Unreliable and Limited Science
The arrival of DNA evidence in American courtrooms in the late 1980s and 1990s changed the criminal justice system forever. The widely accepted strength of DNA testing has led experts to call into question the reliability of other forms of forensics. Where these older forms of forensic science could indicate that someone might have committed a crime, DNA can show whether someone is actually guilty or innocent.
Inexact sciences
For centuries, scientific and pseudo-scientific evidence has been used in courts of law. From blood typing and hair comparison to bite marks and ballistics, many questionable and limited forms of science and technical evidence have been – and still are – used to convict defendants in the United States.
Evidence that would traditionally call for comparisons of hair, bite marks and blood types can now be tested for DNA. While older forms of forensic sciences can narrow possibilities, DNA can provide definitive proof of innocence or guilt. Because of its power to conclusively reveal the truth, DNA testing should be used whenever it can be probative.
Misplaced trust
If evidence and lab tests are mishandled and not presented fairly to a jury, scientific evidence can carry a false appearance of reliability and lead to wrongful conviction. A jury hears that science supports the prosecution’s theory – but it doesn’t always hear that the science may be unreliable or inexact. The Innocence Project has worked on cases in which:
- Hair evidence was said to microscopically match the defendant and only 1 in 10,000 people. It was impossible to prove this statistic.
- A jury was told by a scientist that biological evidence matched the defendant’s blood type. The jury was not told that 41 percent of the public also matched.
- An incorrect bite mark match led an innocent man to death row.
Faulty science — under a microscope
To this day, the microscopic comparison of hairs, particles and fibers is used in courtrooms as evidence of guilt. In some cases, it is the only evidence used to convict a defendant.
These tests have never stood up to real scientific scrutiny. DNA results have shown again and again that these tests, as performed, lack sufficient reliability and frequently produce erroneous results. The advent of mitochondrial DNA testing has given scientists much stronger evidence, even when comparing only a fragment of a hair.
The limits of a blood type
Before DNA testing, bodily fluids collected from crime scenes were routinely tested for blood type. This practice is known as serology. While this form of forensic science has often been reliable, it has substantial limits.
Conventional serology lacks the discriminating power of DNA. It often failed to exclude suspects who are innocent. Serology could only determine whether a suspect is a member of the population of potential donors who could have deposited biological evidence at a crime scene. In many DNA exoneration cases, at the original trial, serologists either failed to disclose the very limited utility of the test or mistakenly misapplied statistical principles. In both situations, juries were misled and innocent people were convicted.
| Featured Case: Alejandro Dominguez | |
 | Dominguez was 16 years old when he was convicted in Illinois of a rape he didn't commit. In addition to an eyewitness misidentification, the limited science of a blood type match led jurors to believe that evidence against Dominguez was stronger than it actually was. One of several errors in the trial was a reckless omission by a forensic scientist who testified for the prosecution. Semen was found on the victim’s body, the scientist testified, and Dominguez’s blood type matched the semen sample, meaning he could have been the perpetrator. The scientist did not tell the jury, however, that two-thirds of men in America would have matched that sample. Dominguez was convicted and sentenced to nine years in prison. He was released after serving four years and sought DNA testing at his own expense. The tests proved his innocence. His case is one of many in which limited forensic science or erroneous forensic testimony has led to wrongful convictions. |