Debunking myths on genetics and DNA

Monday, August 8, 2011

DNA and the criminal law

One of the things that makes DNA so fascinating is its widespread use not only in science and technology, but also, as you all know, in forensic law. I had the pleasure to discuss DNA evidence with my friend Mark Pryor, an assistant district attorney in Texas and a fellow writer. His novel, THE BOOKSELLER, is currently on submission to publishers and is represented by Ann Collette of the Helen Rees Agency.

I asked Mark to explain what happens after the DNA evidence has been gathered and a suspect has been identified.

 MP: Usually the detective writes an affidavit while consulting the prosecutor. The affidavit is used to lay out the probable cause that the defendant did the crime, then it is presented to the judge.  If the judge thinks there is probable cause, he causes the warrant to be issued.  
EEG: When will  you approve a detective's affidavit based on DNA evidence?

MP: Depends.
I know, poor answer.  But here's why: DNA is used to put someone in a certain place.  That's all.  In a rape case it can put the defendant inside the victim, literally, or just in her vicinity.  So usually we need more than just DNA, unless there is NO reason for the defendant to have been at the scene of the crime.  So your DNA being present when your husband is murdered in your house isn't helpful.  But my DNA there would be, although by itself it's still probably not enough for a warrant.  What would happen is that a detective would pay me a visit and find out if I had a good reason for being at your house.  Remember, DNA by itself can't tell you when someone was at a location, just that they were there.

The one time it would be enough by itself would be in a stranger-on-stranger rape case, where the victim got a good look at her attacker but didn't recognize him, didn't know him.  Even in a date-rape type scenario, where consent is usually the issue, a positive DNA hit will lead the detective to interview the accused before seeking a warrant, most likely.

EEG: What about the opposite scenario: when can DNA evidence be challenged?

MP: It's very hard to argue with DNA.  It's science, and good science, so it's not going to work if you just say, "That's wrong."  What you might be able to do is show an error in the testing.  One interesting case I've seen: semen from a rape/murder was tested against that of the suspect.  It came back negative.  Years later, a detective reopened the case and was convinced it was that suspect, so she resubmitted the sample.  Sure enough, they'd screwed up the testing and it was his.
Now that of course presents a problem when this case comes to trial because the defense lawyer will no doubt argue, "They messed up once, maybe they did twice."  Or even suggest we rigged the result.  

EEG: Interesting. As a scientist, I do know lab contamination is a reality, even when people are extremely careful. Ideally one should always repeat the same analysis multiple times, though DNA testing is still too expensive to allow for that. On a good note, in the five years I've been in this field, I've seen the technology in DNA typing improve exponentially, and the costs lower every year. I'm fairly confident that ten years from now the technology will be much more affordable and errors will be negligible.

Do you believe in the CSI effect, in other words, the fact that after the rise in popularity of shows like CSI prosecutors have a harder time convincing a jury?

MP: Oh yes.  I've blogged about this very thing, in fact. People have much higher expectations of prosecutors these days, and it's definitely something I have to address while picking juries, to make sure they understand what role DNA evidence might or might not play.  For example, I had a murder case recently--it'll be featured on 48 Hours on October 1 of this year--where that was an issue while picking the jury.  This was a cold case, from 1985, and I knew people associate cold cases with DNA, mostly because that's how they are usually solved.  But that wasn't our case. We didn't have DNA and during voir dire I wanted the jurors to understand that there are some instances when DNA testing is pointless.  In our case, the victim and defendant had been dating and so it was not disputed that he'd been at her apartment, where the murder took place.  If they'd collected samples, fingerprints etc., his would likely have been there, but if we'd presented that evidence it would have been a big "So what?" because, as I said above, DNA just tells you someone has been at a location.  Which we all knew.  

Even so, after explaining this, one potential juror insisted we would need to present her with DNA evidence before she'd convict.  I put the hypothetical to her: "What if we had the whole thing on video and you could clearly see the defendant committing the crime?"  She replied that she'd still need DNA.  Needless to say, that disqualified her from the jury panel.  And in the case itself, the defense made a big deal about how we had no DNA evidence of the defendant's guilt!  But those are the types of challenges we face.  DNA is a fantastic development, it's a wonderful tool, for law enforcement but we need to educate people as to its limitations, to the reality of what it tells us about a crime.

EEG: I couldn't agree more. CSI is a wonderful show, but unfortunately too many people watch it thinking they are learning science or forensic law. Watch it for the fun, folks!

Thank you, Mark, for taking the time to answer my questions. I look forward to the 48 Hours show in October!

To find out more about Mark and his book, visit his website, DA Confidential, where he blogs about writing and prosecuting crimes.

Photo: metal knots. Canon 40D, focal length 47mm, exposure time 1/125.


  1. Really great interview! Fascinating stuff, and I too, can't wait for the 48 Hours episode in October.


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