The Role of Expert Testimony in Forensics

By Douglas P. Lyle

Judges typically allow a great deal of leeway with regard to how expert witnesses present information to the jury. Most witnesses are permitted only to answer questions. If they attempt to move too far afield of the question, the judge will rein them in.

Experts, on the other hand, are allowed to depart from the normal Q-and-A format because their often-technical testimony necessitates explanation. Indeed, the expert teaches the judge and jury. For example, understanding the impact of DNA evidence is difficult for the average juror when presented simply from a series of questions with yes or no answers. Enabling the expert to explain what DNA is, how it is tested, and what the results of the testing mean gives the jurors the knowledge they need to understand and evaluate the evidence.

Rarely will an expert witness express testimony as an absolute fact, especially when being challenged by the other side. Instead, the wise expert uses phrases such as “similar to,” “consistent with,” “not dissimilar from,” “compatible with,” or “shares many characteristics with.” Why is this?

The truth is that forensic evidence rarely, if ever, is absolute but rather states probabilities. For example, except for identical twins, no two people have the same DNA, but the testifying expert should never say the DNA “absolutely matches” that of the defendant. Instead, the expert should say that the probability that it matches someone other than the suspect is a billion to one. That is almost, but not quite, absolute.