“All Knowledge Degenerates into Probability”: Why Fingerprint “Matches” Are Opinions, Not Facts

By: John Guidry

“All knowledge degenerates into probability.” — David Hume

Ok, I’m not the biggest Hume fan, but I love that quote. After defending over 60 jury trials in Orlando, I’ve seen more than my fair share of prosecutors presenting scientific evidence as “fact,” when it is really just probability.

• DUI Cases: The State presents a BAL above .08 as a hard number, ignoring the margin of error.
• Forensics: A lab analyst tells the jury that a substance “is” cocaine, or that DNA “matches” my client.

The “Science” of Fingerprints: Fingerprints are the classic example of this deception. Prosecutors present this evidence as if it were gospel. It is common in Burglary, Rape, and Firearm Possession charges to prove a defendant was at the scene. The Myth: Everyone believes that “no two fingerprints are alike.” The Reality: We don’t actually know that.

Did the State claim your fingerprint was found at the scene?

A “match” is an opinion, not a fact. Call John today at (407) 423-1117.

The “Madrid Bombing” Error

A few years ago, a man from Seattle (Brandon Mayfield) was accused of a terrorist act in Madrid, Spain.

• The Evidence: The FBI claimed his fingerprints “matched” those found on the explosive device.
• The Truth: It wasn’t him. The Spanish authorities eventually caught the real bomber.
• The Problem: How can two different people have the same fingerprint? Or rather, how can an expert say they match when they don’t?

It’s All Probability: Fingerprint matching is based on probability theory, not absolute certainty. Scientists observe that the sun rises in the east every day and predict it will probably do so tomorrow. Similarly, they estimate the probability of a fingerprint match.

• The Process: Analysts don’t compare the whole print; they compare “features” (ridges, loops, whorls). They look for points of similarity (e.g., a ridge ending pointing up).
• The Opinion: If they find 6 or 7 matching points, they declare a “match.” But this is just an opinion. No one knows how many other humans might share those exact same 7 points because no large-scale study has ever proven that fingerprints are unique.

The Study: “Accuracy and Reliability” (PNAS 2011)

A major study, “Accuracy and reliability of forensic latent fingerprint decisions” (Ulery, et al., PNAS, April 2011), finally put this to the test. The findings were disturbing:

“The accuracy of decisions made by latent print examiners has not been ascertained in a large-scale study, despite over one hundred years of forensic use…”

The study found that while false positives are rare, they do happen. Even worse, examiners often reach different conclusions when looking at the same print a second time! As Public Enemy said: “Don’t believe the hype.”

John’s 2026 Update: The Ban on “100% Certainty”

Note: In 2011, we were just starting to question the science. In 2026, the rules of testimony have changed.

1. The “Zero Error Rate” Ban In 2026, federal and state guidelines (following the DOJ’s Uniform Language for Testimony and Reports) now prohibit examiners from saying:

• “I am 100% certain.”
• “There is a zero percent error rate.”
• “This print could not have come from anyone else.” The Strategy: If an examiner slips up and uses these absolute terms in court, we move to strike their testimony for violating scientific standards. They can only testify that there is “strong support” for a match—which brings us back to Probability.

2. The “Close Non-Match” Trap (AI Errors) In 2011, humans looked at prints with magnifying glasses. In 2026, AI Algorithms (ABIS) do the initial search.

• The Risk: AI databases are so huge (containing millions of prints) that they often find “Close Non-Matches”—prints from two different people that look almost identical.
• The Study: Recent data shows that when humans are asked to distinguish between these AI-generated “Close Non-Matches,” the error rate skyrockets (some studies show error rates as high as 15-28%).
• The Defense: We demand to know if the “match” was suggested by an AI. If so, we argue that the computer found a “doppelganger” print, and the human examiner just rubber-stamped it due to Cognitive Bias.

3. “Likelihood Ratios” are the New Norm Instead of “Match” or “No Match,” modern forensic reports often use Likelihood Ratios (e.g., “It is 10,000 times more likely this print is the Defendant’s than a random person’s”).

• The Translation: That sounds high, but in a state of 22 million people, “1 in 10,000” means there could be 2,200 other people with that print. We make sure the jury understands that math.

Science is Never “Closed”

Just because a lab coat says it’s a match doesn’t mean it’s the truth. We know how to cross-examine the probability and expose the doubt.

Call me at (407) 423-1117. Let’s inspect the prints.

About John Guidry II

John Guidry II is a seasoned criminal defense attorney and founder of the Law Firm of John P. Guidry II, P.A., located in downtown Orlando next to the Orange County Courthouse, where he has practiced for over 30 years. With more than three decades of experience defending clients throughout Central Florida since 1993, Guidry has successfully defended thousands of cases in Orange, Seminole, Osceola, Brevard, Lake, and Volusia counties. He has built a reputation for his strategic approach to criminal defense, focusing on pretrial motions and case dismissals rather than jury trials.

Guidry earned both his Juris Doctorate and Master of Business Administration from St. Louis University in 1993. He is a member of the Florida Bar and the Florida Association of Criminal Defense Lawyers. His practice encompasses the full spectrum of Florida state criminal charges, with a particular emphasis on achieving favorable outcomes through thorough pretrial preparation and motion practice.

Beyond the courtroom, Guidry is a prolific legal educator who has authored over 400 articles on criminal defense topics. He shares his legal expertise through his popular YouTube channel, Instagram, and TikTok accounts, where he has built a substantial following of people eager to learn about the law. His educational content breaks down complex legal concepts into accessible information for the general public.

When not practicing law, Guidry enjoys tennis and pickleball, and loves to travel. Drawing from his background as a former recording studio owner and music video producer in the Orlando area, he brings a creative perspective to his legal practice and continues to apply his passion for video production to his educational content.