Forensic Evidence
Forensic evidence is scientific evidence used to prove or disprove facts in a criminal case. It includes DNA analysis, toxicology, ballistics, fingerprint comparison, digital forensics, document examination, accident reconstruction, and many other specialized disciplines. Forensic evidence is often presented through expert witnesses and is subject to special rules of admissibility under R v Mohan, 1994 and R v White Burgess Langille Inman, 2015.
Mass Tsang's criminal lawyers work with forensic experts across disciplines to test the Crown's case rigorously. For more on trial evidence, see our blog post on evidence in criminal trials.
Common categories
DNA evidence is used to connect (or exclude) suspects in cases involving biological material — sexual assaults, homicides, break-and-enters. Toxicology proves the presence and concentration of drugs or alcohol — central to impaired driving and many drug cases. Ballistics matches bullets and casings to firearms. Fingerprint and shoeprint evidence places persons at scenes. Digital forensics extracts data from phones, computers, and other devices. Accident reconstruction reconstructs the dynamics of motor vehicle collisions.
Admissibility — the Mohan/White Burgess framework
Expert opinion evidence is admissible where: (1) the evidence is logically relevant; (2) the evidence is necessary to assist the trier of fact; (3) there is no exclusionary rule barring it; (4) the witness is properly qualified; and (5) the underlying science is sufficiently reliable (the White Burgess gatekeeping function). The court must also weigh probative value against prejudicial effect — particularly important for novel or contested scientific techniques.
Challenging forensic evidence
Defence challenges to forensic evidence target: chain of custody (handling, storage, transport of samples); contamination (cross-contamination during collection or analysis); reliability of the underlying methodology (some traditional disciplines, such as bite-mark and hair comparison, have been seriously discredited in recent decades); qualifications of the expert; the analyst's adherence to lab protocols; and the interpretation of the results. A poorly run DNA analysis or a sloppy fingerprint comparison can be dismantled on cross-examination.
Defence experts
Where the Crown's forensic case is strong, the defence often retains its own expert to review and, if appropriate, contest the Crown's analysis. Defence experts can identify analytical errors, propose alternative interpretations, and provide opinion testimony at trial. Legal Aid funding for expert assistance is available in eligible cases.
Related glossary terms
