Forensic Science Techniques

Why This Matters

Forensic science sits at the intersection of science and law—and your exam will test whether you understand how different techniques work, when they're most useful, and what limitations they carry. You're not just memorizing a list of lab methods; you're learning how physical evidence gets transformed into courtroom testimony. The techniques covered here demonstrate core principles like Locard's Exchange Principle (every contact leaves a trace), individualization versus class evidence, and the chain of custody requirements that make evidence legally admissible.

Think of forensic techniques as falling into categories based on what they analyze and what questions they answer. Some identify individuals with near-certainty; others reconstruct events; still others establish connections between people, places, and objects. When you study these methods, focus on their scientific basis, their evidentiary weight, and their limitations. Don't just memorize what each technique does—know which principle it demonstrates and when investigators would choose one method over another.

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Identification Techniques: Who Was There?

These methods answer the fundamental question of identity. They work by comparing unique biological or physical characteristics to known samples, producing either individualizing evidence (pointing to one person) or class evidence (narrowing to a group).

DNA Profiling

• Analyzes short tandem repeats (STRs) in genetic material—produces a statistical probability of a match that can reach 1 in billions
• Requires biological samples like blood, saliva, hair follicles, or skin cells; even touch DNA from handled objects can yield profiles
• Can both convict and exonerate—the Innocence Project has used DNA to overturn hundreds of wrongful convictions, making this technique central to discussions of justice system reform

Fingerprint Analysis

• Compares ridge patterns classified as loops, whorls, or arches—no two individuals share identical fingerprint characteristics
• Latent prints (invisible to the naked eye) require development using powders, chemicals, or alternative light sources before comparison
• Considered individualizing evidence but relies on examiner judgment; the 2009 NAS Report questioned the lack of standardized error rates in this field

Forensic Odontology

• Uses dental records and bite mark analysis for victim identification when fingerprints or DNA aren't available
• Dental characteristics like fillings, missing teeth, and unique wear patterns can survive conditions that destroy other identifying features
• Bite mark analysis remains controversial—multiple wrongful convictions have raised questions about its scientific reliability, making this a frequent exam topic on forensic limitations

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Reconstruction Techniques: What Happened?

These methods help investigators piece together the sequence of events. They analyze physical patterns and distributions to infer actions, positions, and timing—transforming a static crime scene into a narrative.

Blood Spatter Analysis

• Interprets bloodstain patterns to determine victim and assailant positions, number of blows, and type of weapon used
• Pattern characteristics like size, shape, and directionality reveal the angle of impact and force applied
• Requires understanding of physics—blood behaves predictably as a fluid, but interpretation remains somewhat subjective and has faced legal challenges

Arson Investigation

• Determines fire origin and cause by analyzing burn patterns, V-patterns, and char depth
• Accelerant detection uses gas chromatography to identify substances like gasoline; trained dogs can also locate accelerant residue
• Distinguishes accidental from intentional fires—critical for determining whether a crime occurred at all, which affects both criminal charges and insurance claims

Forensic Anthropology

• Analyzes skeletal remains to establish biological profile: age, sex, ancestry, and stature
• Trauma analysis can reveal cause of death, including distinguishing between blunt force, sharp force, and gunshot injuries
• Estimates postmortem interval through bone weathering and associated evidence; essential in cases involving unidentified remains or mass disasters

Forensic Entomology

• Uses insect succession patterns to estimate time since death—different species colonize remains in predictable sequences
• Maggot development stages provide a biological clock; environmental factors like temperature affect growth rates and must be calculated
• Most useful in later decomposition stages when other time-of-death indicators have degraded

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Linkage Techniques: How Are They Connected?

These methods establish connections between suspects, victims, and crime scenes. They apply Locard's Exchange Principle: whenever two objects come into contact, material transfers between them.

Trace Evidence Analysis

• Examines microscopic materials including hair, fibers, glass fragments, soil, and paint chips transferred during contact
• Provides class evidence that narrows possibilities—a fiber match doesn't prove guilt but can place a suspect at a scene
• Requires meticulous collection protocols to prevent contamination; analysts must document chain of custody for courtroom admissibility

Ballistics and Firearms Examination

• Matches bullets and cartridge cases to specific weapons through unique markings left by the barrel, firing pin, and ejector
• Rifling patterns (lands and grooves) are class characteristics; microscopic striations are individualizing
• Determines shooting distance and trajectory—gunshot residue patterns and wound characteristics help reconstruct positioning

Impression Evidence Analysis

• Compares shoe prints, tire tracks, and tool marks found at crime scenes to known exemplars
• Class characteristics (brand, size, tread pattern) narrow the field; individual characteristics (wear patterns, damage) can link to a specific item
• Requires proper casting and photography—three-dimensional impressions in soil or snow need careful preservation before comparison

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Document and Digital Analysis: What Does the Record Show?

These methods extract evidence from written and electronic records. They address authenticity, authorship, and the recovery of hidden or deleted information.

Questioned Document Examination

• Analyzes handwriting characteristics, paper, and ink to determine authenticity or authorship
• Examines alterations and obliterations—infrared and ultraviolet light can reveal erased or overwritten text
• Critical in fraud, forgery, and contested will cases—establishes whether documents are genuine or fabricated

Digital Forensics

• Recovers and analyzes data from electronic devices including computers, smartphones, tablets, and cloud storage
• Metadata analysis reveals when files were created, modified, or accessed—often contradicting suspects' timelines
• Essential for cybercrime and increasingly relevant in traditional crimes where digital evidence (texts, location data, social media) provides alibis or incrimination

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Behavioral and Biological Analysis: Why Did It Happen?

These methods examine the human element—what substances were involved and what psychological factors may have contributed. They bridge the gap between physical evidence and human behavior.

Forensic Toxicology

• Detects drugs, alcohol, and poisons in bodily fluids and tissues through gas chromatography and mass spectrometry
• Determines cause of death or impairment—distinguishes between therapeutic levels, overdose, and poisoning
• Critical in DUI cases, overdose deaths, and suspected poisonings—quantitative results establish whether substances reached harmful concentrations

Forensic Psychology

• Applies psychological principles to legal questions—competency to stand trial, mental state at time of offense, risk assessment
• Supports criminal profiling by analyzing behavioral patterns, though profiling remains more art than science
• Evaluates witness reliability and interrogation methods—increasingly important given research on false confessions and memory distortion

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Crime Scene Foundation: Where Do We Start?

This foundational method underlies all other forensic techniques. Without proper scene processing, even the most sophisticated laboratory analysis becomes worthless in court.

Crime Scene Investigation

• Systematic documentation, collection, and preservation of all potential evidence following established protocols
• Chain of custody must be maintained from scene to courtroom—any gap can result in evidence being ruled inadmissible
• First responders' actions are critical—contamination or improper handling can destroy evidence before specialists arrive

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Quick Reference Table

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Self-Check Questions

1. Which two forensic techniques provide individualizing evidence that can identify a specific person, and what makes their evidentiary standards different?

2. A decomposed body is discovered in a wooded area. Which three forensic specialties would likely be called to the scene, and what specific question would each one answer?

3. Compare and contrast trace evidence analysis and impression evidence analysis—how do both demonstrate Locard's Exchange Principle, and what type of evidence does each typically produce?

4. An FRQ asks you to evaluate the reliability of forensic techniques. Which two methods discussed have faced significant criticism regarding scientific validity, and what concerns have been raised about each?

5. How do forensic toxicology and forensic psychology both address questions of criminal intent, and in what type of case might both be relevant to the same defendant?