What Is Undue Influence in a California Trust or Will?

Published June 20, 2026 · Reviewed by Elizabeth A. Tresp, JD, LL.M., Principal Attorney

The fact pattern is familiar in San Diego probate court. An aging parent executes a restatement. A caregiver, a new spouse, or one child is suddenly the remainder beneficiary. The other children learn about it after the funeral. The question they bring to counsel is almost always the same: was this what Mom wanted, or was it undue influence?

The statutory definition

California did not leave this to common-law adjectives. Welfare & Institutions Code §15610.70 defines undue influence as excessive persuasion that causes another person to act or refrain from acting by overcoming that person’s free will and that results in inequity. The statute tells the court to consider four clusters of facts:

• Vulnerability of the victim. Illness, cognitive impairment, isolation, dependency, recent grief, illiteracy, or a language barrier.
• Apparent authority of the influencer. Fiduciary, family member, care provider, legal professional, spiritual adviser, or someone the victim thought they had to please.
• Actions or tactics. Controlling necessities, medication, or contact; using affection or intimidation; initiating changes in haste or secrecy; making claims of expertise.
• Inequity of the result. The economic consequences, divergence from the victim’s prior intent, and whether the change was the product of an independent decision.

Probate Code §86 incorporates that definition into the Probate Code. The litigation is therefore a statutory case, not a morality play.

The §21380 presumption

Probate Code §21380 presumes that certain donative transfers are the product of fraud or undue influence: transfers to the person who drafted the instrument, to a person in a fiduciary relationship who transcribed it, and to a care custodian of a dependent adult. The presumption is rebuttable. It is also limited. A child who provided care is not automatically a “care custodian” for this purpose, and transfers to relatives described in the statute are carved out. Getting the presumption right is often the difference between a case that survives summary judgment and one that does not.

How these cases are actually proved

Undue influence is proved with a mosaic, not a single email. Typical evidence includes:

• The drafting attorney’s file and who sat in the meetings.
• Medical records around the date of signing — not a general impression that the settlor was “sharp.”
• A pattern of isolation: changed locks, new cell phone, family cut off from visits.
• Prior estate plans, so the court can see the delta.
• Bank and real-property records showing transfers that preceded the amendment.

A competent drafting attorney who met the settlor alone, documented capacity, and declined to take instructions from the beneficiary is powerful defense evidence. The absence of that file is often the petitioner’s best fact.

Undue influence is not the same as a bad outcome

Parents disinherit children. They leave more to the child who stayed. They marry late. None of that is, by itself, undue influence. California law protects the settlor’s right to be unfair. What it does not protect is a plan extracted from a vulnerable person by someone who stood in a position of trust. If you are trying to decide which side of that line you are on, the next step is a review of the documents and the timeline, not a guess. Call (858) 248-2779 or request a consultation.

---

This article is general information about California law, not legal advice, and reading it does not create an attorney–client relationship. Trust and probate deadlines are strict and many rights are lost by missing one — for guidance on your own situation, contact Tresp Law, APC or call (858) 248-2779.