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Hello!
Time for this week's Murder & Mayhem recommendation: Exhibit A, a Netflix true-crime documentary.
Exhibit A is an intriguing twist on true crime shows--it's kinda like the flip side of Forensics Files. Each episode examines a type of forensic evidence and, through a target case, questions to what degree we can really trust that evidence.
By now most people have heard of the CSI Effect: the expectation juries have that all cases should have conclusive scientific forensic evidence, particularly DNA. Failing that, the juries will refuse to convict. Exhibit A looks at the other side of this coin: the assumption that evidence is more conclusive than it really is.
No evidence, not even DNA, is a clear-cut slam-dunk; it all involves interpretation, statistical analysis, and human error. Recently hair and fiber analyses have come under attack, and before that, bite-mark analyses were cast into doubt. My particular pet peeve is this arena is "touch DNA." At first glimpse, touch DNA seems like a godsend for identifying perpetrators--all you need is a few skin cells. But it's far more complicated than it seems on the surface (I talk about this very briefly in The Dancing Girls).
Part of understanding what a DNA profile means is knowing where the sample came from, and what type of sample it is. For example, if you get a DNA profile from a victim's underwear, it makes a huge difference whether that DNA comes from semen, blood, or epithelial cells. If there's visible semen or blood on the underwear, you can be fairly certain the sample came from someone who was in the physical presence of the victim. But even that doesn't tell you for certain you have your perpetrator. Say the victim had relations with her boyfriend before being attacked later in the day; in that case, assuming the semen came from a rapist would be a huge error.
The problem becomes more complex if we're looking at sweat or saliva, and far more complex when we move into epithelial cells (the source of touch DNA). For example, researchers testing new underwear directly out of their original packaging have found DNA profiles, most likely from the factory workers who made or packaged the underwear. So if a victim's underwear gives a DNA profile from epithelial cells, it's difficult (or impossible) to tell where that profile actually originated.
And that's just one issue among many with touch DNA. Exhibit A has an episode that goes into another potential problem with the tiny sample sizes touch DNA often involves.
Does that mean we can't trust touch DNA, or other types of evidence? Not at all. But we do need to understand what we can really take away from any source of evidence, and weight it accordingly when people's lives are at stake. Exhibit A puts these issues squarely out on the table for consideration.
Have you seen Exhibit A? What did you think?
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M. xoxo
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