Probably the most common legal theory to challenge a trust's validity in CA is undue influence. The other is probably lack of capacity for those keeping score at home.
“Undue influence” means excessive persuasion that causes another person to act or refrain from acting by overcoming that person’s free will and results in inequity. Welfare & Institutions Code §15610.70(a).
Various factors are used to determine if the result was the product of influence, i.e. the vulnerability of the victim, the influencer's apparent authority, the actions or tactics used by the influencer and the equity of the result. Welfare & Institutions Code §15610.70(a)(1)-(4).
When challenging a trust or will, the contestant will gather evidence as to what happened during the time that the person wrote the trust or will. The reason being is that the contestant will need to prove that at the time of execution, the trust or will was the product of undue influence. Hence what happened 30 years prior to the trust or will's execution would likely be immaterial. Rather, the focus would be on the months or years leading up to the trust or will's execution. For example, the person was diagnosed with Alzheimer's disease 3 months before they signed their trust, or the person was admitted to a skilled nursing facility 3 months before they signed their will.
It is common for the contestant, also known as the petitioner, to testify that the trust or will was the product of undue influence. For example, the petitioner might state that the person required extensive assistance with life's daily functions, suffered from a physical or mental impairment, etc. These actions contributed to the person executing a trust or will that was the product of undue influence.
Conversely, the respondent, often a spouse or child, will counter petitioner's testimony by downplaying the person's physical or mental impairment and testify that the person loved respondent more than petitioner because respondent took care of the person.
Furthermore, it is common for the petitioner and respondent to each hire their own expert, typically a medical doctor, who will opine as to the person's level of capacity and their corresponding vulnerability. Typically, the medical expert, if hired by the petitioner, will accentuate the person's fragility, or if hired by the respondent, minimize the person's fragility.
A recent unpublished appellate decision addressed the sufficiency of evidence in a undue influence case.
"In 2009, Richard Barnes (decedent Richard) executed a will (2009 will), leaving property to his wife, objector and appellant Celestine Barnes (objector Celestine), and to his daughter from a previous marriage, petitioner and respondent Joann Barnes Williams (petitioner Joann). In 2011, decedent Richard and objector Celestine created a pour-over will and revocable living trust (collectively, 2011 trust), which had the effect of disinheriting petitioner Joann. They restated the trust in 2014 (2014 Trust), without any significant changes to petitioner Joann or objector Celestine's interests. Following decedent Richard's death in 2016, petitioner Joann filed a petition pursuant to Probate Code section 17200 et seq., seeking to determine the validity of the trusts and alleging that decedent Richard suffered from progressive mental decline as the result of dementia and either lacked capacity to execute the trusts or did so as the result of undue influence."
The trial court noted who were the key witnesses in the case:
"Here, in its statement of decision, the trial court expressly stated that it considered the testimony of all 14 witnesses who testified at trial and considered all of the exhibits admitted into evidence. The trial court specifically noted portions of the testimony it found compelling in reaching its findings on undue influence and breach of fiduciary duty and further identified the four witnesses (petitioner Joann, objector Celestine, and their respective medical experts) that it deemed the "principal witnesses."
Estate of Barnes, Riverside County Superior Court case # RIP1600410.
