Lucky criminologists and maladroit criminals

One of the most common ways to find out the culprit of a crime, in fiction, is to let a tiny physical evidence be found on them, or maybe their car, thus linking them undoubtedly to the scene or even the victim; or to find (always unique and very small) evidence on the scene or the victim that is certainty linked to a suspect.
It seems easy to detect such evidence (Abby Sciuto from “NCIS”, in the photo, does it all the time) and, when it happens, this discovery is almost overwhelming, often pushing the suspect to confess.

A typical example is some paint found on a car forced off the road by another vehicle. It is quite common that, following a violent impact, some paint is transferred from one car to another; indeed it almost always happens. Think about all the times you found a little gift on the bumper of your car by someone who leaned to it while parking or who made a mistake while exiting from a parking space.
But is it really possible to trace back the model and manufacturing year of the car? Generally not, because the same widespread paints are used in different models, often different brands, in different periods, and even if the type of paint is determined (besides the colour, I mean), this doesn’t narrow too much the search field. The only exception occurs when you have to do with some very rare paint used in an equally rare vehicle, for example, a limited series or a classic car with the original paint. Usually here a sci-fi database comes out like magic, which in five minutes at most starting from a paint chip pulls out the photo of the car that, incidentally, is so rare that very few people in the whole town (and we often talk about big towns or even cities, like New York) have one, including a potential suspect that shows another small link with the victim.
There you have it: a tenuous link with the victim plus the same paint of their car, and the suspect collapses.
Yes, I killed her!

Sometimes I wonder why these criminals are so maladroit or unfortunate enough to use rare vehicles to carry out their misdeeds.
This so unusual that (almost) never occurs in the real world. Usually even being able to go back to the colour and perhaps the brand of the car is just one more element to substantiate the suspicions about a specific person, but doesn’t add any certainty.
But there is a particular situation where the paint can be important and become an irrefutable proof: when a car has been repainted several times and the same stratification is found as physical evidence.
But a doubt always remains. Are we sure that the transfer took place precisely during the crime and not earlier? Most times we are not.
This is the problem that plagues all kinds of residues, whether it’s paint, glass, dirt, and even the one coming from a gunshot: we cannot establish the precise moment when it settled.

Let’s think about glass. The murderer broke a window to enter the house of the victim. The suspect has a tiny fragment of glass on them. Is it the culprit? Assuming also that the glass is of the same type, it could also be someone who has been on the scene after the crime, or maybe who visited a neighbour of the victim, in the same building, who has an identical glass, which is broken.
Here, as for the paint, applies the case of very rare types of material, but that would be a stroke of luck.

What about the soil? Well, in TV series such as “CSI” it isn’t unusual that from micro-grains of soil found under the soles or on the clothing of a victim they come to find out exactly the primary scene of the crime. There is always some pollen from an exotic plant that is grown only in one place within hundreds or thousands of kilometres, where, despite a size of several hectares, our investigators discover more physical evidence pointing to the culprit. Obviously there is a special database with all pollens: you just have to do a banal search by image. Never once it happens that they find a very common soil or that the place of origin is so broad to be a dead end or that it is then found out that the soil ended up on the victim as a result of a chain of common contamination that leads to absolutely no result.

And then there are gunshot residues.
When firing, a cloud of residues is ejected by the weapon, containing elements such as antimony, barium, and lead, with a composition that is typically specific of a certain weapon with a certain type of bullets.
If a person was killed with a certain weapon, which was found, and the same gunpowder residue produced by it is detected on the hand of the suspect, it is very likely that he/she pulled the trigger.
What if they have used a pair of gloves? Well, residues may still have ended up on their clothes, only that in this case there would be no certainty that they shot. The suspect may also have been in the vicinity, while someone else was shooting, or even got into contact with the murderer at a later time and have nothing to do with the crime. On second thought, if they shook hands with the murderer shortly after the shot, they could have the same gunpowder residue, even if they aren’t the culprit. It gets even more complicated, if we’re dealing with people who use weapons and are routinely subjected to multiple contaminants from gunshot residues.

In short, all the physical evidence that is used very often in fiction (and we love it) to nail the culprit, in reality, most of the time are totally or nearly useless.
But in fiction we like to have fun and authors of crime fiction like me are in their element when using this. For example, I mentioned gunshot residue in the second chapter of “The Mentor”, in which Detective Shaw leads a man suspected of being a killer for hire to confess a murder, after having framed him with false fingerprints on the murder weapon. The suspect, a certain Damien Johnson, is led to think that he cannot prove his innocence (there is also the fact that he isn’t innocent), because, although during the crime he was wearing gloves and so couldn’t possibly leave fingerprints, he has recently used a weapon during his job as security guard and then the gunpowder residue along with the false evidence nails him. To refute it he should say that he used a pair of gloves, but that would still be a confession. Eric knows that the two residues may not match, but Johnson, who is aware to be guilty, is so resented about being framed that he doesn’t even think about it (perhaps he even doesn’t know that the residues may have different compositions: indeed he isn’t a criminologist!) and he decides to negotiate a plea bargain. The story then quickly goes on, the character is put aside and the reader doesn’t think about it anymore.

Similarly no one is scandalised if in “Bones”, when they examine the skeleton of a victim (who knows why, anyway, they always decide to strip the flesh from the victims to find out how they died, even from all those that are found whole, while in other series a normal autopsy is more than enough), they always find only a tiny trace embedded in a bone (of course) from which a series of clues comes out leading to the killer. If we were to reflect on each decision made by the characters, we would soon realise that they could have made many different ones, which would take them completely off the track. What else could I say? They must be geniuses (and Temperance Brennan constantly emphasises that), but on second thought, it seems that they are just very lucky!

Could it be the reason why, in fiction, the cases are solved in a day, but often in the real world after years of investigation, even when they seem to be solved, you are never sure you have found the real culprit?
Yes, even in this case we cannot deny it: although reality often exceeds imagination, fiction is much more fun and, above all, reassuring.