Why This Matters
Forensic evidence is the backbone of modern criminal investigations, and understanding the different types—along with their strengths and limitations—is essential for criminology exams. You're being tested on more than just definitions; you need to understand how evidence is collected, analyzed, and used in court, as well as the scientific principles that make each type reliable (or potentially problematic). These concepts connect directly to broader themes like the scientific method in criminal justice, chain of custody, and the balance between investigative power and civil liberties.
When studying forensic evidence, think in terms of individualization versus class characteristics, transfer theory (Locard's Exchange Principle), and reconstruction of events. Don't just memorize that DNA comes from blood samples—know why DNA evidence revolutionized forensics and how it differs from pattern evidence like fingerprints or tool marks. The exam will ask you to compare evidence types, evaluate their reliability, and apply them to hypothetical scenarios.
Biological Evidence: The Body Tells the Story
Biological evidence relies on materials produced by or shed from the human body. These samples contain genetic information or chemical signatures that can identify individuals or reveal physiological states at the time of a crime.
DNA Evidence
- DNA profiling provides individualization—the statistical probability of two unrelated people sharing a DNA profile is often 1 in billions, making it the gold standard for identification
- Collection sources are diverse: blood, saliva, skin cells, hair roots, and other biological materials all contain nuclear DNA suitable for analysis
- CODIS (Combined DNA Index System) allows comparison of crime scene samples against a national database of convicted offenders and unsolved cases
Toxicology
- Identifies foreign substances in the body—drugs, alcohol, poisons, and medications can be detected and quantified through chemical analysis
- Critical for determining cause of death or establishing impairment in cases involving overdose, poisoning, or intoxicated driving
- Sample types affect detection windows: blood shows recent use, urine extends the window, and hair can reveal months of substance history
Blood Spatter Analysis
- Reconstructs the dynamics of violent events—analysts interpret the size, shape, and distribution of bloodstains to determine positions and movements
- Pattern types indicate mechanism: passive drops fall due to gravity, projected patterns result from arterial spurts, and impact spatter comes from blows or gunshots
- Angle of impact calculations use the shape of individual stains to determine the direction blood was traveling when it struck a surface
Compare: DNA evidence vs. blood spatter analysis—both involve blood, but DNA answers who while spatter analysis answers what happened. On an FRQ about reconstructing a crime scene, you'd use spatter for sequence of events and DNA for suspect identification.
Pattern and Impression Evidence: Marks That Match
Pattern evidence involves comparing questioned marks or impressions to known standards. The underlying principle is that objects with unique surface characteristics leave distinctive marks that can be matched through careful examination.
Fingerprints
- Friction ridge patterns are permanent and unique—they form before birth and remain unchanged throughout life, barring injury or intentional alteration
- Three pattern types (loops, whorls, and arches) provide classification, while minutiae points like ridge endings and bifurcations enable individualization
- AFIS (Automated Fingerprint Identification System) rapidly searches databases but requires human verification of potential matches
- Class characteristics narrow the field—tread pattern, size, and manufacturer can identify the type of shoe or tire present at a scene
- Individual characteristics enable matching: cuts, wear patterns, and embedded debris create unique features that can link a specific item to an impression
- Casting and photography techniques preserve three-dimensional impressions in soft surfaces like mud or snow for laboratory comparison
- Manufacturing and use create unique striations—every tool develops microscopic imperfections that transfer to surfaces it contacts
- Comparison microscopy allows examiners to view questioned marks alongside test marks made by a suspect tool side-by-side
- Common in property crimes: pry marks on doors, cut marks on locks, and screwdriver impressions can connect tools found in a suspect's possession to the crime scene
Compare: Fingerprints vs. tool marks—both rely on unique surface characteristics for matching, but fingerprints identify people while tool marks identify objects. Both require examiner interpretation, raising questions about subjectivity in pattern evidence.
Firearms Evidence: Projectiles and Propellants
Firearms analysis combines physics, chemistry, and pattern matching to connect weapons to crimes. Every firearm leaves distinctive marks on ammunition components, and the discharge process deposits detectable residues.
Ballistics and Firearm Evidence
- Rifling creates unique striations—the spiral grooves inside a gun barrel impart distinctive marks on bullets that can be matched to a specific weapon
- Cartridge casings retain firing pin and ejector marks that are equally individualized and often easier to recover than bullets
- NIBIN (National Integrated Ballistic Information Network) functions like AFIS for firearms, comparing ballistic evidence across jurisdictions
Gunshot Residue (GSR)
- Primer compounds deposit on hands and clothing—barium, antimony, and lead particles form distinctive spherical particles detectable by scanning electron microscopy
- Presence indicates proximity to discharge but cannot definitively prove who fired; residue transfers easily and dissipates within hours
- Collection timing is critical: suspects should be tested as soon as possible, as normal activity removes residue from skin
Compare: Bullet striations vs. GSR—striations can identify the specific weapon used, while GSR only indicates someone was near a discharged firearm. Striations are more probative for linking a gun to a crime; GSR is more useful for placing a suspect at the scene.
Trace Evidence: Locard's Exchange Principle in Action
Trace evidence embodies the principle that every contact leaves a trace. These small, often overlooked materials can establish connections between people, places, and objects.
Trace Evidence (Hair, Fibers, Soil)
- Transfer and persistence determine evidentiary value—fibers from a suspect's clothing on a victim demonstrate contact, while soil on shoes places someone at a location
- Hair analysis provides class characteristics: species, body area, racial origin, and treatment history, though only roots with follicular tissue yield DNA
- Fiber comparison examines color, composition, and microscopic structure; rare or unusual fibers have greater evidentiary weight than common materials
Compare: Trace evidence vs. DNA—trace evidence often provides class characteristics (this fiber is consistent with that carpet), while DNA provides individual identification. Trace evidence excels at establishing contact and corroborating other evidence rather than standing alone.
Modern investigations increasingly rely on evidence that exists as information rather than physical objects. Analysis focuses on authenticity, origin, and the recovery of hidden or deleted data.
Digital Forensics
- Data persistence enables recovery—deleted files often remain on storage media until overwritten, allowing forensic recovery of incriminating evidence
- Metadata reveals context: timestamps, geolocation, device identifiers, and user accounts can establish when, where, and by whom files were created or accessed
- Chain of custody is paramount—forensic imaging creates exact copies of digital media, preserving the original while allowing analysis
Questioned Documents
- Handwriting comparison examines individual characteristics like letter formations, spacing, and pen pressure to determine authorship
- Physical analysis of materials—ink composition, paper fiber content, and printing methods can reveal alterations, date documents, or identify their source
- Indented writing recovery uses specialized lighting or electrostatic detection to reveal impressions from pages that were on top of the document being examined
Compare: Digital forensics vs. questioned documents—both analyze information-based evidence, but digital forensics deals with electronic data while questioned documents examine physical materials. Both require demonstrating authenticity and can reveal attempts at concealment or deception.
Quick Reference Table
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| Individualization (unique ID) | DNA, fingerprints, bullet striations |
| Class characteristics | Fiber type, shoe size, blood type |
| Locard's Exchange Principle | Trace evidence, GSR, fibers |
| Event reconstruction | Blood spatter, ballistics trajectory |
| Physiological state | Toxicology, DNA (for identification) |
| Pattern matching | Fingerprints, tool marks, footwear impressions |
| Digital information recovery | Digital forensics, questioned documents |
| Database searching | CODIS (DNA), AFIS (fingerprints), NIBIN (ballistics) |
Self-Check Questions
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Which two evidence types rely on national databases for comparison, and how do their matching processes differ in terms of human examiner involvement?
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A suspect claims they were never at the crime scene. Which types of forensic evidence would be most useful for placing them there, and what are the limitations of each?
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Compare and contrast DNA evidence and fingerprint evidence in terms of their scientific basis, collection methods, and reliability in court.
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If an FRQ asks you to evaluate the strength of pattern evidence versus biological evidence, which examples would you use to illustrate the distinction between individualization and class characteristics?
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How does Locard's Exchange Principle connect trace evidence, GSR, and fiber analysis? Provide a hypothetical scenario where all three might be relevant to the same investigation.