Working With Forensic Expert Witnesses: A Litigator’s Guide

Christine Funk

Written by
— Updated on June 23, 2020

Working With Forensic Expert Witnesses: A Litigator’s Guide

Forensic Expert Witnesses

If you have a case involving forensic science, it’s a good idea to hire a forensic expert witness. However, before you go out and hire the first forensic scientist you find in the relevant field, consider why you are hiring a forensic expert witness.  Reasons to hire a forensic expert include the following:

  1. Gathering background information about the forensic science in general
  2. Observing testing performed by another forensic scientist
  3. Reviewing work already performed by an expert for the other side
  4. Providing independent testing
  5. Observing or reviewing deposition or trial testimony
  6. Providing deposition or trial testimony

In some cases, you may choose to hire an expert to perform a combination of these functions.  However, these decisions must be made after consideration of how this may benefit your position, or whether it could be to your detriment.

Case Hypothetical

Imagine a case, either criminal or civil, which includes the following facts:

  1. A person shot to death “at close range”
  2. A shotgun which may have both fingerprints and DNA on it, possibly belonging to the shooter
  3. The body discovered in a house that has burned, where the fire may have been intentional or accidental

Background Information

Particularly if you have never had exposure to a given forensic science before, it might be a good idea to retain a forensic expert for background information.  Consider, for example, our hypothetical.  Which areas of the gun might be best to examine for fingerprints?  Are some areas of the gun more likely to yield a DNA profile?  Can you lift a print and then test for DNA evidence, or should you do it the other way around?

By consulting a forensic expert, you can gain general background information which may provide insight on how to proceed with testing, or alternatively, may provide information on whether the testing was done appropriately in your case.

Observing Testing

Observing testing done by the other side is a double-edged sword.  On the positive side, people tend to be a bit more careful when they know they are being observed.  If an analyst misses a step or does something in error, an observing expert may say something to stop the error. Additionally, if an analyst makes a mistake, the observer will see this and document it.

On the negative side, if the testing expert does everything right, sending your own expert to observe the testing renders them a witness the other side may call to confirm the original testing was conducted properly.

When considering retaining an observing witness, some ground rules should be established ahead of time.  If the observer views a possible error (such as failing to change gloves between handling pieces of evidence for DNA testing), does the observing expert stop the testing and say something?  Or are they only allowed to stand and watch without offering input regardless of whether some valuable evidence might be lost? Additionally, a discussion about whether the other side might be able to call the witness to testify should occur prior to observing testing.

Reviewing Another’s Work

Scientists take the view if it isn’t written down, it didn’t happen.  Thus, when scientists perform scientific testing, they document their work.  An arson investigator documents the scene, including the floor plan of the home, potential points of origin for the fire, and other relevant factors.  DNA scientists and fingerprint experts will draw sketches or photograph the firearm, noting where relevant evidence was observed or collected.  A medical examiner takes copious notes, runs lab tests, and photographs the body.

Sometimes it makes sense to hire an expert to review the work of another forensic scientist.  From the collection of evidence at the scene to laboratory testing to conclusions based on the evidence, every step of a scientists’ approach is subject to evaluation, and possible criticism.  In many cases, a lawyer can send the evidence to a consulting forensic science expert without disclosing the expert’s identity, or even that an expert is reviewing the file.  Sometimes, however, the situation requires disclosure of an expert.  For example, if a medical examiner creates laboratory slides, the expert may need to go to the medical examiner’s office to view the slides in person.  In those cases, again, consideration should be given to a discussion wherein both sides agree the consulting expert will not be called by the opposing party as a witness for that side.

Independent Testing


There are two types of independent testing.  First, a lawyer may wish to retest the evidence already tested.  Second, the lawyer may wish to perform different testing than that performed by an expert for the other side.  Particularly with DNA evidence, lawyers often find retesting the evidence a tempting approach.  However, a consultation with an expert may dissuade the lawyer.  This is because of the nature of DNA evidence.

First, DNA is deposited on a sample now in question – for our case, let’s say it’s the shotgun, with presumed DNA from the shooter on the gun.  If one side tests the gun and finds a male DNA profile matching an accused, scientists will tell you the DNA is explained in one of three ways:

  1. The accused deposited the DNA found
  2. The accused’s DNA somehow contaminated the gun sample at some point, either during collection, processing, or testing
  3. A coincidental match explains the evidence

Retesting the gun will not provide any information about which of these three scenarios is correct.  Instead, it will only confirm the presence of the DNA on the gun sample.  On the other hand, evaluating the raw data and the conclusion the DNA matches the accused may yield a different result.  In this type of case, retesting may not be the best use of funds.

New Testing

On the other hand, in some cases, lawyers seek the answer to a different question.  For example, one side may examine the shotgun and determine, based on an analysis of the firing pin and test fires, the shotgun likely fired a certain cartridge casing.  However, imagine the other side wants to know the distance between the deceased and the fired gun.  Perhaps the accused shooter tells a version of events in which he shot in self-defense.  Perhaps the shooter and other witnesses claim the deceased was approaching him with a knife.  An attorney may request an expert determine the width of the pellet spray at 2 feet, 4 feet, 6 feet, etc.  By comparing the width of the fatal wound to the width of the test fired pellet spray, an expert may determine the distance between the shooter and the deceased.

Observing Testimony

Whether during a deposition or testimony at a hearing or trial, there is some merit to hiring a forensic expert to observe the testimony.  Just as when observed while testing, people tend to exercise more care, and choose words more precisely when observed.  Attorneys may also benefit from hiring an expert to review previously given testimony.  The expert provides insight regarding where an expert may have glossed over a favorable theory or failed to mention an important study.  An expert can also assist in developing lines of testimony to advance the theory of the attorney.

Testifying Experts

Finally, an attorney may decide to hire a forensic expert to testify.  Just as with other witnesses, forensic experts are not all created equal when it comes to testifying skills.  When hiring a testifying expert, look for someone who is a teacher.  Attorneys should look for a forensic expert who can translate complex scientific issues into words lay people can understand.  A testifying expert should be confident without being condescending.  They should teach without droning on and on.  They should make their points using data, photos, and graphs where these are helpful to the jury.

Determining Appropriate Experts For A Case

Sometimes, even after careful consideration, attorneys may find themselves uncertain about the appropriate type of expert.  In this case, an open discussion with potential expert witnesses can assist in clarifying the issues.

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