Increasingly, a person’s genetics are playing a role in court cases. You watch CSI and CourtTV. It’s all about DNA evidence.
But are we interpreting the information provided by the tests properly? Some statisticians and lawyers don’t think so.
Mathematics might seem a logical fit for the courts, then. Judges and juries, though, all too often rely on gut feeling. A startling example was the rape trial in 1996 of a British man, Dennis John Adams. Adams hadn’t been identified in a line-up and his girlfriend had provided an alibi. But his DNA was a 1 in 200 million match to semen from the crime scene – evidence seemingly so damning that any jury would be likely to convict him.
But what did that figure actually mean? Not, as courts and the press often assume, that there was only a 1 in 200 million chance that the semen belonged to someone other than Adams, making his innocence implausible.
It actually means there is a 1 in 200 million chance that the DNA of any random member of the public will match that found at the crime scene (see “The prosecutor’s fallacy”). The difference is subtle, but significant. In a population, say, of 10,000 men who could have committed the crime, there would be a 10,000 in 200 million, or 1 in 20,000, chance that someone else is a match too. That still doesn’t look good for Adams, but it’s not nearly as damning.
If all of my reading on rationality is teaching me something, it’s that in a lot of situations, human nature is all about jumping to conclusions. And when someone’s life or liberty is on the line, jumping to conclusions is a dangerous thing to do. I can’t think of a better time to sit back and analyze carefully and rationally what is going on. Go ahead and go with your gut on standardized tests, but turn on those higher functions when it comes to others’ futures or your own.
And then there’s the question of how much at person’s genetics influence his or her actions.
In 2007, Abdelmalek Bayout admitted to stabbing and killing a man and received a sentenced of 9 years and 2 months. Last week, Nature reported that Pier Valerio Reinotti, an appeal court judge in Trieste, Italy, cut Bayout’s sentence by a year after finding out he has gene variants linked to aggression.
Do genes remove or mediate personal responsibility? Where do arguments like this leave free will?