Chapter 1: Introduction to Forensic Science

A central tenet guiding forensic investigation is Locard’s exchange principle, which posits that whenever two objects or persons interact, a cross-transfer of material evidence occurs. The field has developed through the contributions of numerous figures who established key forensic disciplines, including Mathieu Orfila, recognized as the father of forensic toxicology; Alphonse Bertillon, who devised the early identification system known as anthropometry; Francis Galton, who established the foundational methodology for classifying fingerprints; Leone Lattes, who developed a technique for typing dried bloodstains; Calvin Goddard, who pioneered the use of the comparison microscope for firearms analysis; and Albert S. Osborn, whose work led to the judicial acceptance of questioned document examination. The growth of crime laboratories in the United States, which operates a decentralized system of federal, state, and local facilities, has accelerated significantly due to changes in judicial requirements (less reliance on confessions), rising crime rates, massive increases in drug analyses casework, and the demanding needs of DNA profiling. A full-service crime laboratory typically provides essential services across five units: the Physical Science Unit (analyzing drugs, glass, paint, soil using chemistry and physics principles), the Biology Unit (identifying and performing DNA analysis on body fluids, hair, and fibers), the Firearms Unit (examining weapons, ammunition, and tool marks), the Document Examination Unit (analyzing handwriting and paper), and the Photography Unit. Beyond laboratory analysis, a forensic scientist functions as an expert witness in the courtroom, offering opinions based on specialized knowledge, skill, and experience to aid the judge and jury in understanding evidence. The admissibility of scientific evidence has been governed historically by the Frye standard, requiring "general acceptance" within the scientific community. However, the 1993 Supreme Court ruling in Daubert v. Merrell Dow Pharmaceuticals, Inc. established that trial judges act as "gatekeepers," using flexible criteria (such as testability, error rate, and peer review) to determine reliability under the Federal Rules of Evidence. This gatekeeping role was later extended to all technical and specialized expert testimony by the Kumho Tire decision. Recent legal precedent also emphasizes the constitutional right to confrontation, mandating that the original analyst who performed or observed the test must testify in court, not a substitute or affidavit. Finally, law enforcement has access to specialized services external to the traditional lab, including forensic odontology, forensic engineering, and digital/computer analysis.