Drawing The Line Between Expert And Lay Testimony On Peer-To-Peer File Sharing Program

In considering admissibility of testimony about LimeWire peer-to-peer file sharing program, Fifth Circuit notes that “[t]he case law is not completely clear on where to draw the line between expert and lay testimony,” in United States v. Caldwell, 586 F.3d 338 (5th Cir. Oct. 26, 2009) (No. 08-50804)

Discerning the difference between lay and expert testimony can be difficult at best. FRE 701 permits lay testimony “in the form of opinions or inferences … which are (a) rationally based on the perception of the witness, (b) helpful to a clear understanding of the witness’ testimony or the determination of a fact in issue, and (c) not based on scientific, technical, or other specialized knowledge within the scope of Rule 702.” Under FRE 702, testimony involving “scientific, technical, or other specialized knowledge” may be admitted to “assist the trier of fact” where certain requirements are met. A recent case involving an explanation of the LimeWire peer-to-peer file sharing program highlighted this challenge between lay and expert testimony.

In the case, investigators learned that an internet protocol address belonging to defendant Caldwell, an Army specialist stationed at Fort Bliss Army Base, was connected to a computer using a LimeWire peer-to-peer file sharing program and contained child pornography images. In executing a search warrant at the residence, the defendant’s computer was in the process of downloading bestiality pornographic files with the LimeWire program. Seventeen child pornography videos were located on the computer. The defendant was charged with the transportation and receipt of materials involving the sexual exploitation of minors.

After the government noted it planned to call a LimeWire employee to explain how the program operates, the defense moved to exclude the testimony since it did not receive pretrial disclosure of the expert testimony, under Fed. R. Crim. P. 16(a)(1)(G). The prosecutor noted that the witness would not provide expert but fact testimony concerning “how LimeWire works and what a person would see when they turn on and use the program.” With this representation, the defense agreed no expert testimony was being offered. After the witness explained the operation of the LimeWire program, the prosecutor posed two questions to the witness:

Q. Is it possible for somebody else with LimeWire to send you a video file that you haven’t searched for?
A. No, it can’t.
Q. Is it possible for another computer user on the internet to send you and download into your shared folder a file you haven’t searched for?
A. No, they cannot.

No objection was made during this direct examination. During cross-examination, the defense asked how a Trojan could be used with the LimeWire program:

Q. . . . [Is there] a warning system to tell the user that LimeWire was being overtaken by a trojan?
A. I could only speculate what that feature actually is and what it actually means, because those terms are very – I don’t really know what that feature is, but it could be.. . . .
Q. Would it be fair to say that there’s other people at LimeWire that would know about this particular feature, correct?
A. Exactly. If I can explain, there is a huge code base, several hundred thousand lines of code, so I would have to always make myself familiar with the certain area, which I could do.


At trial, the defendant claimed he did not look at the child pornography or bestiality videos or download pornography on the day of the search. The jury convicted the defendant. As one ground on appeal, he challenged the admission of the LimeWire testimony.

The Fifth Circuit affirmed the convictions and found no plain error on the admission of the LimeWire testimony. First, the circuit concluded that the government did not mislead the defense concerning the lay testimony offered at trial since the LimeWire testimony did not “clearly or obviously go beyond explaining how LimeWire works, which is what the Government had said he would testify to.” Caldwell, 586 F.3d at 347. The jury heard other testimony about any role a Trojan may have played. The government’s computer expert “testified that there was no trojan that was responsible for any of the child pornography found on the defendant’s computer.” In contrast, the defense expert that he found a trojan on the defendant’s computer and it was possible that “a trojan of some kind could ‘take over’ the LimeWire program so as to cause a computer to receive LimeWire videos it had not requested.” Caldwell, 586 F.3d at 347 n.4. No objection was made to the “two, isolated questions and answers elicited” at trial. The defense “delved into similar, but defense favorable testimony, on cross examination” and offered its own expert testimony.

Second, the Fifth Circuit found no plain error in admitting possible expert testimony. The circuit noted that “[t]he case law is not completely clear on where to draw the line between expert and lay testimony.” The difference between the two forms of testimony is that “lay testimony results from a process of reasoning familiar in every day life, whereas expert testimony results from a process of reasoning which can only be mastered by experts in the field.” Caldwell, 586 F.3d at 348 (citing FRE 701 ACN; United States v. Yanez Sosa, 513 F.3d 194, 200 (5th Cir. 2008) (adopting the reasoning in the ACN).

The circuit reviewed other cases on the issue of determining lay or expert testimony and noted that “[t]he trend in the circuits seems to turn on whether the testimony falls within the realm of knowledge of the average lay person.” The following cases were noted:

  • United States v. Soto-Beniquez, 356 F.3d 1, 37–38 (1st Cir. 2004) (“The district court correctly determined that none of the eight forensic examiners provided expert testimony,” including pathologists testifying about autopsies. “Witnesses who testify only about their perceptions of an event, or about lay opinions arising out of those perceptions, see Fed R. Evid. 701, are not experts under Rule 702 regardless of any specialized training or experience they may possess. That rule is dispositive here: the court permitted each of the witnesses to testify only about their observations at the various crime scenes they personally investigated.”) (citations omitted)
  • Bryant v. Farmers Ins. Exch., 432 F.3d 1114, 1124 (10th Cir. 2005) (“Taking a simple average of 103 numbers, though technically a statistical determination, is not so complex a task that litigants need to hire experts in order to deem the evidence trustworthy. A mathematical calculation well within the ability of anyone with a grade-school education is, in our opinion, more aptly characterized as a lay opinion under Fed. R. Evid. 701.”)
  • Doddy v. Oxy USA, Inc., 101 F.3d 448, 460-61 (5th Cir. 1996) (in toxic chemicals case, affidavit concerning “‘toxic chemicals and other toxic substances’ ‘contaminat[ing]’ the well and other parts of the East Texas Oil Field” was not lay opinion; “it is rather doubtful that, in the absence of any specialized training or experience in these areas, he — or any other ordinary person — could have personal knowledge about the toxicity of chemicals or substances testimony about the toxicity of chemicals was not lay opinion because it required specialized training and experience”; district court did not abuse its discretion in striking portions of affidavit)
  • United States v. White, 492 F.3d 380, 401-05 (6th Cir. 2007) (in Medicare fraud case, testimony that “contracts contained a significant profit margin which rendered them unreasonable” and that the defendant “had ‘control’ over certain of the businesses” by “fact witnesses directly involved in the transactions at issue in this case” was admissible as lay testimony; however, testimony of fiscal Intermediary witnesses “relied to a significant degree on specialized knowledge acquired over years of experience as Medicare auditors in testifying to the structure and procedures inherent in the Medicare program, as well as their understanding of various terms” was expert testimony but error was harmless)
  • United States v. Garcia, 413 F.3d 201, 215-16 (2d Cir. 2005) (DEA agent’s testimony about the role the defendant played in drug conspiracy was not lay opinion; “Precisely because Rule 701 limits the admissibility of lay opinions at trial to those based only on personal perceptions, an opinion … which appears to have been based on the totality of information gathered by various persons in the course of an investigation, was not admissible before a jury.”; “when a jury hears that an agent’s opinion is based on the total investigation, there is a risk that it may improperly defer to the officer’s opinion, thinking his knowledge of pertinent facts more extensive than its own”; “the government made no attempt to demonstrate that [the agent’s] challenged opinion was informed by reasoning processes familiar to the average person in everyday life rather than by scientific, technical, or other specialized knowledge”; error in admitting testimony was harmless)

On this record, the Fifth Circuit found no plain error:

“Like White, it’s difficult to say that Berger’s testimony was clearly expert testimony. With the prevalence of computer technology, it is not unreasonable to argue this type of information would fall within knowledge of the average lay person. The defendant himself testified to the same type of information. Furthermore, whether LimeWire allows the user to download files not specifically requested concerns how the program works. In contrast, the questions the defense asked, such as how a trojan may interact with LimeWire, go beyond that. The testimony elicited by the defense is closer to expert testimony than is the testimony of which it complains. We hold that the trial court did not commit plain error in admitting the testimony of Felix Berger, and that error, if any, in this respect did not seriously affect the fairness, integrity or public reputation of judicial proceedings.”
Caldwell, 586 F.3d at 348.


As Caldwell shows, the line between lay and expert testimony is very hard to discern. A closer question would have been raised in the case if an objection had been made at trial and review was under the less deferential abuse of discretion standard. The issue of lay versus expert testimony arises in other contexts, including on computer forensic testimony, as noted in these prior posts: Drawing The Line On Computer Forensic Expert And Lay Testimony (Part I); Drawing The Line On Computer Forensic Expert And Lay Testimony (Part II).

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