might not. Two famous hypotheticals show how harsh that could be.
The Fertile Octogenarian
Under the traditional rule, courts assumed any living person could still have children, regardless of age. An 80-year-old grandmother was presumed capable of having another baby. If a trust said “income to my children for life, then the principal to my grandchildren,” a theoretical after-born child would not be a life in being, and the grandchildren’s interest could be measured against that hypothetical child. The whole gift to the grandchildren could be voided based on a biological impossibility.
The Unborn Widow
If a trust said “to my son for life, then to his widow for life, then to his children,” courts noted that the son’s future widow might be someone not yet born. If she was not a life in being, her life estate could not serve as a measuring life, and the gift to the children after her death might vest more than 21 years after everyone alive at creation had died. Again, the gift failed on a theoretical possibility that rarely matched reality.
Modern Reforms
These traps are the reason the traditional rule has been softened almost everywhere. Most states have moved away from the strict common law approach in one way or another.
Wait and See
The biggest conceptual shift is the wait-and-see approach. Instead of asking whether an interest could possibly vest too late under some imagined scenario, courts simply wait and see whether it actually does. If the interest vests within the allowed period, it is valid. If it does not, it is void. This resolves the fertile octogenarian and unborn widow problems entirely, because courts look at what actually happened rather than what might theoretically happen.
The Uniform Statutory Rule
Many states have adopted the Uniform Statutory Rule Against Perpetuities, which adds a flat 90-year alternative period. An interest is valid if it either satisfies the traditional common law rule or actually vests within 90 years of its creation. The 90-year window was designed to approximate the time that would elapse under the traditional rule in a typical family situation, without the complexity of identifying measuring lives.
Judicial Reformation (Cy Pres)
Some courts use a doctrine called cy pres, from the French for “as near as possible,” to fix invalid interests rather than void them. A court rewrites the offending provision to come as close as possible to the creator’s intent while staying inside the rule. If a trust set a 30-year termination period that violated the rule, a court might simply reduce it to 21 years. Two requirements apply: the reformed interest has to comply with the rule, and it has to preserve the creator’s intent as closely as possible.
Savings Clauses
Estate planning attorneys routinely add a savings clause as a safety net. A typical clause states that if any interest has not vested by the end of the perpetuities period, the trust terminates and the assets are distributed to the beneficiaries then living. The backstop prevents a violation from ever occurring. Any competently drafted trust from recent decades should contain one.
States That Have Abolished the Rule
Roughly half the states have repealed the Rule Against Perpetuities entirely or extended trust duration to centuries. Florida and Wyoming, for example, allow trusts to last up to 1,000 years. Others permit truly perpetual trusts with no expiration date.
That shift has fueled the rise of dynasty trusts, irrevocable trusts designed to pass wealth from generation to generation without ever triggering estate or generation-skipping transfer taxes. Because the trust itself never terminates, and the assets technically belong to the trust rather than to any individual beneficiary, wealth inside can grow and compound free of transfer taxes indefinitely.1Internal Revenue Service. Whats New – Estate and Gift Tax
The result is a notable tension in current law. The rule was originally designed to prevent exactly the kind of permanent wealth concentration that these trusts enable. The practical takeaway for anyone creating a trust today is that the choice of which state’s law governs it is a meaningful planning decision, because the perpetuities period, or the lack of one, will shape how long the structure can last.