Forensic Science Fundamentals

Forensic Science Fundamentals

• Forensic science is any scientific application used in a legal context, public or court-based.
• Criminalistics, a sub-discipline of forensic science, focuses on analyzing crime scene evidence to reconstruct events.

Case Studies and Courts

• DNA analysis helped solve the Penny Parker case, linking Don Jennings' DNA to the semen found. This link was exceptionally strong, 10,000 times more probable than a random match.
• Civil courts aim to compensate victims (plaintiffs) for harm by defendants. Guilt is established by a "preponderance of evidence" (more than 50% likely).
• Criminal courts aim to uphold societal order, punishing wrongdoers. Guilt requires "proof beyond a reasonable doubt" (estimated 90% or greater likelihood).
• Courts function on an adversarial system where opposing attorneys present evidence and arguments before a jury or judge.

Expert Witnesses

• Expert witnesses provide impartial interpretations of evidence. Their expertise can come from special knowledge, skills, experience, training, or education.
• Expert testimony should be based on established scientific principles, methods, and literature. They must remain within their areas of expertise.
• Expert witnesses can be challenged by opposing counsel within established legal frameworks.

Crime Laboratories

• Crime labs have proliferated without unified national/regional planning. Over 350 public labs exist at various governmental levels.
• Factors driving crime lab growth include Supreme Court rulings enhancing scientific evidence, escalated drug abuse, and DNA profiling.
• Crime labs perform services like toxicology analysis (alcohol/drugs/poisons), DNA analysis (identification/databases), firearms analysis, and trace evidence analysis.

Criminalist Specializations

• DNA: Analyze biological samples (blood, bone, hair) using STR technology and equipment like thermocyclers and capillary electrophoresis. Expert testimony and DNA statistics are crucial tasks.
• Toxicology/Alcohol: Analyze blood/breath/urine for alcohol and drugs, explaining impairment and safety risks, often in court.
• Trace Evidence: Analyze diverse materials (paints, fibers, soils) using sophisticated instruments (SEM, laser ablation, GC-MS). This often the sole source of evidence in many homicide cases.
• Firearms: Analyze bullets/cartridges, reconstruct shootings, and identify gunshot residue, using tools like microscopes and ballistics comparison systems.

Educational Requirements

• A science degree (biology, chemistry, biochemistry, etc.) is essential, along with significant chemistry and quantitative analysis coursework.
• Specificity in forensic science degrees is less useful for lab careers compared to rigorous science training.

The CSI Effect

• The "CSI effect" impacts jury perceptions of forensic science, demanding more forensic evidence due to unrealistic portrayals.

Crime Scene Investigation Methods

• The goal of criminalistics is establishing associations between evidence and suspects.
• The Locard Exchange Principle posits that every contact leaves a trace.
• A forensic diamond (victim, scene, suspect, evidence) is used to connect evidence.

Types of Evidence & Analysis

• A wide range of evidence types can be analyzed, from animal hairs to digital forensics. Different types of analyses include DNA, explosives, fingerprints and various other materials (glass, hair, ink, paint, polymers, rope, soil, textiles, toolmarks, etc).
• Controls and comparison are essential for establishing associations.

Estimating Time of Death

• Rigor mortis (muscle stiffening), Livor mortis(blood pooling) and Algor mortis(cooling) are unreliable markers of elapsed time.
• Stomach content (stage of digestion) can roughly estimate the time since the last meal.
• Vitreous humor potassium (rapid increase in potassium after death) can be used as a more reliable measure, with greatest accuracy about 12 hours after death.
• Extensive additional factors, including environmental clues, daily routines, or microbiome analysis, contribute to time-of-death estimation.
• These factors cannot stand alone for determining time-of-death alone.

Scientific Methodology

• Scientific method principles (testability, repeatability) guide forensic investigations.
• Bloodstain pattern analysis (BPA) reconstructs events by analyzing patterns, sizes, and directions of bloodstains.
• Key points in bloodstain pattern analysis: passive, spattered, altered patterns.

Evidence in a Legal Context

• Evidence forms the basis for legal investigations and trial decisions (judges, juries).
• Real and demonstrative evidence are types of evidence.
• The Locard Exchange Principle suggests that contact results in evidence exchange.
• Evidence persistence, the ability to transfer and remain in a state, may undergo numerous changes as well as loss before investigation.
• Evidence transfer and persistence depend on various factors.
• Essential concepts of forensic association include classifying items based on identity, class or individualisation, and using known and questioned materials. Type 1 and Type II errors are important factors to consider in forensic science.