The general rule is that you need an expert witness to testify when the subject is “sufficiently beyond common experience that the opinion of that expert would assist the trier of fact.” Evid C §801(a). When it comes to a person’s sanity, you don’t always need an expert.
Under Evid C §870, there are three categories in which a lay witness may testify on the sanity of a person:
- Testimony of an intimate acquaintance. Under this category, a witness may testify about his or her opinion of the sanity of the person whose sanity is in issue. Evid C §870(a). A trial judge must determine the preliminary fact of whether the proffered witness comes within the category of an intimate acquaintance. The justification for this rule of evidence is that an intimate acquaintance presumably will have had sufficient observation of the words and conduct of the person whose sanity is in question to express a meaningful opinion about that person’s sanity. By contrast, a casual acquaintance can’t testify to an opinion on sanity, because such an acquaintance’s observations are too limited to serve as an adequate basis for such an opinion.
- Testimony of a subscribing witness. A subscribing witness to a will may, for example, have been a total stranger to the testator, but the witness is permitted to testify about his or her opinion of the testator’s sanity. Evid C §870(b). The subscribing witness’s opinion, however, is limited to the person’s sanity as of the time the writing was signed.
- Testimony of a lay witness on sanity who qualifies under Evid C §800. Section 800 sets out the rule on when a lay person may give opinion testimony, which includes, but is not limited to, an opinion that is:
- Rationally based on the perception of the witness; and
- Helpful to a clear understanding of his or her testimony.
How can a lay witness who isn’t an “intimate acquaintance” qualify under §800 to testify to an opinion on sanity? It’s conceivable that an individual might not be an intimate acquaintance but still might have had enough contacts with the person whose sanity is in issue to be able to testify to that person’s words and actions, and that opinion might be helpful to a clear understanding of his or her testimony.
Keep in mind that Evid C §870 deals solely with the admissibility of lay opinion testimony about the sanity of a person. It doesn’t prohibit a witness’s testimony about his or her observations of the words and conduct of a person, from which the trier of fact may draw inferences about the sanity of that person. See Comment to Evid C §870.
When you need to determine whether to get opinion testimony from an expert or a lay witness, turn to CEB’s Jefferson’s California Evidence Benchbook, chap 30.
Other articles you may find useful:
- 7 Questions to Answer Before Using Opinion Character Evidence
- 4 Ways to Attack Expert Testimony
- Do You Need an Expert Witness, and If So, What Type?
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