In the U.S., a person can be convicted and executed for murder via circumstantial evidence — without witnesses to the crime. The quality of this form of evidence can vary widely.
The uniqueness of fingerprints has been claimed to be as high as one in 64 billion. This has come into judicial question with the observation that there is no uniquely established way to compare them. It depends upon the expert. Nevertheless, if we divide the uniqueness by an arbitrary million, we get 64,000, still useful as circumstantial evidence.
DNA, another form of circumstantial evidence, also relies on experts. As with fingerprints, admissibility was established by common law precedent. The human genetic code is more unique than fingerprints, which are actually determined by DNA. Alas, DNA has a drawback. The molecule is extremely stable, resistant to sterilization methods for pathogens. Unlike fingerprints, DNA can travel, contaminating objects distant from the source. Hence, labs use disposable plastic containers and tools. Improper collection techniques can implicate the innocent.
Despite the debates, which center around one-in-a-million versus one-in-a-billion, these are reliable, well studied tools, which obey the law of large numbers. The shortcomings are largely removed by the judicial process. What remains would be worrisome in isolation.
Fortunately, the justice system does not rely exclusively on technical methods. The gold standard is the testimony of witnesses. It is frequently supplemented by other forms of circumstantial evidence. These include expert testimony, which encapsulates technical expertise that, unlike fingerprints and DNA, have not been validated as standard tools.
Robert Roberson was convicted of shaking his child, causing her death. The conviction relied on expert testimony that the injuries were caused by shaking, not by a fall from a bed. Expert testimony which is not validated by the law of large numbers is frequently contradicted by other experts, with spectacle in the courtroom.
It satisfied the jury. Should it? If the testimony of experts were validated in the same way as fingerprints and DNA, you would have to shake 64,000 babies, and drop another 64,000. The question before all of Texas is not whether it is likely Roberson is guilty. It is whether he is guilty “beyond reasonable doubt.”
Neither is it about the correctness of judicial procedure. By gun or by needle, Texas is cavalier about human life. Let the justice who believes justice has been served throw the switch.