Skip to main content
Handbook Cover for Treatise on Internet Sex Crimes

Chapter 4:
Pretrial Strategy & Motions in Limine


Motion to Exclude Expert Testimony


RULE 701 – LAY WITNESS OPINIONS

Rule 701 states that an opinion in testimony by a lay witness must be limited to one that is rationally based on the witness’s perception, helpful to clearly understanding the witness’s testimony or to determining a fact in issue, and not based on scientific, technical, or other specialized knowledge within the scope of Rule 702 (which pertains to expert witnesses).[1] In other words, a lay witness can testify in the form of an opinion so long as their opinion is based on their personal knowledge and is helpful to understanding their testimony or determining a fact in issue. Lay opinions on matters such as a witness’s credibility, the defendant’s guilt, or a medical diagnosis are not permitted. 

Expert testimony may be required to help the fact finder determine the age of actors in pornographic materials if their age is difficult to determine.[2] For example, the Fifth Circuit heard a case “in which the government must prove that  a model, who is post-puberty but appears quite young, is less than eighteen years old, expert testimony may well be necessary …” However, at the same time, “it is sometimes possible for the fact finder to decide the issue of age in a child pornography case without hearing any expert testimony.”[3] This is because “age is a matter on which everyone has an opinion. Knowingly or unknowingly, we all form conclusions about people’s ages every day. It is therefore particularly appropriate for a lay witness to express an opinion on the subject.”[4] Many courts have admitted lay witness’ opinions as proof of subjects’ ages.[5] Whether or not expert testimony is required to determine a subject’s age is determined on a case by case basis.

Lay witnesses may also offer opinion testimony for facts other than age. For example, the Eleventh Circuit found that a lay witness could testify that a website’s purpose was to facilitate casual sexual encounters because they determined that any person looking at the website could perceive it that way without specialized knowledge.[6] That same witness was allowed to offer testimony interpreting the typographical errors in emails sent by the defendant, as this was based solely on the witness’s perceptions of the conversations.

Other opinions are outside of the scope of lay witnesses’ perception, and cannot be admitted into evidence. For example, the New Hampshire District Court determined that lay witnesses could not testify to the process of creating virtual images and discerning whether that process had been used to create a particular image, because this process is not familiar in everyday life.[7]

Testimony is unhelpful to the jury if it “answer[s] the same question that the trier of fact is to consider in its deliberations.”[8] For example, the First Circuit determined that an agent’s testimony was improper “because the determination of whether [the defendant] was the man in the webcam video could have been properly reached only by considering evidence available to the jury,” meaning that the agent’s testimony “usurped the jury’s role instead of being helpful to it.”[9] In this case, the testimony is not admissible under Rule 701.


[1] Fed. R. Evid. 701

[2] United States v. Katz, 178 F.3d 368, 373 (5th Cir. 1999)

[3] United States v. Katz, 178 F.3d 368, 373 (5th Cir. 1999)

[4] United States v. Yazzie, 976 F.2d 1252, 1256 (9th Cir. 1992)

[5] United States v. Davis, 41 F. App’x 566, 571 (3d Cir. 2002) (unpublished); United States v. Stanley, 896 F.2d 450, 452 (10th Cir. 1990); United States v. O’Malley, 854 F.2d 1085 (8th Cir. 1988); United States v. Kain, 589 F.3d 945, 952 (8th Cir. 2009); United States v. Stahlman, 934 F.3d 1199 (11th Cir. 2019)

[6] United States v. Stahlman, 934 F.3d 1199 (11th Cir. 2019)

[7] United States v. Tanguay, 895 F. Supp. 2d 284 (D.N.H. 2012)

[8] 4 J. Weinstein & M. Berger, Weinstein’s Federal Evidence § 701.05 (Joseph M. McLaughlin, ed., Matthew Bender 2d ed. 2011)

[9] United States v. Vazquez-Rivera, 665 F.3d 351, 361 (1st Cir. 2011)

RULE 702, 703 & 704 – EXPERT WITNESS TESTIMONY

Rule 702 states that a qualified expert witness may provide testimony in the form of an opinion if it is more likely than not that their specialized knowledge will aid the fact-finder in understanding the evidence or determining a relevant fact, the testimony is grounded in sufficient facts or data, the testimony is derived from reliable principles and methods, and their opinion reflects a reliable application of the principles and methods to the specific facts of the case.[10]

The Advisory Committee Notes from the rule’s proposal in 1972 states that “the rule is broadly phrased” in the sense that the fields of knowledge which may be drawn upon are “extended to all “specialized” knowledge, and not merely limited to the “scientific” and “technical,” and similarly, “the expert is viewed, not in a narrow sense, but as a person qualified by ‘knowledge, skill, experience, training or education.”[11] A 2023 amendment specifies that the proponent must demonstrate that the proffered testimony meets the rule’s admissibility requirements by a preponderance of the evidence.

Before admitting expert testimony, the district court “must perform a ‘gatekeeping role’ of ensuring that the testimony is both ‘relevant’ and ‘reliable’ under Rule 702.”[12] This is to “ensure the reliability and relevancy of expert testimony … to make certain that an expert … employs in the courtroom the same level of intellectual rigor that characterizes the practice of an expert in the relevant field.”[13] The government bears the burden of establishing the reliability of the principles and methods used to draw a conclusion “regarding the particular matter to which the expert testimony was directly relevant.” [14]

When assessing whether expert testimony is based on scientific knowledge, courts may consider factors such as (1) whether the expert’s technique can be tested, (2) whether the technique “has been subjected to peer review and publication,” (3) whether there is a “known or potential rate of error,” and (4) whether the technique is generally accepted within the relevant scientific community.[15] These same factors may also be relevant in assessing the admissibility of an expert’s testimony on the basis of “technical, or other specialized knowledge.”[16]

The Supreme Court further describes how courts should apply Rule 702 to exclude unreliable or unhelpful expert testimony: “the requirement that an expert’s testimony pertain to ‘scientific knowledge’ establishes a standard of evidentiary reliability,”and the “helpfulness” standard “requires a valid scientific connection to the pertinent inquiry as a precondition to admissibility.”[17] If the Government cannot explain how a witness’s experience led to the conclusions they reached, why the experience was a sufficient basis for their opinions, and how the experience was reliably applied to the facts of the case, the expert’s testimony opinions are not reliable and must be excluded from trial. 

If the Government has not explained the substance of their expert’s opinions, and the bases and reasons for these opinions, or the methodologies that the expert employed, it has not met its burden of establishing that the expert’s opinions are relevant and reliable under Rule 702 and Daubert, and the defense may file a motion to exclude that expert’s testimony. Furthermore, if the Government argues that the proposed testimony is not subject to Rule 702 and Daubert, but the testimony indeed goes beyond ordinary knowledge and requires technical or specialized knowledge, the defense may file a motion to exclude the witness’s testimony. 

Rule 703 states that an expert can form an opinion based on facts or data presented in the case that they are aware of or have personally observed. If such facts or data are the type that experts in the field would reasonably rely on, they do not necessarily have to be admissible for the opinion to be accepted. However, if the facts or data would typically be considered inadmissible, the proponent of the opinion can only present them to the jury if their probative value in assisting the jury in evaluating the opinion significantly outweighs their prejudicial effect.[18] In other words, in most instances, the side opposing an expert cannot object and seek to exclude an opinion because the expert used inadmissible evidence to reach the conclusion. The opinion itself may still be admissible. For example, if a psychological expert relied upon statements from interviews with the defendant’s relatives former teachers in concluding that the defendant had a learning disability, their overall opinion would be permissible under Rule 703 even though it was based in part on hearsay interviews that themselves would be inadmissible at a trial.

Furthermore, the expert may give the final opinion but not the inadmissible evidence behind it to the jury unless giving them that information is necessary to understand how the expert reached the conclusion and if doing so substantially outweighs the prejudice of letting the jury hear that information.[19] If this is the case, the judge should provide a jury instruction that the evidence is only to be used to explain how the expert formulated their opinion and should not be taken as fact.

The Seventh Circuit ruled that psychiatrists are not required to personally examine people in order to testify about them because Rule 703 permits experts to testify based on facts that they have personally observed or been made aware of.[20]

In some cases, the government’s expert witness may present a differing opinion than the defense’s expert witness. In this case, “the jury [is] free to evaluate both experts’ testimony” and conclude which is “more reliable and credible.”[21[ By questioning the experts on their specific training, experiences, or methodologies, one can be made to appear more knowledgeable and credible with regard to the specific matter at hand than the other.

Rule 704(b) provides that “in a criminal case, an expert witness must not state an opinion about whether the defendant did or did not have a mental state or condition that constitutes an element of the crime charged or of a defense. Those matters are for the trier of fact alone.”[22] Experts may provide testimony supporting an inference or conclusion regarding the defendant’s mental state, but cannot draw “the ultimate inference or conclusion for the jury,” and should ensure that “the ultimate inference or conclusion does not necessarily follow from the testimony.”[23] For example, experts cannot explain how a defendant distributed/received/possessed/produced child pornography without commenting directly on their mental state, and such testimony must be excluded from trial. 


[10] Fed. R. Evid. 702

[11] Id.

[12] United States v. Ruvalcaba-Garcia, 923 F.3d 1183, 1188 (9th Cir. 2019) (quoting Daubert v. Merrel Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993))

[13] Kumho Tire Co. v. Carmichael, 526 U.S. 152 (1999)

[14] United States v. Valencia-Lopez, 971 F.3d 891, 900 (9th Cir. 2020)

[15] Daubert v. Merrell Dow Pharms., Inc., 509 U.S. 579, 593, 113 S. Ct. 2786, 2796, 125 L. Ed. 2d 469 (1993)

[16] Kumho Tire Co. v. Carmichael, 526 U.S. 137, 149, 119 S. Ct. 1167, 1174, 143 L. Ed. 2d 238 (1999)

[17] Daubert v. Merrell Dow Pharms., Inc., 509 U.S. 579, 592, 113 S. Ct. 2786, 2796, 125 L. Ed. 2d 469 (1993)

[18] Fed. R. Evid. 703

[19] 2000 Amendment to Rule 703: “Rule 703 has been amended to emphasize that when an expert reasonably relies on inadmissible information to form an opinion or inference, the underlying information is not admissible simply because the opinion or inference is admitted.”

[20] United States v. Gmoser, 30 F.4th 646, 647 (7th Cir. 2022)

[21] United States v. Anderton, 136 F.3d 747, 750 (11th Cir. 1998)

[22] Fed. R. Evid. 704

[23] United States v. Younger, 398 F.3d 1179, 1189 (9th Cir. 2005)

Johnson Citronberg Logo

Call us at 703-684-8000
or send us an email below.