I don’t throw in a lot of information about evidence in the D&R books. This is intentional. Despite what the media would have you believe, most forensic evidence isn’t all that solid. Circumstantial evidence is actually still the strongest evidence regardless of what TV and movies and popular crime novels would have you believe. Unless of course the idiot that committed the crime was nice enough to video or livestream themselves doing it, that is… Which sadly happens more than you might realize.
Even then, circumstantial evidence has flaws. Since these books are more about the chase than the prosecution of criminals, I tend to ignore it. Here’s some things we’ve learned about forensic evidence in the last decade or so.
DNA is fragile and resilient at the same time. This means a killer might touch a hammer and not leave any DNA, but if you touched the hammer while it was on shelves in the store, it is entirely possible your DNA is still on the hammer when crime scene techs collect it. Sweat is an interesting thing… It can leave DNA and it can remove DNA and it can destroy DNA. It is actually person dependent.
Then there’s the problem of matching a DNA sample to someone. We did this when I was in high school. DNA is put in a sterile container and spun in a centrifuge, breaking it up and unravelling it. It is then placed in wells in a block of auger and stained blue. The created by the stain travels up and down the auger and it is these that actually get compared (there isn’t a fancy computer that does it). The first thing I realized when we spun cow DNA in my genetics class was that there were no lines on the block of auger to match up which blue lines went with which blue lines. Now imagine trying to compare two blocks of auger for mostly identical blue lines in all the same places as each other. It is awesome in a “holy shit my eyes are exhausted” kind of way. Yep, DNA comparison relies on human eyesight to match up blue lines in blocks of auger. The results are entered into a computer after the technician finishes the comparisons and a print out of that comparison is given to those that need it.
A tech with a headache or allergies could easily screw it up and they do… This is why multiple tests are done and different technicians are used and sometimes different labs are used. It’s also why one lab might say it matches Person X with 99% certainty and another lab might say it doesn’t match with 99% certainty.
What’s worse, we don’t match all the genes. Considering how much DNA is considered “junk DNA” or genes shared by almost all humans or genes shared between humans and turtles, it becomes even sketchier to rely solely on DNA evidence.
Most eyewitness testimony is like slamming your foot repeatedly in a door. Eyewitnesses are rarely reliable. Even in recounting it later. Person Z might be 100% certain two days after witnessing a crime, but two hours after the crime, not so much and two months after the crime, yeah that reliability didn’t get better. A study done in Norway or Sweden, one of those Nordic countries with too much snow, found that criminals are actually better witnesses than witnesses and criminals are better at identifying witnesses than witnesses are at identifying criminals. It also found that the recollection skills related to a crime were better by criminals than by witnesses. And that criminals that witness crimes are more reliable than the general public as far as how well they remember details, but are far less likely to tell the truth. But have no fear, you can train yourself to be a better eyewitness. Unfortunately, you either have to hone this skill to use all the time, which is cumbersome, or hone it and hope the days your brain is just randomly practicing it, you witness a crime… It essentially requires you to be more connected to your surroundings, being aware of everyone, even those in the periphery of your vision, and understanding how to commit all those details to memory. The easiest thing is to try to relate it to someone you know. So if you walk into a gas station and see it being robbed and manage to get out, if you can remember the robber looked like your Uncle John, you’ll remember more about him and why you thought he looked like your uncle. Unfortunately, that also creates a bit of a memory bias in the sense that you might remember why he looked like Uncle John, but not the details that don’t look like Uncle John, in which case the police might be knocking on Uncle John’s door.
Bite mark evidence has been almost completely disproved. With many experts revisiting cases and wondering if the “bite marks” were actually bite marks at all. Meaning if the clerk points to his wrist and says the robber bit him and there’s an obvious bruise, that bruise is now bite mark evidence and even if Uncle John doesn’t have teeth, he could still be found guilty using bite mark evidence, because there has a been a serious spate of cases overturned due to bite mark evidence that turned out not to be bitemarks at all. There have even been a handful of cases overturned because the bite mark evidence that matched the bite mark of the suspect that was most likely convicted based on the bite mark evidence turned out to be bitemarks from animals that were misidentified as being primate bite marks.
Expectation bias is also a problem in forensic science. If a scientist has an expectation of what the evidence will tell them, ie; Uncle John robbed a gas station and there were some fingerprints recovered from the cash register, if they resemble Uncle John’s fingerprints, the fingerprint technician’s brain might go ahead fill in the gaps where Uncle John’s fingerprints don’t match the suspects so that in the end, Uncle John ends up robbing a gas station while also sitting at home watching TV in his underwear while eating Twinkies and drinking beer, by himself.
Expectation bias is a serious issue as it can cloud almost all forensic evidence from fingerprints to fibers to DNA. And forensic scientists are bombarded by bias every day of their working careers. They get it from detectives, they get it from prosecutors, they get it from grieving families, and all of it, especially if there is a suspect in custody can create a problem. We are actually starting to understand that not having a suspect in custody and not having a detective breathe down the neck of a forensic scientist, leads to better forensic science.
So I leave most of it out, letting the mystery of detectiving continue to be a mystery. I hate perpetuating unrealistic expectations and stereotypes.