In any court case, the prosecution and defense both rely on a variety of evidence to convince the judge or jury of their argument. Some forms of evidence are real (like a bloody weapon found at the crime scene) and others are demonstrative, like photographs or written documents. And then there’s forensic evidence—the results of scientific tests and analysis that can help prove or disprove the case. Forensic evidence often comes in the form of trace evidence (like hair or fibers), blood spatter, fingerprints and ballistics evidence (bullets and casings).
A forensic scientist will use various laboratory methods to test, examine, compare, process and analyze evidence. For example, they may use a technique called Fourier transform infrared spectrometry to look at the chemical composition of paint samples. They can also conduct a serology analysis to identify the presence of body fluids such as blood or semen on a piece of evidence and then perform DNA analysis to find matches in suspect profiles.
Often, forensic scientists are called to testify at trial. They will give their expert opinion under oath about the observed, factual results of testing or analysis (i.e., the evidence) and what it means. This is known as testimony and forensic experts are required to follow Department of Justice guidance documents known as the Uniform Language for Testimony and Reports or ULTRs.
However, even expert witnesses can have biases that influence their judgment. It is the job of a skilled legal team to expose these biases during cross-examination and weaken the credibility of the evidence that is presented at trial.
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