Battle of Expert Witnesses: How Criminal Courts Handle Conflicting Science

Criminal courts can hear conflicting expert opinions because qualified experts may interpret the same evidence differently. The judge decides whether an expert is qualified and whether the testimony satisfies the rules for admissibility. Once the testimony reaches the jury, jurors do not have to accept an expert’s opinion. They can consider the expert’s qualifications, methods, reasoning, and other evidence before deciding how much weight to give the testimony.

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Why Are There so Many Expert Witnesses Saying Opposite Things?

More than one reasonable interpretation of scientific evidence can exist. An expert might reach a different conclusion because of the assumptions used, the facts relied upon, or the methodology selected. An expert’s specialized knowledge also matters. A toxicologist, physician, DNA analyst, or other professional may approach the same evidence from a different area of training or experience.

That does not necessarily mean any expert is wrong. Under the federal rules, competing opinions alone are not enough to keep an expert from testifying. Courts can consider whether the expert’s method has been tested, subjected to peer review, or has a known or potential error rate, among other factors. The focus is on whether the opinion is sufficiently reliable for the jury to consider, rather than whether the judge agrees with the expert’s ultimate conclusion.

Once both sides have an opportunity to present expert evidence, they can challenge each other’s conclusions. An attorney might use cross-examination to question how the expert reached an opinion, what information the expert relied on, whether another explanation fits the evidence, or whether the method has known limitations. This is especially important when forensic science plays a major role in a criminal case.

Before the jury decides which expert it finds more persuasive, though, the judge must first decide whether the competing opinions are admissible.

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Who Decides Whether Someone Is Qualified as an Expert?

A person does not become an expert witness simply because they have an impressive resume. In federal court, Rule 702 governs the basic requirements for expert testimony. Under the Federal Rules of Evidence, a witness may qualify through knowledge, skill, experience, training, or education.

No blanket rule requires every expert to have a particular degree or job title. The important question is whether the witness has specialized knowledge that will help the jury understand the evidence or decide a disputed fact.

The trial judge makes that initial call. Before expert testimony reaches the jury, the judge considers whether the opinion is based on sufficient facts or data, whether the expert’s principles and methods are reliable, and whether the expert reliably applied those methods to the facts of the case. Rule 702 also requires the testimony to be relevant and helpful to the jury.

The judge’s role is often described as gatekeeping. The U.S. Supreme Court addressed this role in Daubert v. Merrell Dow Pharmaceuticals, Inc. Courts applying the Daubert factors may consider whether:

• A scientific theory or technique can be tested
• It has been subjected to peer review
• Its known or potential error rate
• Standards or controls govern the technique
• It has gained general acceptance

However, these factors are flexible. A judge doesn’t have to follow this outline exactly or consider every factor.

When an expert’s reliability or qualifications are seriously disputed, the court may hold a Daubert hearing. This hearing can take place before trial or during the proceedings. The purpose is to give the judge a closer look at the proposed testimony. The judge will decide whether to admit it.

State Court Rules on Expert Testimony

Rules can differ in state court. Some states follow Daubert. Others use the Frye standard, from Frye v. United States, which focuses on whether the scientific principle or technique has achieved general acceptance in the relevant scientific community. For that reason, the standard governing the admissibility of expert testimony depends in part on which court is hearing the case.

Does the Jury Have To Believe an Expert Witness?

A jury does not have to believe an expert witness simply because the judge allowed that person to testify. Expert testimony is evidence for the jury to consider. Jurors remain responsible for deciding what facts they find persuasive. That means a jury can accept an expert’s opinion, reject it, or give it less weight than other evidence presented at trial.

Jurors can consider the expert’s qualifications and experience. They can look at the facts and data supporting the opinion. They can consider the methodology the expert used. Their analysis can include whether the methodology was applied reliably.

Cross-examination can give jurors additional information. An attorney may question an expert about assumptions, gaps in the data, limitations of the methodology, or inconsistencies between the opinion and other evidence. The jury can then compare that testimony with the rest of the case and decide whether the opinion holds up.

What Happens When Experts Disagree?

Opposing sides often present experts who disagree. That is part of the adversarial process. Each side can develop its own interpretation of the evidence, challenge the other side’s expert witness testimony through cross-examination, and point out problems with the opposing methodology. These disputes can arise over forensic evidence in criminal cases and expert testimony in civil litigation.

The court still has to draw a line between a legitimate difference of opinion and an unreliable opinion. The judge serves as the gatekeeper for expert testimony and considers whether the witness is qualified and whether the opinion rests on a reliable foundation rather than junk science. If both sides’ experts are allowed to testify, the jury does not have to accept either opinion.

It can weigh the facts, methodology, reasoning, and other evidence before deciding which conclusions, if any, are persuasive.

Seek Legal Advice

Conflicting expert testimony can make a criminal case difficult to understand, particularly when both sides present professionals with impressive credentials. When expert testimony could affect the outcome of your criminal case, an attorney can review the expert’s qualifications, methods, and conclusions and determine how to challenge the testimony.

Use the Super Lawyers directory to find a criminal defense lawyer who can evaluate your case.