The Netflix series Making A Murderer drew attention to a number of failures in the justice system, from police and prosecutorial misconduct, to the collection and scientific analysis of the physical evidence. A group of four forensic experts have been conducting a year-long evaluation of forensic evidence from the Steven Avery case, and recently wrote an article for the Pittsburgh Post-Gazette which points out a few “striking examples” of potential shortfalls. “Many forensic science methods are inadequately validated, which means they have not been sufficiently tested to establish how well they work and how often and under what conditions they fail,” the experts write. “Avery’s case provides a dramatic example of the uncertainty and confusion that can arise when experts rely on such methods, especially when the court allows the findings to be introduced as scientific evidence at trial.”
Here are the two ways that forensic evidence in the Avery case might have been mishandled.
Forensic Pitfall #1 – Inadequate Validation
A DNA test confirmed that the blood found in Teresa Halbach’s car was a match for Steven Avery. However, his trial attorneys theorized that the blood could have been planted from a vial of Avery’s blood collected by police years before. Blood from the vial would contain a chemical preservative known as EDTA, therefore, if EDTA was detected in the blood found in Halbach’s car, it had to have been planted. The FBI, at the request of the prosecution, quickly developed a new test to detect EDTA in dried blood. FBI expert Mark LeBeau testified that he tested three out of the six blood stains in Halbach’s car and did not detect EDTA, and therefore concluded “within a reasonable degree of scientific certainty” that all of the blood in Halbach’s car did not contain EDTA.
One issue with LeBeau’s conclusion, the experts write, is that the swiftness with which the test was developed. There was no time for the extensive research and analysis needed to even know how often and under what conditions EDTA could be detected in dried blood taken from a decades old vial. Without that research, the experts write, assessing the validity of LeBeau’s conclusion is difficult.
The fact that LeBeau tested only 50 percent of the available samples is also cause for concern. “The absence of evidence isn’t evidence of absence,” the experts write. “If the quantity of EDTA present in the stains was near the threshold of detection, it might have been detected in some of the stains but not others. In fact, if only one of the samples had EDTA above the threshold of detection, there was only a 50 percent chance that it would have been detected in the three analyzed samples.”
Forensic Pitfall #2 – Contextual Bias
The idea of contextual bias acknowledges scientific experts are human. It’s possible for their analysis to be influenced by non-scientific aspects of a case. That’s why, the experts write, “forensic scientists sometimes ‘blind’ themselves to information that is not needed for their tests, such as information about what the police expect and hope they will find.” The researchers pointed to two examples of contextual bias that might have affected how the experts in the Avery case conducted their research.
For starters, the prosecution gave the FBI an impartial reason for devising the EDTA test. A letter requesting the analysis explicitly states that the purpose of testing the blood was “eliminating the allegation that [Avery]’s vial was used to plant evidence.” There’s no way to know whether this influenced the results: FBI expert LeBeau testified that the blood stains did not contain EDTA, albeit with a degree of certainty that the test itself did not support.
LeBeau wasn’t the only analyst potentially affected by contextual bias. Sherry Culhane, the DNA analyst tasked with testing a bullet found in Avery’s garage, wrote in her laboratory notes that she should “try to put [Halbach] in [Avery’s] house or garage.” While she testified that the expectation “had no bearing on my analysis at all,” the experts write that they find this claim “difficult to accept” due to the fact that her analysis broke critical aspects of scientific protocol.
Culhane’s report found traces of Halbach’s DNA on the bullet from Avery’s garage. However, she also noted that the control samples revealed traces of her own DNA. In other words, the test was tainted. Normal procedure would have had her run the test again, but she couldn’t in this case because she had already used the entire sample. That led the experts to wonder whether “she would have decided to ignore her own protocol and report the highly problematic results of this test had she not known of the pressing desire of the police to ‘put’ Halbach in Avery’s garage.” In other words, because the botched test revealed what the prosecution wanted it to, was Culhane inclined to ignore her own scientific procedures?
Ultimately, the experts write, the failures in forensic analysis exhibited in the Avery case are not unique. The popularity of Making A Murderer has helped to illustrate how essential it is that such problems be addressed. “When the liberty of a human being is on the line,” they conclude, “the scientific evidence on which we rely must be as valid and unbiased as possible.”