Every crime show ever.
Setting—messy crime scene
Attractive Detective 1: Wow, look at all this blood and semen so helpfully spread all over the crime scene!
Attractive Detective 2: I will collect it and rush it to the crime lab, pronto!
5 minutes later, setting—gleaming hi-tech crime lab
AD 2, handing evidence to Attractive Lab Tech: This case is super important. We need results pronto!
ALT: Right on it, Detective.
5 minutes later
ALT: Here you go, Detective. Yep, all the DNA definitely belongs to Slimy Suspect. I’m 100% sure.
AD2: Great, thank you!
5 minutes later, setting—courtroom
Stern Judge: I sentence you, Slimy Suspect, to life in prison!
How many times have you watched TV shows with essentially this scenario? There are many inaccuracies here, but today I’m going to focus on one, involving that crime lab.
On TV, the lab gets the evidence and analyzes it, and the suspect promptly finds himself whisked off to jail. In reality, though, evidence analysis takes time. Most of this is due to high demand and limited capacity. It’s estimated that the current backlog may include well over 350,000 cases. And it’s not just DNA from crime scenes, but also drug testing and evidence from rape kits. Depending on the jurisdiction and the case, it may take over a year to get results back from the lab.
Time isn’t the only problem. Labs range in quality and not all are accredited. Their employees also vary in accuracy. There have been numerous reported cases of lab employees making errors in their work, either by accident or on purpose.
And even the best employees of the best labs can’t always get wonderful results. Forensic evidence can degrade or be contaminated. To give a personal example of this, when I was in college and working at a deli, I was robbed at gunpoint. When the police came, they dusted the counter for prints, but we all knew that was a pretty useless activity. Over the course of the day, dozens of people had touched that counter, so distinguishing the robber’s prints from everyone else’s would have been impossible. (They never caught anyone for the crime.)
And even when the evidence is in great shape, an analysis can never deliver absolute certainty. The best it can do is offer a probability of a match. And if the (innocent) suspect has an evil identical twin, he may be out of luck.
Finally it should be noted that DNA and similar evidence isn’t present at most crime scenes. If someone robs a bank, she’s probably not going to leave behind blood or anything else that is an obvious source of DNA evidence.
So those pretty detectives and lab techs might make for fun TV viewing, but they have little to do with law enforcement reality.