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Growing Awareness of Wrongful Convictions by DNA Sheds Light on the Weakness of Many Forensic Sciences and is Forcing Changes in Forensic Subjective Comparison

Michael J. Saks and Jonathan Koehler
The Coming Paradigm Shift in Forensic Identification.
Science 309: 892-95 (2005, August 5)

Forensic science, aimed at individualizing samples based largely on subjective comparisons, is being subjected to severe challenges as a result of: • DNA profiling, • the discovery of wrongful convictions, • changed legal admissibility standards, and • error-rate studies across the forensic sciences.  A coming paradigm shift will replace semi-informed guesswork with scientifically grounded methods.

DNA exonerations showed – unexpectedly – that forensic individualization sciences are among the leading factors associated with wrongful convictions.  Forensic science testing errors were found in 63% of the first 86 DNA exoneration cases, and false or misleading testimony by forensic scientists in 27%.  News of erroneous forensic identification of hair, bullets, handwriting, footprints, bite marks, and even venerated fingerprints have shaken the once-complacent world of forensic individualization scientists.


The traditional forensic individualization sciences are based on the central assumption of “discernible uniqueness” – • marks produced by different people or objects are observably different, • experts can observe the differences, and • when a pair of markings is not observably different, criminalists conclude that the marks were made by the same person or object.

This assumption, however, lacks theoretical or empirical foundation. Forensic examiners mostly hold bachelors’ degrees or less (3% have masters’ degrees; 1% PhDs), are not steeped in skeptical culture of scientific rigor, and are subject to pressures to fudge data or even to fabricate.   

Changes in the law of evidence regarding the acceptance of expert testimony also pressured forensic individualization scientists to improve their approaches. Older legal tests emphasized the expert’s credentials or the widespread acceptance of a method. In the 1990s federal courts, followed by many state courts, adopted a standard that required experts to show that their testimony rested on a reliable scientific foundation. A court found that handwriting identification had no scientific basis.  Such expert testimony is admissible as non-scientific expertise, but only if it passes appropriate tests of validity.

Error rate studies based on proficiency tests of forensic examiners found spectrographic voice identification error rates as high as 63%; handwriting error rates around 40% and even higher; false positive error rates for bite marks as high as 64%; and a microscopic hair comparison error rate of about 12%. Although fingerprint examiners generally fare better, data from a forensic testing program contradict boasts of perfection. Since 1995, about one-fourth of fingerprint examiners have failed to correctly identify all latent prints in a proficiency test.

DNA profiling, which was subject to intense scrutiny by several sciences, offers a model for other forensic sciences. All forensic sciences and methods ought to engage in examiner proficiency testing, root out unscientific practices, and develop data-based, probabilistic assessments of meaning of evidentiary “matches.”

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