Forensics

1. Forensic Science: The Intersection of Law and Science

法醫科學是科學與法律的交集。

Defining forensics. Forensic science applies scientific principles and techniques to legal matters, particularly criminal investigations and evidence analysis for court presentation. The term "forensic" originates from the Latin "forum," the ancient Roman public assembly place used for debate and dispensing justice. This highlights the historical link between public discourse, law, and the eventual integration of scientific methods.

Beyond the lab. While often associated with crime labs, forensic science encompasses diverse fields like medicine, chemistry, biology, physics, anthropology, entomology, and psychology. Forensic scientists, or criminalists, specialize in areas such as toxicology, serology, ballistics, and document examination. Their work provides objective evidence to link suspects to crimes, corroborate or refute testimonies, and reconstruct events.

Historical roots. The application of science to law dates back centuries, with early examples like 13th-century China using medical knowledge to solve crimes. Modern forensic science built upon scientific discoveries, such as:

• Karl Landsteiner's ABO blood typing (1901)
• Leone Lattes' method for dried bloodstain typing (1915)
• Calvin Goddard's comparison microscope for ballistics (early 20th century)
• Albert Osborn's foundational work in document examination (1910)
  These advancements, coupled with the organizational principles of pioneers like Hans Gross and Edmond Locard, laid the groundwork for contemporary forensic systems.

2. Evidence: The Silent Witness and Heart of Forensics

凡走過、碰過，甚至是不自覺的，無論他留下什麼，都將成為指證他的無聲證人。

Locard's principle. Edmond Locard's Exchange Principle is the cornerstone of forensic science: every contact leaves a trace. When a person interacts with another person, object, or location, a transfer of material occurs. This exchange, whether visible or microscopic, provides physical evidence that can link individuals to crime scenes.

Types of evidence. Evidence is broadly categorized as direct or indirect, and physical or biological.

• Direct evidence (e.g., eyewitness testimony) is subjective and prone to error.
• Indirect evidence (e.g., blood, fibers, DNA) is objective and often more reliable.
• Physical evidence includes fingerprints, tool marks, fibers, paint, and firearms.
• Biological evidence includes blood, semen, hair, bones, and plant matter.
  All forensic science evidence is inherently indirect, requiring inference to establish facts.

Evidence functions. Evidence serves multiple crucial functions in an investigation:

• Establishing the corpus delicti (facts of the crime)
• Identifying the modus operandi (MO) of the perpetrator
• Linking suspects to victims, locations, or objects
• Verifying or refuting testimonies and alibis
• Identifying perpetrators or victims
• Reconstructing the crime scene
• Providing investigative leads
  The analysis of evidence aims to individualize it, narrowing down the possible sources and ultimately identifying the perpetrator by excluding all others.

3. Body Examination: Autopsy and Identification

死者的身分辨識至關重要。

The autopsy. An autopsy (or post-mortem examination) is a systematic examination of a body to determine the cause, mechanism, and manner of death, and estimate the time of death. Forensic pathologists, specially trained physicians, perform these examinations, often involving both external and internal inspection, microscopy, and ancillary tests like toxicology. The process includes documenting injuries, collecting trace evidence, and identifying the deceased.

Identifying the unknown. Identifying a body is paramount, especially in criminal cases, as most victims are known to their perpetrators. Identification methods vary based on the body's condition:

• Intact bodies: Visual recognition by family/friends, photographs, fingerprints, scars, tattoos, clothing.
• Decomposed/Skeletal remains: Requires specialized expertise from forensic anthropologists and odontologists. Methods include:
  • Estimating age, sex, stature, and ancestry from bones.
  • Analyzing skeletal trauma or disease.
  • Comparing dental records (X-rays, molds) or unique dental features.
  • Using DNA analysis (nuclear or mitochondrial) from bone or teeth.
  • Facial reconstruction from the skull.
    Cases like the O.J. Simpson trial (dental records) and the Xiana Fairchild case (DNA from a tooth) highlight the importance of these techniques.

Challenges. Decomposition, environmental factors (temperature, moisture, predators), and attempts to destroy the body (fire, chemicals) complicate identification. Forensic taphonomy studies how bodies decompose in various environments, aiding in estimating post-mortem interval and locating remains. Despite challenges, forensic scientists can often extract valuable information even from severely altered remains.

4. Time of Death: A Critical Timeline Element

準確估計的死亡時間可以是揭發加害者身分的依據。

Defining death. Determining the precise moment of death is complex, involving physiological (cessation of vital functions), legal (recorded on death certificate), and estimated (forensic pathologist's best guess) times. Only witnessed deaths allow for absolute certainty. In most cases, the forensic pathologist provides an estimated time of death, crucial for establishing timelines and evaluating alibis.

Estimating the interval. Estimating the post-mortem interval (PMI) relies on observing predictable changes in the body after death, though these are highly variable:

• Algor mortis (body cooling): Body temperature cools or warms towards ambient temperature. Rate is affected by environment, body size, clothing. Formula: PMI ≈ (98.6°F - body temp) / 1.5°F per hour.
• Rigor mortis (body stiffening): Muscles stiffen due to ATP depletion. Typically starts within 2 hours, complete by 12 hours, lasts 12 hours, and resolves over the next 12 hours (12-12-12 rule). Influenced by temperature, activity before death.
• Livor mortis (lividity): Blood settles in dependent areas due to gravity, causing discoloration. Starts within 30 mins-2 hours, fixed by 6-8 hours. Can indicate if a body has been moved.
• Decomposition: Breakdown by enzymes (autolysis) and bacteria (putrefaction). Rate depends heavily on temperature, moisture, environment (air, water, buried). General rule: 1 week in air ≈ 2 weeks in water ≈ 8 weeks buried.

Other factors. Additional clues include:

• Stomach contents: Digestion stage can provide a rough estimate of time since last meal.
• Eye changes: Corneal clouding and potassium levels in vitreous humor change predictably.
• Entomology: Insect activity (types, life stages) on the body is a valuable indicator, especially after 72 hours. Forensic entomologists study insect colonization patterns.
• Scene markers: Uncollected mail, dated receipts, stopped clocks, or witness accounts of last sighting can help narrow the timeframe.

5. Bodily Harm and Asphyxia: Interpreting Injuries

法醫務必盡力準確分類死亡方式，因為接下來的發展取決於他的判斷。

Wound analysis. Forensic pathologists analyze injuries (wounds) to determine the cause and mechanism of death and contribute to determining the manner of death (natural, accidental, suicide, homicide, undetermined). Wound characteristics can reveal the type of weapon used, the force applied, the sequence of events, and the relative positions of victim and perpetrator.

Types of injuries:

• Gunshot wounds: Entrance/exit wounds, tattooing, stippling, charring indicate distance. Bullet path helps reconstruct events.
• Sharp force injuries: Stab wounds (deeper than wide), incised wounds (cuts, longer than deep), chop wounds. Characteristics (depth, width, shape, hesitation marks, defensive wounds) suggest weapon type and intent.
• Blunt force injuries: Abrasions (scrapes), contusions (bruises), lacerations (tears), fractures (bone breaks). Patterns can indicate weapon shape. Bruise color changes estimate age. Fractures show force direction and age (callus formation).
• Electrical injuries: Burns at entry/exit points, internal damage. Low voltage can cause fatal arrhythmias; high voltage causes severe burns and respiratory paralysis. Lichtenberg figures (tree-like skin patterns) are unique to lightning strikes.
• Bite marks: Can link a suspect to a victim or object. Unique dental features aid identification (e.g., Ted Bundy case). Can distinguish pre-mortem (bruising) from post-mortem bites.

Asphyxia. Asphyxia is death due to oxygen deprivation. Mechanisms include:

• Suffocation: Environmental (low oxygen), smothering (blocking nose/mouth), choking (airway obstruction), mechanical (external compression), toxic gases (displacing oxygen).
• Strangulation: External pressure on the neck. Manual (throttling), ligature (using a cord), hanging (body weight). Obstructs airways and blood flow to the brain. Petechiae (pinpoint hemorrhages) in eyes are common. Hyoid bone/thyroid cartilage fractures suggest manual strangulation.
• Toxic gases: Interfere with oxygen transport/utilization (e.g., carbon monoxide, cyanide, hydrogen sulfide). CO binds to hemoglobin, turning blood cherry red. Cyanide inhibits cellular respiration.

6. Toxicology: Unmasking Drugs, Poisons, and Toxins

所有一切都有可能是毒物。

The toxicologist's role. Forensic toxicologists identify and quantify drugs, poisons, and toxins in biological samples (blood, urine, tissues) from living individuals or the deceased. Their findings help determine if substances influenced behavior (e.g., impaired driving) or played a role in injury or death. The dose makes the poison; even therapeutic drugs can be lethal at high concentrations.

Investigation process. When poisoning is suspected, toxicologists aim to answer:

• Was a poison involved in the death?
• How was it administered?
• Was the exposure accidental, suicidal, or homicidal?
  Samples are collected from various sites (blood, urine, stomach contents, liver, vitreous humor, hair) to track absorption, distribution, metabolism, and excretion. Samples must be collected before embalming, which can interfere with analysis.

Detection methods. A two-tiered approach is common:

• Presumptive tests: Quick, inexpensive screening tests (e.g., color tests, immunoassays, thin layer chromatography) indicate the possible presence of a substance or class of substances.
• Confirmatory tests: More specific and expensive tests (e.g., gas chromatography-mass spectrometry, infrared spectroscopy) precisely identify and quantify the substance.
  These methods can detect thousands of substances and their metabolites (products of biotransformation).

Interpreting results. Interpreting toxicological findings is complex, considering factors like:

• Route of administration: Oral, injection, inhalation, dermal absorption.
• Concentration: Therapeutic, toxic, or lethal levels. These vary greatly based on individual factors (age, health, tolerance) and drug interactions.
• Acute vs. chronic exposure: High dose vs. repeated low doses. Hair analysis can reveal a timeline of chronic exposure to heavy metals like arsenic.

7. DNA: Your Unique Personal Code

事實上，只有DNA和指紋...是絕對個別化的特徵，從來沒有兩個人擁有相同的DNA或指紋。

The blueprint of life. Deoxyribonucleic acid (DNA) is the genetic material found in the nucleus of nearly every cell. It's a double-stranded molecule forming a double helix, composed of four bases (A, T, C, G). The sequence of these bases forms genes (coding DNA, ~5%) and non-coding DNA (~95%). While most DNA is shared among humans, the non-coding regions contain highly variable segments called polymorphisms, making each individual's DNA profile unique (except identical twins).

DNA fingerprinting. Developed by Alec Jeffreys in 1984, DNA fingerprinting (or typing) analyzes these variable regions to distinguish individuals. Early methods like Restriction Fragment Length Polymorphism (RFLP) required large, intact samples. Modern techniques, like Polymerase Chain Reaction (PCR) combined with Short Tandem Repeats (STR), can amplify tiny or degraded samples, making DNA analysis faster, more sensitive, and widely applicable. STR analysis examines the number of repeating units at specific locations (loci) on the DNA.

Power of identification. By analyzing multiple STR loci (FBI uses 13 core loci), the probability of two unrelated individuals having the same profile becomes astronomically low (trillions to one). A match between a crime scene sample and a suspect's profile is powerful evidence. DNA can be obtained from various biological sources:

• Blood (white blood cells)
• Semen (sperm, epithelial cells)
• Saliva (epithelial cells)
• Hair (follicle cells, mitochondrial DNA in shaft)
• Bones and teeth (osteocytes, pulp)
• Skin cells (touch DNA)

Beyond nuclear DNA.

• Mitochondrial DNA (mtDNA): Found in mitochondria, inherited only from the mother. Useful for degraded samples (hair shafts, old bones) and tracing maternal lineage.
• Y-chromosomal DNA: Found only on the Y chromosome, inherited only from the father. Useful for tracing paternal lineage and identifying male contributors in mixed samples.

Databases. National databases like CODIS (Combined DNA Index System) store DNA profiles from convicted offenders and crime scenes, enabling investigators to link cases and identify suspects across jurisdictions. Cases like Colin Pitchfork (first mass DNA screening) and the Green River Killer (cold case solved with STR/PCR) demonstrate the impact of DNA technology.

8. Fingerprints: A Reliable Identification Tool

脊線用於識別身分是依據三個原則：獨特性、穩定性、一般模式。

Unique patterns. Fingerprints, palm prints, and foot prints are formed by friction ridges on the skin. These patterns are unique to each individual (even identical twins), remain unchanged throughout life (unless deep injury occurs), and can be systematically classified. This makes them invaluable for identification.

Historical development. Fingerprints were used as signatures in ancient China. Scientific study began in the 17th century (Malpighi). Pioneers like Purkinje, Herschel, Faulds, Galton, and Henry developed classification systems and recognized their forensic potential. The Bertillonage anthropometry system was an early attempt at identification but was eventually superseded by fingerprints due to cases like Will West and the Mona Lisa theft.

Classification. Fingerprint patterns fall into three basic types:

• Arches (5%): Ridges rise in the center (plain or tented).
• Loops (60%): Ridges curve back on themselves (radial or ulnar, single or double).
• Whorls (35%): Ridges form circular or spiral patterns (plain, central pocket loop, double loop, accidental).
  The Henry Classification System, based on the presence and type of whorls on each finger, allows for systematic filing and searching of large fingerprint databases.

Finding and collecting prints. Fingerprints can be visible (patent, e.g., in blood), three-dimensional (plastic, e.g., in putty), or invisible (latent). Latent prints are left by the natural oils and sweat on the ridges. Detection methods include:

• Oblique lighting or UV/laser light.
• Fingerprint powders (black, gray, magnetic, fluorescent) adhere to residues.
• Chemical treatments (cyanoacrylate fuming, iodine fuming, ninhydrin, silver nitrate) react with residues on porous surfaces.
• Specialized techniques for blood prints (luminol, amido black).
• Digital enhancement improves visibility of faint prints.

Databases. Automated Fingerprint Identification Systems (AFIS) digitize and store fingerprint images, allowing rapid searching and comparison of unknown prints against vast databases. While AFIS provides potential matches, a trained examiner performs the final comparison to confirm identity. Cases like the Night Stalker demonstrate AFIS's speed and effectiveness.

9. Impressions and Trace Evidence: The Small Details That Link Crimes

羅卡交換定律在細微證據的位置、採集和分析上，顯得非常有力。

Impressions. Impressions are patterns left by objects, such as shoes, tires, or tools. Like fingerprints, they can be patent (visible), plastic (3D), or latent (invisible). Impressions link suspects to crime scenes, help reconstruct events, connect multiple crime scenes, and estimate the number of perpetrators.

Shoe prints: Shoe soles have unique patterns (tread design) that identify manufacturer and size (class evidence). Wear patterns, cuts, or embedded debris create
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Last updated: May 22, 2025