3.07 Expert Witness Testimony

Expert Witness Testimony

Forensic scientists are often called upon to give testimony in the court of law as an expert witness. In the courtroom, experts present their findings to the jury and may be questioned by lawyers from both the prosecutor and defendant. The purpose of an expert witness is to explain complex evidence or testing in a manner that is easy for jurors to understand.

The image below shows an outline of a typical courtroom. The Judicial Learning Center (Who Are the Players in the Courtroom? | The Judicial Learning Center) provides more information about each of the players in the courtroom.

To serve as an expert witness before a jury, scientists must demonstrate a rigorous application of the scientific method. Each state has its own laws that determine what evidence and expert witnesses are admissible (acceptable as evidence) in court, but most require experts to meet the Frye Standard, Federal Rules of Evidence 702, the Daubert Ruling, or a combination of all three.

The Frye Standard

A 1923 court case, Frye v. United States, established the standard for polygraph tests. The defendant, John Frye, was accused of murder. An expert claimed that he could prove that Frye was lying by measuring changes in blood pressure while Frye was asked questions. The dubious reliability of this early polygraph necessitated the creation of new standards of scientific evidence for determining whether or not such a test would be admissible in court.

The Frye Standard establishes rules for the admissibility of scientific experiments and examinations in US law. The key elements to the Frye standard are that the data collected from these methods must be reliable and reproducible under similar conditions and be generally accepted by the scientific community. For example, blood alcohol testing has been shown to be consistent and scientifically reliable, so it is admissible under the Frye Standard; on the other hand, polygraph “lie detector” tests are often not admissible, because they have not been proven to meet this standard for reliability and reproducibility. Some people have been able to “beat” the polygraph test.

Which of these is not admissible in court?

1. lie detector tests
2. DNA evidence
3. toolmark evidence
4. blood alcohol levels

Answer: a. lie detector tests.

Polygraph tests are not allowed in court because they are not ________blank.

Answer: reliable.

Federal Rules of Evidence, Rule 702

Written in 1975, Federal Rule of Evidence 702 states that a witness who is qualified as an expert by knowledge, skill, experience, training, or education may testify in the form of an opinion or otherwise if:

• the expert’s scientific, technical, or other specialized knowledge will help the jury to understand the evidence or to determine a fact in issue.
• the testimony is based on sufficient facts or data.
• the testimony is the product of reliable principles and methods; and
• the expert has reliably applied the principles and methods to the facts of the case.

The Daubert Ruling

In 1993, plaintiffs Jason Daubert and Eric Schuller sued a pharmaceutical company in Daubert v. Merrell Dow Pharmaceuticals. Both men were born with birth defects after their mothers had been prescribed a drug called Bendectin while they were pregnant. Daubert and Schuller claimed the drug was responsible for their birth defects. At the trial, expert witness Dr. William McBride (an Australian obstetrician) claimed that the drug caused numerous serious birth defects such as limb and skeletal malformations, cancer, and brain damage, and other defects and illnesses. However, independent sources examined Dr. McBride’s claims, and found that his data were fabricated.

The Daubert Ruling revises the Frye Standard in cases where scientific evidence is being presented by experts. In order to avoid the risk of experts like Dr. McBride falsifying evidence or giving dubious testimony, the Daubert standard requires a rigorous application of the scientific method, including hypothesis testing, error rate estimates, peer-reviewed publication, and acceptance by the general scientific community.

Standards Review

The appropriate standards for expert witnesses are matched to the descriptions below.

Frye Standard

• 1923
• Least restrictive standard
• Requires methods to be reliable, reproducible, and generally accepted.

Federal Rules of Evidence, Rule 702

• 1975
• Requires the expert to have knowledge, skill, training, or experience.
• Requires the testimony to be helpful to the jury in understanding the case.

Daubert Ruling

• 1993
• Most restrictive standard
• Requires estimates of error rates.

Based on these standards, who can provide their opinion in the court of law? According to Federal Rule 702, experts must be qualified by knowledge, skill, training, or education. Therefore, witnesses should have a significant level of training and experience. Certification from a professional organization can bolster the credibility of an expert witness.

What evidence can be presented to the jury? The findings of the expert witness must be based in science that has been reviewed by peers and is generally accepted in the scientific community. Some forms of forensic analysis have wide acceptance in the court of law—such as DNA testing and fingerprint evidence. However, experts using newer techniques will have to prove the reliability of their methods.

Expert Testimony

What does an expert witness say? Expert witnesses can give their opinion, but they should support it with reasoning, referencing facts and specific evidence from the case. Photos and diagrams are often used to help the jury see the evidence or understand the expert’s reasoning.

In general, an expert testimony would go something like this:

• Introduction — the expert will state who they are, including how they are qualified to testify as an expert in this case (employment, training, certifications, experience)
• Assignment — the expert will explain the assignment they were given, including specific pieces of evidence, and what tools and techniques they used to analyze the evidence.
• Factual findings — the expert will present the scientific findings from the case, this may include pictures, measurements, and data from various instruments.
• Opinions — the expert will give their opinion on the evidence (“This particular screwdriver was/was not used to pry open this door”)
• Reasoning — the expert will support that opinion by referring to specific evidence and factual findings.

Expert witnesses can be cross-examined by the opposing attorney. They should be prepared to answer additional questions concerning their opinion and findings.