"Junk Science" in Texas Courts: Bite-Marks, Blood-Spatter, and Field Sobriety

What Makes a Forensic Discipline "Junk Science"?

“Junk science” is shorthand for forensic testimony that looks scientific but rests on no validated method or known error rate. The reliability screen built into Rule of Evidence 702 exists precisely to weed it out, and Texas courts have grown more willing to test disciplines that were once accepted on faith.

The modern skepticism has a source. The 2009 National Academy of Sciences report, Strengthening Forensic Science in the United States, concluded that many pattern-comparison fields — disciplines that ask an examiner to eyeball two samples and declare a match — lacked rigorous studies of their accuracy. The Court of Criminal Appeals has cited that report when re-examining forensic evidence, including in Ex parte Chaney. The table below shows how several contested disciplines currently fare in Texas courts.

The unifying lesson is that a courtroom label like “forensic science” is not proof of reliability. Each discipline stands or falls on whether it has been tested, whether it has a measurable error rate, and whether the examiner’s conclusion is more than a subjective impression dressed up as expertise.

Texas judges have a specific tool for this problem. The reliability inquiry under Rule 702 is, as the Court of Criminal Appeals has put it, where trial courts “weed out” testimony pertaining to so-called junk science by insisting the evidence rest on sound methodology. That screen matters most for disciplines that generate a confident conclusion — a “match,” a cause of death, a blood-alcohol number — from a process the scientific community has never rigorously validated.

Bite-Mark Comparison: From Courtroom Staple to Discredited

Bite-mark comparison once let an examiner “match” marks on a victim to a defendant’s teeth and testify to a near-certain identification. That claim has collapsed. Studies now show the same teeth do not leave identical marks, and Texas has granted relief where a conviction rested on discredited bite-mark testimony.

In Ex parte Chaney, the Court of Criminal Appeals set aside a decades-old murder conviction because the field of bite-mark comparison had evolved in a way that discredited almost all of the probabilistic bite-mark evidence used at trial. Research showed that the same dentition produced dramatically distorted and different marks depending on tissue, angle, and movement, undermining the premise that a mark can be traced to one person’s teeth.

The professional bodies have followed the science. The American Board of Forensic Odontology now cautions against “individualization” testimony — the claim that a particular person, to the exclusion of all others, made a mark. In Texas, that means a defense challenge to bite-mark evidence today attacks the first Kelly prong head-on: if the underlying theory that a bite can be reliably individualized is no longer valid, the technique cannot be reliable no matter how experienced the examiner. Older cases that admitted bite-mark testimony must now be read against this changed scientific record.

The shift has consequences for pending cases, not just old convictions. Because the premise of individualization is now widely doubted, a defendant facing bite-mark testimony today can demand a full gatekeeping hearing and put the State to its proof on the first Kelly prong. Where the examiner can only offer subjective “consistency” language rather than a validated method, the testimony is exposed as opinion, and the defense can ask the court to limit or exclude it and to bar any claim of a unique match.

Blood-Spatter and "Shifted-Science" Medical Causation

Blood-spatter interpretation and medical-causation opinions — how a wound was inflicted, whether an infant’s injuries were accidental — can look authoritative yet rest on assumptions that later research undermines. When the scientific consensus shifts after trial, Texas recognizes that the change can be newly discovered evidence.

In Ex parte Henderson, the Court of Criminal Appeals granted a new trial where the medical examiner’s original testimony — that an infant’s fatal head injuries could not have come from an accidental fall — was undercut by advances in biomechanical science, leading the examiner to revise the cause of death to “undetermined.” The court deferred to the trial judge’s finding that the verdict had rested on now-unreliable science.

The principle reaches beyond one field. A causation opinion is only as good as the data and methodology behind it, and disciplines that depend on an examiner’s interpretation of physical patterns are vulnerable when the literature moves. Practically, the defense builds this challenge with current peer-reviewed research and, where possible, testimony from the State’s own former expert. The goal is to show the jury or the habeas court that the “science” presented as settled was contested or has since been abandoned.

Not every spatter opinion is unreliable, and the defense should be precise about which conclusions it attacks. Describing the mechanics of a visible stain is different from opining, to a scientific certainty, about a victim’s position or the sequence of events. The further an analyst moves from documented observation toward interpretive reconstruction, the more the opinion depends on assumptions that must be validated. Isolating that overreach, and pairing it with current literature, is what makes the challenge work.

Field-Sobriety Science: HGN and Retrograde Extrapolation

DWI cases turn on forensic claims that carry real limits. The horizontal gaze nystagmus (HGN) eye test is treated as reliable evidence of intoxication, but not of a specific alcohol level, and estimating a driver’s past blood-alcohol concentration from a later sample is reliable only under careful conditions.

In Emerson v. State, the court took judicial notice that the theory and technique behind HGN are reliable, but held that a certified officer may testify only that HGN indicates intoxication — the officer may not correlate the result to a precise blood-alcohol concentration, because the margin of error is too high. An officer who crosses that line into quantifying a BAC from eye movement offers testimony the science cannot support.

Retrograde extrapolation — working backward from a breath or blood test to estimate the driver’s BAC at the time of driving — drew a searching look in Mata v. State. The court held the technique can be reliable, but only when the expert understands its subtleties and accounts for factors such as the time between driving and testing, the number of tests, and the driver’s individual characteristics like weight, drinking pattern, and last meal. A single test taken hours later, with no knowledge of those variables, will not support a reliable extrapolation. Both cases show that a DWI defense often attacks not the machine but the expert’s overreach beyond what the data allows.

These limits translate into concrete cross-examination. Counsel can confirm that the officer is certified only to administer and score the HGN test, not to diagnose a specific alcohol level, and can hold the retrograde-extrapolation witness to the variables the science requires — the timing and number of tests and the driver’s individual profile. When the State’s expert lacks that information yet still offers a confident number, the gap between the claim and the data becomes the heart of the defense.

The Article 11.073 Junk-Science Writ

Some convictions rest on science that only later turns out to be wrong. Texas is one of the few states with a dedicated remedy: Code of Criminal Procedure article 11.073 lets a convicted person seek habeas relief when relevant scientific evidence was unavailable at trial or contradicts the science the State relied on.

To obtain relief, the applicant must show the scientific evidence is currently available and was not ascertainable through reasonable diligence at trial, that it would be admissible today, and that, had it been presented, on the preponderance of the evidence the person would not have been convicted. The statute directs courts to ask whether the field of scientific knowledge, a testifying expert’s knowledge, or the scientific method itself has changed since trial.

The Court of Criminal Appeals read that language broadly in Ex parte Robbins, holding that an individual expert’s revised opinion — there, a medical examiner who changed a cause-of-death conclusion based on new scientific knowledge — qualifies as a change in “scientific knowledge” within the statute. Chaney applied the same statute to a discredited field. Together they show article 11.073 reaches both a shift in an entire discipline and a single expert’s honest re-evaluation. This is a post-conviction remedy; it does not replace the pretrial reliability challenge, but it is a safety valve when the science moves after the verdict.

The remedy has real reach but real limits. An applicant must still show, on the preponderance of the evidence, that the new or changed science would have produced a different result — that but for the discredited evidence, the person likely would not have been convicted. Article 11.073 does not reopen a case merely because an expert could be cross-examined differently today; it targets convictions whose foundation has genuinely shifted. That is why careful documentation of the scientific change is the core of any 11.073 application.

How the Defense Challenges Suspect Forensic Science

Whether the target is bite-mark testimony, a spatter opinion, or a DWI extrapolation, the mechanics are similar: force a reliability hearing, expose the missing validation, and hold the expert to what the data actually shows. The earlier the challenge is raised, the more room the court has to consider it.

The first step is a written motion to exclude the expert, paired with a request for a gatekeeping hearing under Rule 702 so the judge evaluates reliability outside the jury’s presence. Because the proponent bears the burden by clear and convincing evidence, the defense can win simply by showing the State cannot establish a valid theory, a valid technique, or proper application. Rule 705(b) lets counsel voir dire the expert about the underlying facts and data before any opinion reaches the jury.

The record is built from the discipline’s own weaknesses. Counsel gathers the peer-reviewed literature, the error-rate studies, and reports such as the National Academy of Sciences review, and retains a defense expert where the issue warrants one. On cross-examination, the questions are pointed: What is the error rate? What studies validate this method? Can you exclude every other explanation? When the answers are vague, the testimony is exposed as opinion masquerading as science. If a conviction has already occurred and the science has since changed, the fight moves to an article 11.073 writ.

Timing and preservation shape the whole effort. Raising the reliability objection early, in a written motion with supporting literature, gives the court room to hold a meaningful hearing and preserves the issue if the evidence comes in. A vague or late objection can forfeit the complaint. Where the challenged discipline has already been limited by an appellate court — as with bite-mark individualization or HGN-to-BAC correlation — counsel should cite that authority directly so the trial court is not writing on a blank slate.

Oversight, Crime Labs, and Independent Review

Texas does not leave forensic reliability entirely to individual trial judges. A statewide oversight body reviews complaints about forensic disciplines and laboratories, and its findings can supply powerful, citable support when the defense argues that a particular method or lab practice is unreliable.

The Texas Forensic Science Commission investigates allegations of professional negligence or misconduct in forensic analysis and studies the soundness of forensic disciplines. Its reports and advisory determinations — on matters ranging from crime-lab practices to whether a discipline meets scientific standards — give the defense authoritative, on-point material to present at a reliability hearing, beyond the general academic literature.

That oversight dovetails with the courtroom fight. When the Commission or a national body such as the National Academy of Sciences has questioned a method’s foundation, the defense can hand the trial court a concrete basis for skepticism rather than an abstract argument. It also helps identify laboratories with documented quality problems, which supports both a reliability challenge and a case-specific attack on how the analysis was performed.

Independent review is the throughline of every junk-science challenge. Whether the tool is a defense expert, a peer-reviewed study, a Commission finding, or a changed-science writ under article 11.073, the goal is the same: to test the State’s forensic claim against something other than the examiner’s own confidence. Science that cannot survive that outside scrutiny is exactly what the reliability rules are designed to keep away from a jury.

Where This Fits

This guide is one of four situations that arise under the Motion to Exclude Expert / Kelly-Daubert Challenge. Start with the parent motion for the overall framework, or move to a related fact pattern:

What the Case Law Says

These decisions—verified against primary sources—control how this issue is litigated. Every case still turns on its own facts.

• Ex parte Chaney, 563 S.W.3d 239 (Tex. Crim. App. 2018) — Bite-mark comparison science evolved to discredit individualization, entitling the applicant to habeas relief under article 11.073 for changed forensic science.
• Ex parte Henderson, 384 S.W.3d 833 (Tex. Crim. App. 2012) — A shift in the scientific consensus on the cause of an infant's head injuries was newly discovered evidence warranting a new trial.
• Emerson v. State, 880 S.W.2d 759 (Tex. Crim. App. 1994) — The HGN test is reliable to show intoxication, but a certified officer may not correlate performance to a precise blood-alcohol concentration.
• Mata v. State, 46 S.W.3d 902 (Tex. Crim. App. 2001) — Retrograde extrapolation is reliable only when the expert accounts for elapsed time, number of tests, and the defendant's individual characteristics.
• Ex parte Robbins, 478 S.W.3d 678 (Tex. Crim. App. 2014) — Article 11.073 grants relief where an expert's revised opinion, based on changed scientific knowledge, shows the applicant likely would not have been convicted.

General summaries of published opinions for information only — not predictions about any specific case.