Forensics Explained

Forensic science is the use of scientific methods to examine physical evidence and answer questions in a criminal or legal case, from identifying a substance to linking a trace back to its source.[1] It covers work as different as autopsies, DNA testing, fingerprint comparison, and recovering deleted files from a phone.

Most cases follow the same path. Evidence is found and documented at a scene, packaged and logged, analyzed in a laboratory, and then explained by an expert witness in court. This page walks through each stage, the main branches of the field, and where the science is strong or still contested. For definitions of specific terms, see the forensic glossary, and for method-by-method detail, see techniques A to Z.

What forensic science is and how it works

Forensic science works by treating physical evidence as data. A stain, a bullet, a fingerprint, or a hard drive carries information, and the job of the forensic scientist is to recover that information reliably and describe how much confidence anyone can place in it.[1]

The field sits between two worlds. One is the laboratory, where methods are meant to be tested and repeatable. The other is the courtroom, where a judge and jury weigh what the results actually prove. Careful forensic work keeps those two honest with each other, so a lab result is not oversold once it reaches a trial.

The main branches of forensic science

Forensic science covers a group of specialties, each with its own training and tools. The national standards program that NIST runs recognizes 22 forensic disciplines.[1] These are the ones that come up most often in criminal cases:

• Forensic pathology. A medical examiner or forensic pathologist performs autopsies to determine the cause of death (the medical reason, such as a gunshot wound) and the manner of death (for example homicide, suicide, accident, or natural).
• DNA and serology. Serology identifies body fluids such as blood, semen, and saliva. DNA analysis reads the genetic profile in those samples and compares it against a known person or a database.
• Trace evidence. The study of small transfers between people, objects, and places: fibers, hair, glass fragments, paint, soil, and gunshot residue.
• Forensic toxicology. Testing blood, urine, and tissue for alcohol, drugs, and poisons, often to help explain a death or an impaired-driving charge.
• Firearms and toolmarks. Also called ballistics, this examines bullets, cartridge cases, and the marks a tool leaves, then asks whether they came from a particular gun or implement.
• Latent prints. The recovery and comparison of fingerprints, palm prints, and footprints left at a scene.
• Digital forensics. Recovering and analyzing data from phones, computers, and networks, including deleted files, location history, and message logs.
• Forensic entomology. Using the insects that colonize remains to estimate how long a person has been dead.
• Questioned documents. Examining handwriting, signatures, ink, paper, and printing to test whether a document is genuine or altered.

The forensic team page explains who does each of these jobs.

What a medical examiner does

A medical examiner is a physician, usually a board-certified forensic pathologist, who investigates sudden, violent, or unexplained deaths. The examination combines an external inspection, an internal autopsy, and laboratory tests such as toxicology.

Two findings carry the most weight. The cause of death is the injury or disease that ended the life. The manner of death sorts that into a category: homicide, suicide, accident, natural, or undetermined. Those two words often decide whether a case becomes a criminal investigation at all.

How DNA evidence is collected and tested

DNA evidence starts at the scene, not in the lab. An investigator locates a biological sample, collects or swabs it, and protects it from heat and sunlight, since warmth and direct light break DNA down and can ruin a sample before it is ever tested.[8]

In the laboratory, an analyst extracts the DNA and reads a profile from specific points in the genetic code. That profile is then compared against a known individual or run through a database to look for a match. A reported match is expressed as a probability, which is part of why DNA carries more scientific weight than most comparison methods.

From crime scene to courtroom

The path from evidence to verdict has a few fixed steps, and each one can decide whether a result ever reaches a jury.

Collection and chain of custody. Evidence is photographed, logged, and packaged at the scene. From that point it needs a chain of custody, which is a record of every person who has had physical possession of the item.[7] If that record has a gap, the evidence can be excluded from trial or given less weight by the jury.[6]

Laboratory analysis. In the lab, an analyst runs the test the evidence calls for, whether that is a DNA profile, a drug screen, or a fingerprint comparison, and writes a report describing the result and its limits.

Expert testimony and admissibility. Before a jury hears any of it, a judge decides whether the testimony can come in. Under Federal Rule of Evidence 702, an expert opinion has to rest on sufficient facts, use reliable methods, and apply those methods reliably to the case, and the side offering it must show that is more likely than not.[4] Many courts also apply the Daubert standard, which asks whether a method can be tested, has been peer reviewed, has a known error rate, and is generally accepted, with the judge acting as a gatekeeper.[5]

The limits of forensic science

Not every forensic method is as solid as television makes it look, and two major reviews spelled out where the gaps are.

In 2009 the National Academy of Sciences reported that, apart from nuclear DNA analysis, no forensic method had been rigorously shown to consistently, and with a high degree of certainty, connect a piece of evidence to a specific individual or source.[2] The review found that many techniques had grown up inside crime labs and police work without the kind of testing an outside scientist would expect.

In 2016 a report from the President's Council of Advisors on Science and Technology applied validity criteria to six feature-comparison methods, the ones that try to match a sample to a source: two categories of DNA analysis, bitemark comparison, latent fingerprints, firearms identification, and footwear analysis (it reviewed hair comparison separately). It found that bitemark comparison had not been shown to be foundationally valid, that firearms identification fell short of that bar on the evidence available, and that latent fingerprint comparison was foundationally valid but carried error rates higher than examiners often claimed.[3]

One recurring issue is proof. Validating a whole discipline takes well-designed studies run by more than one independent group, and a couple of studies cannot stand in for a field as broad as firearms and toolmarks.[9] None of this makes forensic evidence worthless. It means a careful court weighs how well a given method has actually been tested. You can follow how the field is closing these gaps on the latest breakthroughs page.

Where to go next

For plain-language definitions, use the forensic glossary. For a method-by-method reference, see techniques A to Z. If you are weighing the work itself, the careers page covers training, roles, and pay.

Sources

1. National Institute of Standards and Technology (NIST), Forensic Science
2. National Research Council of the National Academies, Strengthening Forensic Science in the United States: A Path Forward (2009)
3. President's Council of Advisors on Science and Technology (PCAST), Forensic Science in Criminal Courts: Ensuring Scientific Validity of Feature-Comparison Methods (2016)
4. Legal Information Institute, Cornell Law School, Federal Rule of Evidence 702: Testimony by Expert Witnesses
5. Legal Information Institute, Cornell Law School, Daubert Standard
6. National Institute of Justice, Law 101: Legal Guide for the Forensic Expert, Chain of Custody
7. National Institute of Justice, What Every Law Enforcement Officer Should Know About DNA Evidence, Chain of Custody of Evidence
8. National Institute of Justice, What Every Law Enforcement Officer Should Know About DNA Evidence, Contamination: Transportation and Storage
9. Scurich, Faigman, and Albright, Scientific guidelines for evaluating the validity of forensic feature-comparison methods, Proceedings of the National Academy of Sciences (2023)