The Role of the Expert Witness in a Criminal Case

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Television and movies have dramatized the role of witnesses in a criminal trial. The eyewitness with less than perfect vision or the witness who cracks under intense cross-examination to give the prosecutor or defense counsel the key piece of evidence needed to convince the jurors of the defendant’s guilt or innocence have become familiar characters that turn up again and again.

Expert witnesses, however, do not get as much interest from screenwriters as they do from the attorneys who prosecute or defend people charged by committing crimes. Experts in ballistics, psychology or blood splatter analysis sometimes prove to be the deciding factor in determining whether a criminal jury votes to convict or acquit a person standing trial in a California courtroom.

Witness in Criminal Case

What is an expert witness?

Most witnesses testify about factual information they acquired through the use of their five senses. They do not, however, offer opinions. A witness simply states the facts as he or she knows them, and it is then up to the jury or the judge in the case of bench trials held without a jury to decide what conclusions to draw from the facts.

An expert witness is someone whose education, training, experience, knowledge or skill in a particular subject area gives him or her expertise beyond that possessed by the average person. Because of this superior knowledge, an expert witness may offer an opinion.

For example, a witness testifies about seeing a car identified as belonging to the defendant in a criminal case being driven at a high rate of speed. If the prosecutor asks the witness to estimate the speed of the vehicle, there would probably be a defense objection on the ground the witness is not qualified to offer such an opinion. However, if the witness was a police officer whose training included the estimation of vehicle speeds, the officer could be qualified to be an expert and offer an opinion on the speed of the vehicle.


Types of expert witnesses

Regardless of the subject area, there is sure to be someone with the knowledge and experience to qualify as an expert witness. Commonly used experts include the following:

  • Medical examiners
  • Blood spatter analysts
  • Ballistic experts
  • Chemists
  • Pharmacologists
  • Police and law enforcement procedures experts
  • Psychologists and psychiatrists
  • Forensic accountants
  • Criminologists


How experts are used in California criminal cases

Expert witnesses are used by the prosecution or defense to offer an opinion that is beyond what is considered to be common knowledge. An expert’s testimony should assist jurors in understanding something that requires specialized knowledge. For instance, it is the sole province of jurors to decide if a witness is credible, so an expert offered for the sole purpose of testifying about the body language of a previous witness as proof the person was not telling the truth would not be allowed.

Before an expert witness can begin testifying, a series of questions must be asked to establish the person’s qualifications. The attorney for the party seeking to use the expert will ask questions about education, training, scholarly research and experience to prove the expertise of the witness.  Only after the judge presiding over the trial is convinced the person possesses the knowledge and skills to be recognized as an expert on the subject will he or she be allowed to testify.


Author Photo

Kerry L. Armstrong


Attorney Kerry Armstrong opened up his law firm in June 2007. Mr. Armstrong attended Thomas Jefferson School of Law, San Diego, California, and received his B.S. from Middle Tennessee State University. Kerry L. Armstrong became certified by the State Bar of California’s Board of Legal Specialization for criminal law in August 2020, making him one of the few criminal defense attorneys with a criminal law legal specialization certificate in San Diego County.  Between 2014 – 2019, Mr. Armstrong was selected for inclusion in the California Super Lawyers list, an honor only awarded to 5% of the nation’s attorneys.

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