With the emergence of DNA evidence in the courtroom, experts have begun to call into question the reliability of other forms of science. These older forms of forensic science can show if someone may have committed a crime, but there can never be absolute certainty. Often in cases, forensic science is falsely presented to juries to be 100% reliable, when that is not necessarily the case. Juries are presented with the evidence that matches the prosecutor’s argument, but are not told that the evidence may be inexact or inaccurate.

The Facts

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; even though it was impossible to prove that statistic, it was nevertheless presented to the jury.
• A scientist told a jury that biological evidence matched the defendant’s blood type. The jury was not told that 41% of the public also matched the blood type.
• “Expert” testimony about a bite mark match led an innocent man to death row.

Before DNA, body fluid from crime scenes was collected and tested for blood type, in a practice known as serology. It could only determine whether a suspect was a member of the population of potential donors who could have deposited fluids at a crime scene. In many DNA exonerated cases, serologists failed to disclose the limited reliability of the test or misapplied statistics. In such cases, juries were misled by the testimony and innocent people were convicted. The use of DNA has proven that these tests are unreliable and frequently produce erroneous results.

Learn about other innocence related issues.

 

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