As I understand it, courts sometimes use DNA identification to incriminate or clear suspects. Material found at the scene of a crime is found to contain human cells, the DNA is identified, and then compared with the DNA of a suspect.
Popular sources seem to suggest that it is very, very unlikely that two people have identical DNA, or that the wrong person will be identified from a DNA test.
Lets say that the laboratory reports back that the suspect's DNA matches that of the sample from the scene. What does that actually mean?
According to Wikipedia:
Monozygotic twinning occurs in birthing at a rate of about three in every 1000 deliveries worldwide.So, as I understand it, 0.3 percent of individuals in the population share identical DNA with an identical twin. Unless the existence of such an identical twin can be ruled out, the most one can say is that there in not more than one chance in 333 that someone else left that specific DNA trace at the scene of the crime.
Say that investigators try to rule out the possible existence of a monozygotic sibling. We know that mistakes happen in hospitals and that there are other ways that twins are separated at birth without their ever later learning of that occurrence. So, even if efforts have been made to assure that the individual in question does not have an identical sibling somewhere, that investigation should not be considered absolutely conclusive.
Add in the possibility of error in the lab -- mixing up the crime scene sample with that from another crime scene, mixing up the suspect's sample with that from another person, mistakes in reading the DNA test results, deliberate falsification of the findings. The confidence provided by DNA testimony goes down further.
How about the possibility that the DNA sample taken from the crime scene is not that of the person who committed the crime, but that of some other person? Is it not possible that that does occur? How would you estimate its probability?
How many judges or jurors would be able to really understand the testimony of a DNA witness, properly estimate the probability of a suspect being responsible for the crime from the DNA evidence alone, or able to combine the probabilities arising from the DNA evidence with that from other sources (eye witness testimony, circumstantial evidence, exculpatory evidence offered by the defendant, etc.?
I am not comfortable with the accuracy that the judicial process might achieve in cases involving DNA evidence. Still, such evidence obviously should influence verdicts if available. The question is how to be sure that it appropriately influences verdicts.