Basic Laws of Forensics

A forensic scientist examines physical evidence

Forensic science is the collection, identification and analysis of scientific evidence pertaining to legal criminal and civil cases. It's been an integral part of the legal system for centuries. The word "forensic" is derived from the Latin "foris" or outside, from which we get "forum." In ancient Rome, the forum was the public space where trials were held. Though the science covers a broad range of disciplines--from forensic DNA analysis to forensic entomology to basic criminalistics--there are a few strong principals on which the entire field rests. Professionals return to these essentials again and again throughout their careers to aid in their investigative analysis.

The Exchange Principle

The Locard Exchange principle underlies all trace evidence investigations. The principle states: "With contact between two items, there will be an exchange." In 1915, Edmond Locard founded the first crime lab in history in Lyon, France. He stated that every contact at a crime scene leaves a trace behind, no matter how infinitesimal.

Successful Conclusions

Successful forensic conclusions are reached through the objective application of time-tested scientific principals and logical reasoning. Hunches and intuition are wonderful in Hollywood movies and television shows, but in real life most cases are resolved through careful deductive and inductive reasoning. Induction is making a conclusion from the specific case and applying it to the general, while deduction begins with the general principle and ends with the specific case.

The Chain of Evidence

The chain of evidence must be protected. It doesn't matter if you find the "smoking gun" if the District Attorney can't use it. A Chain of Custody form is an essential document; when properly completed, it provides information about who handled the physical evidence each step of the investigation. The collection of physical evidence is the most critical step in the investigative process; it must be carried out "by the book." Failure to handle objects carefully, to properly and meticulously tag, bag, photograph and document each step of the way, will result in evidence that has no value in the court of law.

Forensic Reports

Forensic reports are imperfect because they are written by people. Even a DNA analysis match can be incorrect. The collection of evidence depends on many professionals' competence, objectivity, integrity and skill. A piece of evidence can be damaged or planted at the scene. It can be tainted by a careless lab assistant. A crime lab can fail to carry out important tests in an effort to save time or money. Pressure from law enforcement authorities to rush results and make an arrest can poorly influence a scientist's conclusions.

References

• Promega: Solving a Crime Using DNA Analysis and Chemistry (Basic Forensics) Instructor's Guide
• Forensic Sciences and Forensics: Science in Law
• "Forensic Science and Law: Investigative Applications in Criminal, Civil, and Family Justice"; Cyril H. Wecht and John T. Rago; 2005

Resources

• American Academy of Forensic Sciences
• Scientific Testimony: A Beginner's Primer on the Investigation of Forensic Evidence