DNA Evidence

In recent years, there has been lots of speculation on the potential intersection of neuroscience and the legal system. Will brain imaging became a fool-proof lie detector? Are some violent offenders suffering from a defective emotional brain that's beyond their control? Should we replace the insanity defense with a less rationalist account of human morality? etc, etc. The assumption is that the latest tools of science can help us refine our squishy concepts of justice, which we've inherited from Plato, the Old Testament and the 18th century British legal system. Needless to say, Plato didn't have fMRI.

But this obsession with progress comes with a hidden cost. For starters, we tend to overestimate the accuracy and utility of the latest gadgets, so that even bad neuroscientific explanations (as long as they are filled with abstruse anatomical references generated by brain imaging) are seen, by non-experts, as valid. We are intimidated by the jargon so that, as a recent paper in the Journal of Cognitive Neuroscience put it, "Even irrelevant neuroscience information in an explanation of a psychological phenomenon may interfere with people's abilities to critically consider the underlying logic of this explanation."

And then there's this problem, which is even more pernicious. We assume that there is something inherently objective about statistics and technologically mediated forms of evidence, that they are less tainted by human bias. But that's not necessarily the case. The LA Times recently ran a great article on how DNA evidence ("the gold standard of evidence") is often willfully misinterpreted by prosecutors and crime labs:

Police found the naked body of Diana Sylvester near her Christmas tree.

The 22-year-old San Francisco nurse had been sexually assaulted and stabbed in the heart. She lay on her back, her neck laced with scratches and her mouth open as if frozen in a scream.

Man convicted in sex assault should be freed or retried, court rules
For more than three decades, Sylvester's slaying went unsolved. Then, in 2004, a search of California's DNA database of criminal offenders yielded an apparent breakthrough: Badly deteriorated DNA from the assailant's sperm was linked to John Puckett, an obese, wheelchair-bound 70-year-old with a history of rape.

The DNA "match" was based on fewer than half of the genetic markers typically used to connect someone to a crime, and there was no other physical evidence.

Puckett insisted he was innocent, saying that although DNA at the crime scene happened to match his, it belonged to someone else.

At Puckett's trial earlier this year, the prosecutor told the jury that the chance of such a coincidence was 1 in 1.1 million.

Jurors were not told, however, the statistic that leading scientists consider the most significant: the probability that the database search had hit upon an innocent person.

In Puckett's case, it was 1 in 3.

The case is emblematic of a national problem, The Times has found.

Prosecutors and crime labs across the country routinely use numbers that exaggerate the significance of DNA matches in "cold hit" cases, in which a suspect is identified through a database search.

Jurors are often told that the odds of a coincidental match are hundreds of thousands of times more remote than they actually are, according to a review of scientific literature and interviews with leading authorities in the field.

Read the whole thing.