Expert Testimony and How It Differs From Lay Testimony

When a person answers questions under oath during a deposition or trial, the person is generally referred to as a “witness” and their statements as “testimony.” Yet whether the person is categorized as a lay witness or an expert witness has a profound effect on which testimony is permissible and under what circumstances.

Furthermore, the line between “lay” and “expert” witnesses and testimony varies among jurisdictions. For instance, in US v. Jayyousi, 657 F.3d 1085 (11th Cir. 2011), the Eleventh Circuit treated the lay witness testimony admissibility requirements broadly, allowing an FBI agent to testify as to the meaning of certain alleged code words in Arabic spoken by the defendants, even though the agent did not speak Arabic.

By contrast, in US v. Freeman, 730 F.3d 590 (6th Cir. 2013), the Sixth Circuit reversed the trial court’s decision to allow an FBI agent to testify as a lay witness about words spoken during phone calls between the defendants. The court held that the agent lacked the knowledge necessary to support a lay witness opinion under Federal Rule of Evidence 701(a).

As demonstrated by the differing results reached by federal courts even in similar cases, questions about lay versus expert witness testimony don’t always have ‘black or white’ answers. Therefore, understanding the potential hurdles for admissibility and disclosure is essential.


Lay Witness Testimony

Generally speaking, the law divides witnesses into two categories: Fact Witnesses and Opinion Witnesses. Fact witnesses, or lay witnesses, testify about their first-hand knowledge: what they heard, saw, said, or did. Fact witnesses are often closely related to the case in some way. They may be family members, business partners, or eyewitnesses at the scene.

Expert Witness Testimony

An expert witness, or an opinion witness, is a person with specialized skill sets whose opinion may help a jury make sense of the factual evidence of a case. Expert witnesses testify as to their opinion about certain facts or events. Because they don’t have first-hand knowledge of the facts or events, expert witnesses use their technical knowledge, experience, skills, and expert methodologies to form their opinions on the case.


While an opinion witness may be either lay or expert witness, which opinions a witness will be allowed to share will depend on their classification as lay or expert. Lay witness and expert witness testimony are subject to two separate sets of rules governing admissibility in federal courts. Federal Rule of Evidence 701 governs the admissibility of lay witness testimony; 702, of expert witnesses.

Under FRE 701, a lay witness may provide any opinion that:

  • Is rationally based on the witness’s perception

  • Is helpful to clearly understanding the witness’s testimony or to determining a fact in issue

  • Is not based on scientific, technical, or other specialized knowledge within the scope of FRE 702

Lay witnesses are typically restricted to testifying about things they personally perceived and about which the average reasonable person could also form an opinion. For instance, a lay witness to a car accident might testify that she saw the defendant’s car run a red light at a high rate of speed, and that as a result, her opinion was that the driver was speeding.

Rule 702 sets out different standards for expert witnesses and for their testimony. An expert opinion may only be admitted if the witness is “qualified as an expert by knowledge, skill, experience, training or education,” and if the witness’s opinion meets the four requirements laid out in FRE 702:

  • The expert’s knowledge will help the trier of fact understand the evidence or determine a fact in issue

  • The testimony is based on sufficient facts or data

  • The testimony is the product of reliable principles and methods

  • The expert has reliably applied the principles and methods to the facts of the case

Unlike a lay witness, an expert witness does not have to have first-hand knowledge of the case in order to form or to testify to an opinion. Instead, the expert witness’s opinion may be based on the witness’s application of reliable principles and methods to the facts or data in the case. Experts are not limited by Rule 702 to testifying as to their opinion. For instance, an expert witness may also testify generally about principles, methods, or other information relevant to the topic and case.



Federal Rule of Civil Procedure 26(a)(2) (and similar rules in many states) requires lawyers to meet disclosure requirements for expert witnesses above and beyond those required for lay witnesses.

Rule 26(a)(2) requires retained experts to submit an expert report if they will testify. Non-testifying experts may also be required to be disclosed or to submit a report in certain circumstances. When they do not, their work is generally considered protected by attorney work product rules.

By contrast, lay witness opinions generally do not need to be summarized in a report or otherwise disclosed prior to trial. When a lay witness’ testimony approaches the line between lay observations and expert opinions, however, lawyers may need to consider ways to ensure the testimony remains usable at trial. Typically, lawyers have two options. The first is to disclose the lay witness’ opinion. The second is to reach an agreement with opposing counsel regarding the precise disclosures required.

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