State crime labs and sloppy forensic science

Science comes up routinely in court - DNA testing, breath tests for alcohol, chemical tests or gas spectrometer tests to determine what a substance is made of, often from a police-run crime lab. The lawyers aren’t usually experts in the science, and neither is the judge, nor the jurors, and so the science is usually explained through the testimony of a scientific expert such as an employee of the state crime lab who performed the test.

When scientific testing is performed by industry, or by academia, the people doing the test have a reason to get it right. The Ford Motor Company really cares about wind-tunnel tests and aerodynamics. Chemistry professors write papers that will be reviewed by other chemists. But those incentives don’t apply the same way in a state crime lab. Those labs are usually run by the police, and the police want to catch bad guys. If the bad guy goes free because a lab result shows that the DNA doesn’t match, or the blood has no intoxicating drugs, or whatever, I don’t think the folks at the state crime lab high-five one another for not accidentally convicting an innocent; I bet they scowl that another one got away. So, I don’t think they want rigorous scientific precision; I think they want to get it close enough to catch the bad guy. Stories about crime lab errors show up occasionally; the San Francisco crime lab has been in the news recently both over mixing up DNA samples and employee theft. But crime labs are secretive; for every time the press or a defense attorney figures out an error, how many more errors never see the light of day?

Ideally, crime labs would be neutral, maybe part of the court system rather than part of the police system. Police crime labs will always put catching bad guys ahead of careful lab procedures. I doubt, however, that it would really help to move crime labs from police supervision to court supervision; pre-sentence report writers, who are usually court employees, tend to have prosecutorial leanings. Regardless, crime-lab procedures need to be transparent, and error rates publicized. If we can’t trust the crime lab results, then innocent people will go to jail based on sloppy lab work.