Forensic Science

Forensic science applies scientific methods to legal investigations.

• Pioneers include Locard, whose exchange principle states "every contact leaves a trace."
• Careers include forensic scientist, medical examiner, crime scene investigator, and toxicologist.
• Evidence must follow the chain of custody to be admissible in court.
• Forensic work must be objective, documented, and reproducible.

Careful processing of a scene preserves evidence and reveals what happened.

• Scenes are secured, documented (photos, notes, sketches), and searched systematically.
• Physical evidence is collected, labeled, and preserved to avoid contamination.
• Manner of death: natural, accidental, suicide, homicide, or undetermined.
• Postmortem changes — livor, rigor, and algor mortis — help estimate time of death.

Toxicology detects drugs and poisons in the body.

• Toxicology identifies substances in blood, urine, and tissue.
• The principle "the dose makes the poison" governs toxicity.
• Analysts study absorption, distribution, metabolism, and excretion of substances.
• Findings help determine cause of death or impairment.

Forensic anthropology examines skeletal remains.

• Bones can reveal age, sex, height (stature), and ancestry.
• Trauma on bone can indicate cause of injury or death.
• The pelvis and skull are especially useful for determining sex.
• It helps identify remains and estimate time since death.

Insects on a body help estimate time and circumstances of death.

• Blowflies are often first to colonize remains.
• Insect life-cycle stages (egg, larva, pupa) help estimate the postmortem interval.
• Species present can indicate season, location, and whether a body was moved.
• Temperature affects insect development and must be accounted for.

Blood and body-fluid evidence can link people to a scene.

• Presumptive tests (e.g., Kastle-Meyer/luminol) screen for blood.
• Bloodstain pattern analysis reveals the direction, force, and position of events.
• Other fluids — saliva, semen, sweat — can carry identifying DNA.
• Careful collection prevents degradation and contamination.

DNA is a powerful tool for identifying individuals.

• Except for identical twins, each person's DNA is unique.
• PCR amplifies tiny samples; STR analysis compares repeated sequences.
• A DNA profile can be matched to a suspect or database (CODIS).
• DNA can also exonerate the wrongly accused.

Forensic psychology applies behavioral science to the law.

• Professionals assess competency to stand trial and criminal responsibility.
• Criminal profiling infers offender traits from behavior and evidence.
• They evaluate mental state and risk, and may serve as expert witnesses.
• The insanity defense hinges on a defendant's mental state at the time of the crime.