The advent of nuclear DNA typing was a watershed in criminal justice. It has helped to convict the guilty, while also permitting the exoneration of wrongfully convicted individuals—more than 300, according to a 2014 study[i].
In its 2009 report, the National Research Council declared that nuclear DNA typing is the only “forensic method [that] has been rigorously shown to have the capacity to consistently, and with a high degree of certainty, demonstrate a connection between evidence and a specific individual or source.”
But the exonerations have raised questions about why those wrongful convictions occurred in the first place. In many of them, the conviction rested in part on expert testimony that later scientific research exposed as flawed. In one study of 156 DNA exonerations, overstated expert testimony was presented at 60% of the trials.
These developments have prompted state legislatures to amend their post-conviction statutes to grant relief to defendants convicted on the basis of subsequently invalidated expert testimony.
While there is a growing consensus that there should be relief in some circumstances, articulating the circumstances in statutory language is challenging.
A California statute which came into effect this year appears to grant relief whenever the expert later “repudiate[s]” her opinion” without an express requirement that the expert specify a basis for the repudiation.
A 2015 Texas statute seems to grant relief only when new research flatly “contradicts” prior expert testimony. In the final analysis, these statutes must balance two fundamental interests: our abhorrence of wrongful convictions, and the finality of convictions that represent a substantial investment of time and resources.
How should that balance be struck? Consider three examples:
In some cases, later analysis almost completely discredits prior testimony. The FBI once employed Comparative Bullet Lead Analysis (CBLA). When the police recover an intact crime scene bullet, an expert can compare its striations to those on bullets fired from the defendant’s weapon.
However, when the crime scene bullet was too deformed, the FBI previously resorted to CBLA, analyzing seven elements in the crime scene bullet and bullets recovered from the defendant’s possession.
An expert often relied on CBLA as a basis for opining that the bullets came from the same batch (a single day’s manufacturing production) or the same box from the defendant’s possession.
But CBLA critics pointed out that even the limited testimony about a batch is valid only if each batch is unique and uniform. Later analyses of bullet-manufacturer data indicated that neither assumption was true. A 2004 National Research Council report endorsed that criticism, and the FBI discontinued the use of CBLA.
Situations involving the use of CBLA are rare.
It is unrealistic to demand conclusive proof of the invalidity of a scientific technique or theory.
In its 1993 Daubert decision, the Supreme Court remarked that “arguably, there are no certainties in science.”
The most important aspect of the Daubert decision is its frank acknowledgment of the limits of the scientific enterprise. A requirement that the prior expert testimony be utterly discredited will deny relief to most defendants convicted on the basis of flawed expert testimony.
Shaken Baby Syndrome
However, it is not enough for the defense to show that post trial, a competing school of thought emerged. Shaken baby syndrome (SBS) is a case in point.
There was formerly a consensus, especially among pediatricians and pathologists, that violently shaking an infant can cause fatal brain injury. In many cases, the autopsy revealed such injuries, a caregiver acknowledged shaking the child, but there was no evidence that the child’s head had struck a surface or object.
Later, biomechanical experts conducted experiments with primates and anthropomorphic models of infant necks. The experiments suggested that shaking alone cannot generate enough force to cause fatal brain injury.
Nevertheless, in 2016 the Ninth Circuit Court of Appeals ruled that the biomechanical research had not invalidated the SBS theory to the extent that a defendant convicted on the basis of SBS was entitled to relief. In the court’s words, although the new research had prompted “a vigorous debate” over SBS, the research did not discredit SBS to the same extent that CBLA has been exposed.
There are doubts about whether the biomechanical findings can be extrapolated to human infants. And further research is complicated by the fact that medical ethics precludes subjecting infants to violent shaking to test the premise.
In the official Note accompanying the 2000 amendment to Federal Evidence Rule 702, the Advisory Committee stated that if proponents of two “competing scientific theories” can each satisfy 702’s reliability threshold, the judge should allow both to testify. In other words, the advent of a competing theory does not automatically invalidate a prior theory or mandate a new trial.
In the previous example, the emergence of a new theory merely raises doubts about the prior theory. But in some cases, new scientific research seriously undermines the theory.
An example is microscopic analysis of hair. Although trace evidence experts have recently given more circumscribed testimony, in the past they frequently went to the brink of opining that two hairs came from the same person because the hairs were “microscopically indistinguishable.”
In an FBI study of 268 microscopic hair analysis cases, reviewers found that prosecution experts had overstated at 96% of the trials. Another FBI study compared microscopic hair analysis opinions with mtDNA test results. In 11% of the cases in which the analysts opined that the defendant was a possible source of the two “microscopically indistinguishable” hair samples, the DNA established that the defendant was not the source.
One jury decided to convict after an expert testified that “it’s my experience in 10 years [that] it’s extremely rare I will have known hair samples from two different people I can’t tell apart.” In 2016, a Massachusetts Superior Court granted a new trial because the mtDNA research had gravely undermined confidence in microscopic hair analysis.
To gain post-conviction relief, a defendant ought to be required to demonstrate more than the emergence of a new school of thought. Moreover, even when the new scientific research is at odds with prior testimony, the defendant must show that the expert testimony played a major role in securing the conviction.
We must balance the competing interests.
A defendant should not receive a new trial simply because a new school of thought has emerged. Yet we cannot be illiberal and revise post-conviction relief statutes to grant new trials only when later research “certainly” debunks the prior testimony.
The standard ought to be whether new scientific insights seriously undermine our confidence in testimony that prompted an earlier conviction.
That standard is the best test for identifying and correcting convictions that are miscarriages of justice.
Edward Imwinkelried is the Edward L. Barrett, Jr. Professor of Law Emeritus at the University of California, Davis. He is a co-author of Giannelli, Imwinkelried, Roth & Campbell Moriarty, “Scientific Evidence” (5th ed. 2012) and author of the “Methods of Attacking Scientific Evidence” (5th ed. 2014). He welcomes readers’ comments.
[i] Beldock, Feldman & Steel, The Death of the “Hurricane” and the Criminal Justice System’s Failures, NAT’L L.J., Apr. 28, 2014, at 47.