Failed Evidence: Why Law Enforcement Resists Science

JURIST Guest Columnist David Harris of the University of Pittsburgh School of Law discusses his new book, which looks at the tendency of law enforcement to resist advances in scientific knowledge that undermine more "traditional" methods of police investigation...

In the US today, consumers of news and popular culture cannot avoid noticing a new era in police work: science seems to be the handmaiden of law enforcement at almost every level. One need not work very hard to find news stories centering on the wonders of DNA. This incredible identification tool solves cases, both current and cold, by identifying perpetrators with a degree of accuracy undreamed of in years past. And the world of entertainment is alive with television programs and movies about police work based on high-tech science and gadgetry. Take a look at any television show about police these days in the US, and you'll see far more images of detectives and technicians handling test tubes and sample swabs than guns and handcuffs.

But this image of a deep alliance between police work and modern science is misleading at best. With the exception of DNA work and certain kinds of classic chemical analysis, law enforcement generally does not embrace existing scientific work. In fact, police and prosecutors in the US resist science. The scientific work I am referring to involves the testing of the more traditional techniques of law enforcement investigation and prosecution: not the high-tech sheen of the DNA lab, but scientific testing of eyewitness identification, the interrogation of suspects and the more traditional forensic methods such as fingerprint identification. This is the daily bread and butter of law enforcement, and scientists have found it wanting. The science on these basic police investigation methods has existed for years — some of it for decades. It is rigorous, and has undergone peer review, publication and replication. It tells us what the problems with traditional police work are, and also gives us some straightforward ways of solving these problems. Yet, most — not all, to be sure, but most — of American law enforcement continues to resist this science and refuses to change its basic tactics to reflect the best of what science has to offer.

The question at the heart of my book, Failed Evidence: Why Law Enforcement Resists Science, is why this resistance takes place. And, if we understand the reasons for this resistance, are there ways to overcome it in order to have better police work and surer prosecution of crimes?

Let's take a step back. It has now been more than 22 years since the advent of DNA as a crime-fighting tool. It gave police and prosecutors an incredibly powerful method for identifying traces of biological matter left at crime scenes. Properly used, DNA allowed scientists to calculate the probability that anyone other than the accused was the source of the evidence, with the odds of an error in the tens of millions or even billions to one. There had never been anything like DNA for proving guilt. But quickly, the other edge of the sword came into view: DNA's precision not only proved some accused people guilty; it also proved convicted people not guilty. And many of those debunked convictions rested on the more traditional police methods like eyewitness identification, confessions from suspects and non-DNA forensic work.

To date, there have been 300 DNA-based, post-conviction exonerations in the US, These cases have exposed just how much we thought we knew about police investigation and methods turns out to be wrong. There is testable DNA in roughly five percent of all cases, so we know we're looking at the tip of the iceberg. Thus, the 300 DNA-based exonerations reveal startling patterns while hinting at much bigger problems. Among those 300 cases, 72 percent contain an incorrect eyewitness identification. Half of the cases contain faulty (non-DNA) forensic work. And 27 percent feature a false confession or statement of guilt at some stage. (These numbers add up to more than 100 percent because many cases have more than one cause of error.)

All of this has pushed us to examine the science that we now have — in some cases, for decades — on these problems. Here are some of the things we know now:

  1. Eyewitness identification is among the least accurate types of evidence. When we use simultaneous lineups — the traditional group of individuals or a group of pictures displayed together for the witness — we introduce an increased risk of error into the process. When we show the individuals or photos to the witness one at a time, this error virtually disappears.
  2. Despite what almost everyone believes, innocent people do sometimes confess to crimes — even serious crimes — that they did not commit, and they do this in the absence of physical abuse, mental illness or intoxication. This is largely because of the enormous psychological pressure brought to bear on people who are interrogated. We can counter these pressures by requiring video and audio recording of full interrogations, prohibiting police lies to suspects about scientific testing and forbidding threats of harsh punishment.
  3. Certain types of forensic methods, such as fingerprint or tool mark identification, work reasonably well but are not data-based. Rather, they are based on human interpretation and experience, making them fallible and subject to cognitive biases. Other types of forensic methods — so-called bite mark analysis, for one — are little more than junk science and should be eliminated. The National Academy of Sciences' 2009 report, Strengthening Forensic Science in the United States, makes all of this clear and points the way to better police laboratory work.
If we know the problems, and we can also see the answers, why the resistance? Two sets of reasons emerge. First, there are cognitive barriers: aspects of human thinking that make change difficult or impossible. For example, cognitive dissonance makes it hard for people to acknowledge fundamentally conflicting information; instead, they rationalize one part of it away. For law enforcement, this is a powerful force. Police and prosecutors believe that they are the "good guys" — protecting the innocent and punishing the guilty. Thus, how do they deal with the opposing fact that traditional methods sometimes result in disastrous miscarriages of justice? They rationalize the second idea away. Three hundred exonerations? That's not many in the grand scheme of things. Or: the fact that these convicted people were exonerated proves the system works. This and other cognitive barriers make change very tough. Second, there are institutional barriers. Police and prosecutors all work within an institutional framework. For police, that framework demands arrests; for prosecutors, it demands convictions. Anything that might threaten the achievement of those goals will be scorned.

But there is a way around these obstacles, and toward better practices based on the science we have. First, every prosecutor's office should have a dedicated conviction integrity unit: a lawyer or group of lawyers whose job is to investigate legitimate claims of wrongful conviction in past cases. (Small prosecutor's offices might join others to form one unit for a group of counties.) This would create a regular mechanism to investigate real claims of innocence into routine practice. This was pioneered by Craig Watkins in the Dallas, Texas prosecutor's office, and has since been instituted in prosecutor's offices in Houston, New York City and Baltimore, among others. Second, with conviction integrity units in place to address the mistakes of the past, reform efforts should focus on the future: how can we change today's practices so as to avoid further mistakes? This will avoid cognitive dissonance by focusing attention on improvement and away from errors already made, which challenge the good guy image of law enforcement. Third, the small but growing number of police and prosecutors who have experience with some of these better practices — police and prosecutors in Minnesota, for example, who have been recording interrogations for almost two decades with great success — should become the spokespersons for these reforms. They are the ones who can change the minds of their brothers and sisters in law enforcement.

This type of reform will not be easy, and it will not be quick. But it is absolutely necessary. The object is not to have a perfect justice system. In a system run and created by human beings, we should not expect perfection. But we owe it to our fellow citizens, and to ourselves, to do the best that our brains can come up with to make sure that we come as close to justice as we can in every case.

David A. Harris is Distinguished Faculty Scholar and Associate Dean for Research at the University of Pittsburgh School of Law. His new book, Failed Evidence: Why Law Enforcement Resists Science, has just been published by New York University Press. Read his blog at

Suggested citation: David Harris, Failed Evidence: Why Law Enforcement Resists Science, JURIST - Forum, Nov. 27, 2012,

This article was prepared for publication by the staff of JURIST's academic commentary service. Please direct any questions or comments to them at

Support JURIST

We rely on our readers to keep JURIST running

 Donate now!

About Academic Commentary

Academic Commentary is JURIST's platform for legal academics, offering perspectives by law professors on national and international legal developments. JURIST Forum welcomes submissions (about 1000 words in length - no footnotes, please), inquiries and comments at

© Copyright JURIST Legal News and Research Services, Inc., 2013.