After the home shop, and this old rag, the Washington Monthly is my favorite magazine, and I live in fear that someday it won't be around. A couple of weeks ago I linked to their awesome investigation into how nut-jobs in Texas were helping make American kids dumber.
Here's another good piece that looks at how prosecutors are misusing DNA evidence to "close" cold cases. The magazine focuses on the case of John Puckett, a 70-year old man recently convicted, wholly on DNA evidence, of first-degree murder:
Typically, law enforcement and prosecutors rely on FBI estimates for the rarity of a given DNA profile--a figure can be as remote as one in many trillions when investigators have all thirteen markers to work with. In Puckett's case, where there were only five and a half markers available, the San Francisco crime lab put the figure at one in 1.1 million--still remote enough to erase any reasonable doubt of his guilt. The problem is that, according to most scientists, this statistic is only relevant when DNA material is used to link a crime directly to a suspect identified through eyewitness testimony or other evidence. In cases where a suspect is found by searching through large databases, the chances of accidentally hitting on the wrong person are orders of magnitude higher.
The reasons for this aren't difficult to grasp: consider what happens when you take a DNA profile that has a rarity of one in a million and run it through a database that contains a million people; chances are you'll get a coincidental match. Given this fact, the two leading scientific bodies that have studied the issue--the National Research Council and the FBI's DNA advisory board--have recommended that law enforcement and prosecutors calculate the probability of a coincidental match differently in cold-hit cases. In particular, they recommend multiplying the FBI's rarity statistic by the number of profiles in the database, to arrive at a figure known as the Database Match Probability. When this formula is applied to Puckett's case (where a profile with a rarity of one in 1.1 million was run through a database of 338,000 offenders) the chances of a coincidental match climb to one in three.
In all of us, there are strong motives for not thinking. Just getting through the day as tax-paying, spouse-loving, child-rearing adult is hard enough. Thinking through issues like this is hard--If DNA automatically exonerates, why can't it automatically convict? It takes some mental work to see that the math of scanning one persons DNA against the crime scene, is very different than the logic of scanning 300,000 people's DNA. The chance of an error when scanning one person's DNA is very low--in this case one in a million. But scan 300,000 people's DNA and the chance of an error rises to one in three. The truly scary thing about this technique as the numbers in the database rise, you start to approach a point of near certainty for getting a DNA match on a innocent person.
Going back to the motives for not thinking, this is all about the search for a magic bullet, something to heal the pain of victims and the collective pain of society. It is so hard to reconcile ourselves to the fact that there is great evil in this world, and we're ill-equipped to balance the scales.