29 June 2014

How scientific is forensic science?

[UPDATE 19/04/15:] Similar problems affect forensic hair matches.

A recent New Yorker article discusses the use of cell-phone call records in criminal trials, and in particular the precision with which a user can be located. Unsurprisingly, this precision is much lower than claimed by some prosecutors and, when overestimated, can lead to wrongful convictions.

Some days ago I read [via Soylent News and Slate] about a 2009 report of an NAS Committee: Strengthening Forensic Science in the United States: A Path Forward. The Slate article also mentions the numerous convictions overturned by DNA tests and draws bleak conclusions about the current state of forensics (in the US, at least.) How did we get here?

One obvious answer is that the courts of law are ill-equipped to deal with scientific subtleties (in the same way scientists are not prepared to interpret fine legal points.) In particular, it is quite difficult for judges to identify sound scientific evidence (although some standards do exist) and to assess its reliability. A very useful introduction to this point is "How Science Works", by David Goodstein, in Reference Manual on Scientific Evidence (2nd ed.)

Another possible reason is the lack of a "checks and balances" mechanism. Scientific results (important ones, at least) are scrutinized by an entire community, with similar expertise and resources as the authors. During trial, evidence introduced by the prosecution should be questioned by experts for the defendant, but the latter may not have the necessary resources. This disparity is even stronger in the case of plea bargains (as in the New Yorker story), where the evidence is never actually introduced.



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