DNA seals the deal. Period. Maybe.
Or does it? Perhaps not. DNA evidence is powerfully convincing perhaps because “we’ve come to accept the idea that DNA is a sort of individual genetic ‘serial number’ that just needs to be ‘read off’ from a biological sample — but the reality is far more complex”.
Vaughan Bell (who usually writes on the Mind Hacks blog) has an article in the Examiner that alerted us to a study showing how even DNA forensic matching results are often decided in a “judgment call” and not by simply reading off the ‘genetic serial number’. DNA samples obtained at crime scenes may be the DNA of two or more people mixed together. It isn’t simply the victim’s DNA, the perpetrator’s DNA and the roommate’s DNA. It may be all three mixed together. These are called, fairly predictably, “mixed samples”. Vaughan describes the study by Itiel Dror and Greg Hampikian:
In Dror’s study, DNA experts were given results from a mixed sample that was drawn, unknown to them, from a previous real-world case that hinged on whether suspects were present at the scene. After analysing the samples, they not only disagreed with one another but also came to different conclusions depending on whether they had information about the case, or whether they had nothing to go on but the genetic data.
It is worth noting that these findings do not invalidate forensic evidence. Studies also show that despite biases, identification is mostly done reliably, but the fact that outside information can affect decisions remains a worry for the justice system.
Perhaps, not surprisingly, Vaughan’s explanation is much easier to understand than the actual article itself. It’s a good reminder that just like forensic evaluations of legal sanity (or insanity) are somewhat idiosyncratic and reflect the individual examiner’s judgment calls–so, apparently, are the analyses of mixed DNA samples. The story (aka case narrative) needs to make sense without the DNA evidence. Often, what seems to happen, is that the story is tweaked to match the DNA evidence since “DNA doesn’t lie”.
That’s right. DNA doesn’t lie. But it’s clear we don’t yet know exactly how to talk DNAs complex language so we might misinterpret.
If you want to reinforce the credibility of the DNA evidence, refer to it as fact, and frame your trial story with that ‘fact’ as a premise for the other aspects of the story. Jurors are likely to use hindsight bias, based on knowledge of the DNA evidence, to reinforce their views of the non-DNA facets of the story.
Dror IE, & Hampikian G (2011). Subjectivity and bias in forensic DNA mixture interpretation. Science & Justice : Journal of the Forensic Science Society, 51 (4), 204-8 PMID: 22137054