What is Evidence? A Guide to Types in Forensic Science
In a legal context, evidence is any form of proof—including objects, materials, testimony, or scientific findings—presented to establish or disprove a fact during a legal proceeding. The primary purpose of evidence is to reconstruct events, test hypotheses, and link or exclude individuals from a crime scene. Forensic science is primarily concerned with the analysis and interpretation of physical evidence.
Primary Categories of Evidence
Evidence is generally classified into several key types, each with its role and weight in an investigation and trial:
- Physical Evidence: Any tangible object or material, large or microscopic, that is relevant to a crime. This is the domain of forensic science, which includes items such as fingerprints, DNA, weapons, fibers, soil, documents, and digital files.
- Testimonial Evidence: Statements made under oath by a witness in response to questioning. While crucial to the legal process, this differs from the physical evidence analyzed in a laboratory.
- Direct Evidence: Evidence that, if true, directly proves a key fact without needing any inference. An eyewitness who testifies they saw the defendant commit the crime is providing direct evidence.
- Circumstantial Evidence (Indirect Evidence): Evidence that requires an inference to connect it to a conclusion. Most forensic evidence is circumstantial. For example, a suspect’s fingerprint at the scene doesn’t directly prove they committed the crime, but it requires an inference that they were present at some point. A strong case can be built from multiple pieces of circumstantial evidence.
Characteristics of Physical Evidence
Forensic scientists further classify physical evidence based on its characteristics to determine its significance:
- Class Characteristics: Features that are common to a group of similar objects. For example, a new Nike Air Force 1 shoe, size 10, has class characteristics shared by all other shoes of that same make and model.
- Individual Characteristics: Features that are unique to a single source, to the exclusion of all others. A worn-down tread on the same Nike shoe, with a unique pattern of scratches and nicks, a specific fingerprint, or a complete DNA profile, all possess individual characteristics. The goal of many forensic analyses is to find and compare these unique identifiers.
