The concept of an insane delusion appears in several branches of the, law, including contracts, gifts, and wills. Critics of the traditional doctrine have made compelling arguments in favor of its modification or abolition in the context of wills, given that it is often used as an excuse to substitute the values of jurors for those of the testator. Moreover, recent scientific studies have shown correlations between delusions and other cognitive impairments, calling into question the need for an independent doctrine of insane delusion. Nevertheless, there is evidence that not all deluded individuals have additional cognitive biases, and those who do may have some impairments while lacking others. Due to the nature of gratuitous transfers, adoption of the fairness-based approach to mental illness in the Restatement (Second) of Contracts is not a feasible alternative to the traditional insane delusion doctrine for wills. This Article accordingly proposes a new use for the concept of a delusion in making legal determinations regarding mental capacity in the context of wills. The concept would be better formulated as a doctrine of partial sanity, used when a testator is found to lack general mental capacity, and only as a basis for upholding all or part of a will. Under such a rule, the issue of a testator's general mental capacity would be decided first. If the person in question had general mental capacity, the will would be held valid. But if the person did lack general mental capacity, the court would then consider whether the testator was capable of making some rational decisions. To the extent that a particular decision was the product of rational decision making, the testator would be deemed to have had the capacity to make that decision. This would preserve, in modified form, a legal concept that has existed for centuries and remains relevant in modern science, without giving excessive license to courts and juries to second-guess the lifestyles and eccentricities of individuals.
Tate, Joshua C., "Personal Reality: Delusion in Law and Science" (2017). Connecticut Law Review. 362.