The confirmation bias in the forensic sciences.

James and Nicki always wanted to work in the forensic sciences. Whilst reading towards their undergraduate degrees they would borrow as many books from the library as they could on forensics and watch the popular television programmes about ‘forensic experts.’ One day when looking though an interesting book about case studies in forensics Nicki came across an interesting case study.

In 1988, Barry Laughman confessed during interrogation to the charges of rape and murder of his neighbour. The following day tests revealed that the person who committed the crime had Type A blood whilst Laughman had Type B. Aware that Laughman had confessed to the crimes the state forensic chemists proposed four theories (none of which were scientific) to dismiss the mismatch. Laughman was in due course convicted and sentenced to 16 years in prison. He was eventually released in November 2003 after a re-examination of the DNA evidence.

The case of Barry Laughman gives us a clear example of the influence of confirmation bias in the forensic sciences. The confirmation bias is shown when an individual ignores evidence that goes against what they believe whilst trying to confirm the belief (Dror, 2006). In Barry’s case the Virginian state forensic chemist ignored contradictory evidence and persisted in dismissing the mismatch in DNA evidence.

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The confirmation bias causes problems in all areas of decision-making. In the forensic sciences errors in decision-making, as caused by the confirmation bias can have severe consequences innocent people can spend a lifetime in prison, and the actual criminal can go on to reoffend. In the forensic sciences, the confirmation bias has been reported by the National Academy of Sciences (2009) in firearms, hair and fibre analysis, blood splatter, hand-writing and fingerprints (Kossin et al., 2013; Garrett et al., 2011).

In a recent study investigators found an interesting example of how the confirmation bias can influence the outcome of a forensic analysis (Ulery et al., 2012). The investigators gave forensic fingerprint examiners the same evidence twice, at approximately 10% of the time the examiners reached different conclusions (Ulery et al., 2012). Three of the reasons as to why the examiners reached differing conclusions are (i) examiners often receive direct communication from the police (e.g., letters, phone calls etc), (ii) cross-communication between examiners, and (iii) examiners overstating the strength of evidence.

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There are measures that can be taken to prevent the confirmation bias. The FBI’s Latent Print Unit revised their Standard Operating Procedures (SOP) (Cole et al., 2005). They adopted a programme of masked verification whereby fingerprint comparisons that involve a single print are masked-verified (i.e., in isolation with no further information about the print). The change in SOP prevents the second examiner from inferring the first examiner’s conclusion when two examiners individually examine the evidence (Office of the Inspector General, 2011).

Other measures that can be undertaken to prevent the confirmation bias include training all forensic examiners so that they know about cognitive biases. Just two of the courses help to install knowledge of cognitive biases are the FBI’s week-long Facial Comparison and Identification Training and the Australian government’s 2-day long facial comparison course. The linear examination of evidence by multiple examiners (Heyer et al., 2013), cross-laboratory verification (Kossin et al., 2013) and peer verification (Heyer et al., 2011) can all help in reducing the impact of the confirmation bias in the forensic sciences.

So, like James and Nicki if you are interested in working in the forensic sciences it is important to learn about the influence of cognitive biases on decision-making. Some private forensic companies have begun to provide training for their employees, and some governments have started to provide training. With adequate training one day we may be able avoid false convictions.


One thought on “The confirmation bias in the forensic sciences.

  1. Nice blog. With 32 years of experience in Fingerprints and related fields, I also encountered many a cognitive biases.
    For a fingerprint expert, comparing a chance print with the specimen prints of inmates, suspects or that on the records have different values.

    Liked by 1 person

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