This article deals with the sometimes elusive line between expert and lay opinion testimony. Justice Holmes once remarked that the law is constantly drawing lines. That remark certainly holds true in Evidence law. On a daily basis trial judges must distinguish between character and noncharacter uses of evidence and differentiate hearsay from nonhearsay theories of logical relevance. The topic of this article is another evidentiary distinction: that between lay opinions and expert opinions.

That distinction has assumed tremendous importance since 1993. In that year, Federal Rule of Civil Procedure 26 was amended to prescribe mandatory pre-discovery disclosures. One of the most important parts of that scheme is a mandate that the proponent of a testifying expert provide the opposition with a detailed report previewing the expert’s opinion. Federal Rule of Criminal Procedure 16 imposes a similar requirement for an expert report. However, there is no requirement under either rule for such a report when the opinion is lay in nature. In the same year, the Supreme Court handed down its celebrated decision in Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993). Daubert announced a new empirical validation test for the admissibility of scientific testimony. In 1999 in Kumho Tire Co. v. Carmichael, 526 U.S. 137 (1999), the Court expanded Daubert and held that the proponent of any type of expert testimony must establish objective indicia of the reliability of the testimony. Like the Civil Rule 26 and Criminal Rule 16 amendments, Daubert and Kumho are inapplicable to lay opinion testimony.

These two 1993 developments have created a strong incentive for litigants to characterize their witnesses’ opinions as lay rather than expert. If the judge accepts that characterization, there is no need for the litigant to file a Rule 26 report or lay a Daubert foundation. Unfortunately, as Part I of the enclosed article points out, the courts are experiencing difficulty drawing the line between lay and expert opinions. Part I considers several possible bases for distinguishing the two types of opinions that have been proposed in the past. Part I demonstrates that each of those potential bases is unsatisfactory. Part II argues that in order to properly differentiate between the two types of opinions, the court should focus on the underlying reasoning processes. In the case of both types of opinions, the witness arrives at the opinion by evaluating a case-specific fact in light of a generalization. However, the two types of opinions differ in both: (1) how the witness obtains the case-specific information; and (2) how they validate and derive the generalization. Part III of the article explains how the courts may use the insights discussed in Part II to both differentiate between the two types of opinion and determine the admissibility of such opinions. Although the courts have struggled to draw the line in these cases, Part III demonstrates that the courts can draw the line with confidence by employing the analysis proposed in Part II.

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