Interview with Dr Jason Chin: Science in a Criminal Trial

By Penelope Bristow

What kind of scientific evidence might be lead in a criminal trial? In Australia, are there particular rules of evidence that apply to such scientific evidence?

As a general rule, scientific evidence is available whenever it makes a fact in issue more or less likely to be true. So that opens the door for any kind of science. For example, a marine biologist might be called to discuss whether a part of the ocean was frequented by sharks at a certain time of year to explain why a body wasn’t found.

But more often, the science comes in the form of “forensic science”, which is a term that just means science as applied to law. Some examples of forensic scientific fields are DNA and fingerprint identification.

In Queensland, the rules come from the common law: the evidence must be beyond common knowledge and from a person qualified in an established field of study. In most other States, uniform evidence law provisions apply. Note, however, that opinion evidence is an exception to the general rule that witnesses can provide only factual evidence. Thus, the party tendering the evidence bears the onus of proving the expert evidence meets the common law rule.

Generally, how do courts evaluate scientific evidence?

Whenever bodies of leading judges and scientists have convened to discuss scientific evidence, they have agreed that trial judges should carefully scrutinize scientific evidence and only admit it if it is demonstrably valid.

Unfortunately, trial judges don’t do that. Instead, they mainly just defer to the expert witnesses. This can be problem because many forensic sciences have never been independently tested. This is why approximately 60% of wrongful convictions contain invalid or misleading scientific evidence.

What role do expert witnesses and lay witnesses play in a criminal trial? What is the difference between an expert witness and a lay witness?

Both laypeople and experts can give opinions. Most commonly, lay witnesses offer eyewitness identification evidence: “That is the person I saw robbing the bank.” Expert witnesses’ opinions are also opinions but they can go beyond their personal observations. For example, the lay witness I just mentioned is comparing a mug shot to her experience seeing someone rob a bank. On the other hand, the expert witness is not relying on just her own observations, but on a body of research (performed by others) that she studies.

What kind issues do courts face in relation to expert witnesses giving opinions at a criminal trial? What can influence an expert witness’ evidence?

Generally, courts are supposed to figure out if the expert’s methodology is valid and reliable enough to go to the jury. It may not be valid, for instance, if the expert has been exposed to case details that could bias her decision. For instance, if she knew that the fingerprints she was matching belonged to the suspect in a high-profile act of terrorism, she might be biased towards declaring a match. This is actually what happened in the investigation of the 2004 Madrid train bombing. An Oregon lawyer, Brandon Mayfield, was identified as the bomber on the basis of a fingerprint match. The FBI examiner declared it an 100% match. In fact, Mayfield wasn’t even in the country at the time. The actual bomber was later identified.

What is the CSI effect and how can it affect a jury?

The CSI Effect is the hypothesized effect of a juror’s familiarity with crime scene dramas on her evaluation of forensic evidence. As the story goes, these dramas portray a very sophisticated science and thus when a juror is not presented such impressive evidence in court, she thinks the police have done a shoddy job. She is thus less likely to convict.

There’s simply no evidence that this is the case and I think people should move on from it and study more important and plausible things.

If you’d like to hear more about any of these topics, take LAWS5182 (Psychology and Law), which is being offered in the first term of 2018.