Jeff Gamso has a fascinating post about the probabilities behind a DNA match.
If you do felony criminal law (from either side of the aisle) and I tell you the number is 6.17 quadrillion, you probably assume that I’m talking about DNA.
The number will reflect just how unlikely it is that the DNA in the whatever left at the scene could have come from anyone other than the defendant. There are four things you need to know about that number.
If you want to know what those four things are, read Jeff’s post. I want to talk about something else. It’s that number, 6.17 quadrillion. There’s an important proviso that’s missing, although Jeff touches on it at the end of his post:
Next time you’re on a bus or a plane or a train or in a restaurant or movie theater or anywhere where there are a bunch of people, look around.
You never know when your not-twin, the one whose DNA profile is the same as yours, might be in the crowd. Despite the odds of 1 in 6.17 quadrillion. Hell, it might be one of the jurors.
If you pick a person at random off the street — or one of the random readers of this blog — the odds of someone else having that person’s DNA are not 1 in 6.17 quadrillion. They’re only about 1 in 500. That’s because about 1 person in 500 was born as one of a pair of identical twins.
Identical twins occur when a single zygote in the mother’s womb splits into two separate zygotes which develop into two separate individuals. Since all the cells in the zygotes trace back to a single zygote and therefore a single fertilized egg, they have essentially the same DNA.
All of this makes me wonder why this problem doesn’t come up more often in news stories about trials. If a criminal defendant has an identical twin, claiming he committed the crime would explain the DNA evidence. As a bonus, it would also explain things like photo arrays, line-ups, and lots of other eyewitness testimony. I guess a defendant might be reluctant to accuse his closest brother of a crime, but I would think that a pair of identical twins creates enough reasonable doubt to protect them both.
I’ve heard that claiming someone else commited the crime is sometimes called the SODDI defense, which stands for Some Other Dude Did It. I’d think we’d occasionally hear about the METDI defense: My Evil Twin Did It.
You know, I wanted to comment on this topic last night on Gamso’s blawg, but was too tired. Seeing this picked up by Windypundit reminded me. There is another aspect to these probability inferences that is often overlooked.
There is an excellent video regarding this topic on YouTube from the TED talks, that shows how totally unable people in general are at reasoning with statistics (http://www.youtube.com/watch?v=kLmzxmRcUTo).
For instance, in the Stephen Myers case, the probability of 1 in 1.6 million may well have been presented as there is only a one in 1.6 million chance that he didn’t do it. But that is not correct, as it is not the whole story. As pointed out in the video, one has to take into account another piece of information; in this case the “prevalence” of other people that have the same combination of DNA segments.
A one in 1.6 million chance, multiplied by, say 300 million as the number of people in the US, is 1 in about 187 that he DID do it. That is, given the large number of people in the US, there will be 187 or so others that have the same combination of DNA segments that were tested as does Stephen Meyers.
The video is very good, well worth watching, and certainly opened my eyes. I hope no one is offended if I post this comment on both blogs…
I don’t know anything about the Myers case, but I think it depends on how the defendant came to the attention of of the police, and what else links him to the crime. If the initial suspect pool consists of the entire population of the United States, and the defendant just popped out of a CODIS hit, then the fact that there are about 190 other people who also have matching DNA is real important.
Even if the defendant lives in the same area as the victim, it’s not too meaningful. With 190 people scattered all over the country that match the DNA, one of them is bound to live within easy driving distance.
On the other hand, if the defendant works with the victim, and he kept asking her out, and she kept saying no, and witnesses heard him threaten her, and his car appears on a traffic camera three blocks from her house around the time of the murder, and he has a registered gun of the same caliber as the murder weapon, and his alibi doesn’t hold up, and then you get the 1 in 1.6 million DNA match…he’s probably the guy.
I wonder what comes after DNA.