Elizabeth Holmes Conviction Stands Despite “Harmless Error”

The “Harmless Error” Problem in Expert Testimony

The harmless error doctrine is meant to ensure that minor procedural missteps do not derail an otherwise valid conviction. However, courts have increasingly used it as a shield to excuse substantive errors—especially in cases involving scientific or expert testimony. When judges declare an error “harmless,” they are essentially engaging in speculation about what influenced the jury, a practice that is fundamentally at odds with due process.

Consider the implications of this approach in the Holmes case:

• The government introduced expert testimony without vetting its reliability under the proper Daubert standard.

• The jury was exposed to scientific conclusions that were never properly challenged for validity.

• The court acknowledged the expert testimony was improperly admitted but dismissed concerns by speculating that the jury would have reached the same verdict anyway.

This is particularly problematic because the reliability of scientific evidence is often opaque to jurors. Jurors are likely to trust scientific testimony as definitive, making it imperative that courts ensure that such testimony is properly vetted before it reaches the jury.

By contrast, courts rarely treat defense evidence with such generosity. If a defense expert were admitted without Daubert scrutiny, the prosecution would rightly object—and most courts would agree that the improper admission required a retrial rather than a mere harmless error finding.

A Speculative Foundation: The Danger of Allowing Courts to “Guess” at Jury Impact

At its core, the harmless error doctrine is speculative when applied to expert testimony. It requires appellate courts to guess at what influenced the jury’s decision-making. This is particularly problematic in fraud cases, where credibility and technical evidence often play a decisive role. If the jury had been presented with a properly challenged expert, perhaps the defense could have undermined the reliability of the conclusions, weakening the prosecution’s case. We’ll never know, because that process never occurred.

Imagine a case where a forensic scientist claims a defendant’s DNA was found at a crime scene, but later it turns out that:

1. The scientist did not follow proper testing protocols.

2. The methodology used was not vetted under Daubert.

3. The scientist’s conclusions were admitted as evidence anyway.

4. The court then says, “Well, there was a lot of other evidence, so this was harmless.”

The problem is obvious: it is impossible to separate the improperly admitted scientific testimony from the jury’s decision-making process. The same applies to expert testimony in fraud cases—once the jury hears an expert claim that a technology was “unsuitable for clinical use,” that statement has already framed the case, whether it was properly vetted or not.

The Need for a Higher Standard of Review in Expert Testimony Cases

The Ninth Circuit’s handling of the expert witness issue in the Holmes and Balwani case is emblematic of a broader failure in the criminal justice system. The harmless error doctrine is too often applied speculatively in cases involving expert testimony, allowing courts to excuse improperly admitted scientific opinions simply because they believe—without definitive proof—that it did not impact the jury’s decision.

This practice must change.

1. Daubert Review Must Be Mandatory – Any expert opinion used in a criminal trial must be vetted under Daubert, and failure to do so should constitute reversible error, not mere harmless error.

2. Burden on the Government – If expert testimony is improperly admitted, the government should bear the burden of proving that it did not affect the jury’s decision—rather than courts simply assuming it didn’t matter.

3. Higher Appellate Scrutiny – In cases involving scientific testimony, courts should be required to assess the record rigorously before concluding an error was harmless, instead of engaging in speculative reasoning about jury impact.

At the heart of criminal justice is the requirement that convictions be based on reliable evidence. Expert testimony—particularly when dealing with scientific or technical subjects—should not be excused from proper scrutiny simply because a court deems the conviction likely valid regardless.

If anything, scientific testimony requires the highest degree of scrutiny, not the lowest. And until courts stop speculating about the impact of improper expert testimony, we risk allowing junk science, unreliable conclusions, and prosecutorial overreach to define the outcomes of criminal trials.

If you found this analysis insightful, you need to read Fight the Feds: Unraveling Criminal Investigations—your essential guide to understanding how federal prosecutions work and how to defend against them.