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Rational Bias in Forensic Science

By Tom W. Bell and Glen Whitman

Somehow I neglected to mention this when it happened. My article on ?Rational Bias in Forensic Science,? coauthored with Roger Koppl (pretty much the only economist out there studying the economics of forensic organization), was published in the journal Law, Probability & Risk. Unfortunately, the full text is gated, but your college or university may have access.

I?m proud of this article because I think it?s one of the most important I?ve done, at least in terms of significance to real people. In the course of writing the paper, I became increasingly appalled by the unscientific and haphazard nature of forensic science. Even DNA profiling, the gold standard of forensic techniques, is not as ironclad as you might think. Many other techniques verge on alchemy. But judges and juries in our Law & Order/CSI/Bones-immersed culture tend not to question the reliability of forensic conclusions and the trustworthiness of forensic analysts. The bottom line? A lot of innocent people are probably getting their lives ruined by the legal system.

The focus of the paper is narrower; it identifies one specific source of forensic bias. Here?s the abstract:

The current organization of forensic science induces biases in the conduct of forensic science even if forensic scientists are perfectly rational. Assuming forensic examiners are flawless Bayesian statisticians helps us to identify structural sources of error that we might otherwise have undervalued or missed altogether. Specifically, forensic examiners? conclusions are affected not just by objective test results but also by two subjective factors: their prior beliefs about a suspect's likely guilt or innocence and the relative importance they attach to convicting the guilty rather than the innocent. The authorities?police and prosecutors?implicitly convey information to forensic examiners by their very decision to submit samples for testing. This information induces the examiners to update their prior beliefs in a manner that results in a greater tendency to provide testimony that incriminates the defendant. Forensic results are in a sense ?contaminated? by the prosecution and thus do not provide jurors with an independent source of information. Structural reforms to address such problems of rational bias include independence from law enforcement, blind proficiency testing and separation of test from interpretation.

I think that?s pretty self-explanatory, but here?s an even simpler explanation. Say you?re a forensic examiner. For the most part, your test samples are provided by the authorities (police or prosecutors). And the authorities generally don?t bother giving you a sample unless they already suspect there?s reason to expect a match. So if you?re a rational forensic examiner, you may infer a somewhat higher probability of guilt by the mere fact that you?re doing a test at all. And that fact will affect your testing process, because there is no such thing as a purely objective test; your subjective beliefs about the likelihood of guilt necessarily affect your interpretation of the evidence.

Full post as published by Agoraphilia on April 06, 2010 (boomark / email).