Undue Influence: Definition and How It Works in California

By Eric Ridley, attorney, Ridley Law. Updated September 2026.

Undue influence is excessive persuasion that overcomes a person’s free will and substitutes another person’s intent for their own, the most common ground for contesting a California will or trust.

How it works in California

Prob. Code, § 86 adopts the definition in Welf. & Inst. Code, § 15610.70: excessive persuasion “that causes another person to act or refrain from acting by overcoming that person’s free will and results in inequity.” What undue influence means under Probate Code section 86 lays out the factors courts weigh, including the vulnerability of the person influenced and the tactics used against them.

Some gifts trigger a legal presumption of undue influence rather than requiring the challenger to prove it directly. Under Prob. Code, § 21380, a gift to the person who drafted the document, a care custodian, or certain other people in a position of trust is presumed the product of undue influence, and the burden shifts to them to disprove it. See the presumption of undue influence under section 21380 for who it covers.

Undue influence is not the same as ordinary persuasion. A spouse or child who argues for a larger share, or who cares for a parent and is later remembered generously, has not necessarily done anything wrong.

Why it matters

For example, a caregiver who isolates an elderly person from family and then becomes the sole beneficiary of a late amendment fits the pattern courts look for, even without direct proof of specific threats or promises.

Common mistakes

Families sometimes label any unequal distribution as undue influence, when unequal is not automatically improper. Others wait too long to raise it, past the deadline that applies to a trust or will contest.

Related terms

• No-Contest Clause: can discourage an undue influence claim that lacks probable cause.
• Testamentary Capacity: a related but distinct ground, often argued alongside undue influence.
• Trust Contest: the proceeding where an undue influence claim is usually raised.
• Will Contest: a court challenge to the validity of a will.

Part of the California estate planning glossary. For the full treatment, see What Is Undue Influence Under Probate Code § 86.