The Frontal Cortex

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.


  1. #1 Duae Quartunciae
    May 6, 2008

    That is appalling. There’s a name for this fallacy. It is called The Prosecutor’s fallacy; and is well known in statistics.

  2. #2 Clark
    May 6, 2008

    The expert the defense called should have brought up the other statistics and the defense attorney searching prior cases should have known these problems.

    What this highlights isn’t the hidden problem of science but the hidden and horrible problem that is throughout our justice system: if you get a bad attorney the chances for justice are pretty slim. Likewise get a really, really good attorney who knows all the loopholes and tricks and you have a better chance of getting off even if you are guilty.

  3. #3 Caledonian
    May 6, 2008

    There’s no way to make the justice system work even with incompetent lawyers without simply replacing the lawyers with something else – and then how do you determine its competence?

  4. #4 Clark
    May 6, 2008

    I’m simply not well versed enough on legal processes to be able to fathom a guess of what would be better. I just note that problems that are said to be tied to technological investigation are really just the old problem of incompetence. If anything technology makes such problems less likely and not more.

    One would hope that in this case where a key fact wasn’t presented that he could appeal. But I just don’t know the details involved in doing that.

  5. #5 Will
    May 10, 2008

    I can’t remember clearly (I also didn’t read the whole “Duped” article – it may have said something about this), but I recently took an abnormal psychology class from William Iacono at the UMN-TC. He is an expert on lie detectors and has been an expert witness a few times. Anyway, he told us that polygraphs have little validity and are more used for confessions. The person performing the polygraph has been trained in how to create stress on the person taking it so as to obtain more confessions. Often times the stress is so intense the person confabulates to relieve it. Also, I think he mentioned P3 waves on an EEG being more predictive of lying. Though I may be confusing that with Schizophrenia.

  6. #6 Will
    May 10, 2008

    Blech! After rereading that I wish I had something to cite besides wikipedia. I only have authority on my side above, not empiricism.

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