Questionable or Questioned Science

The increased use of DNA analysis as a more reliable approach to matching crime scene evidence with suspects and victims has resulted in the reevaluation of older cases that retained biological evidence that could be analyzed by DNA. The number of exonerations resulting from the analysis of DNA has grown across the country in recent years, uncovering a disturbing number of wrongful convictions—some for capital crimes—and exposing serious limitations in some of the forensic science approaches commonly used in the United States.

According to The Innocence Project, there have been 223 postconviction DNA exonerations in the United States since 1989 (as of November 2008).15 Some have contested the percentage of exonerated defendants whose convictions allegedly were based on faulty science. Although the Innocence Project figures are disputed by forensic scientists who have reexamined the data, even those who are critical of the conclusions of The Innocence Project acknowledge that faulty forensic science has, on occasion, contributed to the wrongful conviction of innocent persons.16

The fact is that many forensic tests—such as those used to infer the source of toolmarks or bite marks—have never been exposed to stringent scientific scrutiny. Most of these techniques were developed in crime laboratories to aid in the investigation of evidence from a particular crime scene, and researching their limitations and foundations was never a top priority. There is some logic behind the application of these techniques; practitioners worked hard to improve their methods, and results from other evidence have combined with these tests to give forensic scientists a degree of confidence in their probative value. Before the first offering of the use of DNA in forensic science in 1986, no concerted effort had been made to determine the reliability of these tests, and some in the forensic science and law enforcement communities believed that scientists’ ability to withstand cross-examination in court when giving testimony related to these tests was sufficient to demonstrate the tests’ reliability. However, although the precise error rates of these forensic tests are still unknown, comparison of their results with DNA testing in the same cases has revealed that some of these analyses, as currently performed, produce erroneous results. The


The Innocence Project. Fact Sheet: Facts on Post-Conviction DNA Exonerations. Available at See also B.L. Garrett. Judging innocence. 108 COLUM. L. REV. 55 (2008) (discussing the results of an empirical study of the types of faulty evidence that was admitted in more than 200 cases for which DNA testing subsequently enabled postconviction exonerations).


See J. Collins and J. Jarvis. 2008. The wrongful conviction of forensic science. Crime Lab Report. July 16. Available at See also N. Rudin and K. Inman. 2008. Who speaks for forensic science? News of the California Association of Criminalists. Available at, p. 10.

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