How Judges Decide Whether Expert Testimony Can Be Used at Trial

Update (October 2026): Federal Rule of Evidence 702 was amended effective December 1, 2023. The change makes clear that the party offering an expert must show the judge it is "more likely than not" that the testimony meets each of the rule's requirements. Fed. R. Evid. 702 and 2023 committee note.

A federal judge may let jurors hear expert testimony only if the expert is qualified and the opinion is reliable and relevant. The judge acts as a "gatekeeper" because scientific evidence can strongly sway a jury.

In United States v. Barton, the Eleventh Circuit considered whether DNA evidence presented through an expert was reliable enough to be admitted under the Daubert standard.

The Daubert Standard

In Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993), the Supreme Court held that under Federal Rule of Evidence 702, the trial judge acts as a "gatekeeper" for scientific evidence. Jurors may have trouble judging complex science, and letting junk science in could lead to a wrongful conviction.

Expert Testimony Must Be Reliable and Relevant

The judge must make sure expert testimony is reliable and relevant to the case, and that the expert is qualified to give the opinion. To judge reliability, the court looks at the testing and methods the expert used to reach the conclusion.

The Facts of Barton

Expert Testimony

Barton was convicted under 18 U.S.C. § 922(g) of being a felon in possession of a firearm. At trial, a government expert testified about DNA found on the gun and explained the complex testing process, including how one DNA sample is told apart from others.

Barton argued the DNA testing methods were not reliable. He did not dispute the expert's qualifications or that the evidence was relevant.

The Eleventh Circuit rejected his challenge. It found that even without the expert testimony, there was strong evidence he had the gun. He admitted it when arrested, and he later admitted it to a jailhouse informant. So even if admitting the DNA testimony was a mistake, it was "harmless," meaning it did not change the outcome.

Challenging Expert Testimony

An expert's opinion can decide a criminal trial, so it is important to challenge unreliable expert testimony. This is usually done with a motion in limine, a request before trial asking the judge to keep certain evidence out.

A good way to find weak spots is to consult a defense expert who can review what the government's expert relied on. Ask for the expert's disclosure early; under Federal Rule of Criminal Procedure 16(a)(1)(G), the government must, on request, disclose a written summary of its expert's opinions, the bases for them, and the expert's qualifications. If the judge wrongly admits expert testimony, raise the issue on appeal.

This is one reason to get a criminal defense lawyer involved early in your case. A lawyer can look at the case from every angle and bring in the right experts to help with your defense, including for a Daubert challenge to an expert the government wants to use.

Facing a federal sentence? Book a consultation to talk it through with Brandon Sample.