DNA and fingerprints don’t lie

One of the phrases that you hear most often in TV series that deal with forensic science is that evidence, unlike people, doesn’t lie. A variation of this statement is that the dead (their bodies), unlike the living, don’t lie. The gist of it is always the same: physical evidence includes the answer to find the author of a crime (Mac Taylor from “CSI: NY” seems to have so many of them available in the photo), you just have to find them and interpret them correctly.

In reality, however, most of the physical evidence that is found at the scene of a crime doesn’t allow to uniquely identify a person, is subject to the problem of contamination, and therefore can at most serve as supporting evidence to prosecution, but isn’t sufficient to send someone to jail.

The identification of a person (who is guilty or otherwise involved in a crime) can be successfully accomplished, in the absence of witnesses (which, however, can lie!), only if it involves some items found on the scene that belong exclusively to that person, actually, that are part of that person.

These are the biometric identifiers. There are two categories of them. The first includes physiological characteristics, such as fingerprints, DNA, facial, iris, or retina recognition. The second relates to behavioural characteristics, such as gait, voice, or handwriting.

These identifiers are typical of a specific person, but some of them are even unique and stable throughout life, and may be left at the scene of a crime.

I’m obviously talking about fingerprints and DNA.

In reality, finding this type of physical evidence that can be used to identify the culprit is quite difficult. Fingerprints, in particular, are everywhere at a crime scene and are often so incomplete and numerous that they cannot be used, unless they are found on the murder weapon. DNA is even rarer to spot. For both of them you must still do a comparison to determine their origin (for example, taking a sample of cells from the mouth of a suspect, like Greg Sanders from “CSI” is doing in the photo, next to a character played by Justin Bieber), but there are also databases (however, a still limited number of countries has a DNA database), therefore from a fingerprint or a blood trace found on the scene of a crime you can, at least in theory, trace back to a person, although this person has not yet been connected to the case.

This is a rare case where fiction resembles reality.

Even in fiction it’s unlikely that the culprit leaves behind fingerprints and DNA that can be used, but the reason is that, if they did, the case would be solved too quickly and there would be no story to tell!

This doesn’t mean that this kind of evidence doesn’t appear in TV series, films, and books. Far from it.

In fiction, fingerprints and DNA are almost always found during the investigation, but they belong to someone who is not directly guilty, which, possibly, tends to send off-road criminologists and investigators and to distract the reader/viewer.

But once again fiction highlights the absolute value of this type of evidence (well, it does so even with fibres and paint!), without considering some issues that affect them.

For example, while watching a typical episode of “CSI” you will have the impression that if you enter a fingerprint found on the scene in the computer you just need a short time to get a single finding: the one of the fingerprint’s owner.

It isn’t so at all. Apart from the fact that the time isn’t short at all but depends on the size of the database, you should also consider that the computer could deliver much more than one probable finding and none of them will be a 100% match. This is the moment when the expert intervenes and makes a visual comparison between the footprint available and the one in the database, to determine if they actually match and perhaps exclude some results, if not all. It’s a human being the one who makes this decision and, as such, may make mistakes.

Yet both in fiction and in reality (in this regard I will present some outrageous mistakes in the next article of this series) the fingerprint is considered an overwhelming proof, since there aren’t two people in the world with the same, identical fingerprints.

DNA is even more overwhelming. Here too it depends on its uniqueness (except for monozygotic twins), but not everyone knows that the DNA analysis doesn’t give an absolute result. In this case, the human factor isn’t involved, because the analysis speaks for itself, but its results are statistical in nature, since obviously you don’t analyse the whole available gene pool, but only a certain number of loci. It is likewise true that the probability that samples of two different people (not twins) returns the same profile is as low as to be considered zero.

So yes, DNA is a proof that doesn’t leave room for doubt, whether it is reality or fiction, except that in the latter its detection and its analysis is simplified, and they often take place in a ridiculously short time.

At the end, the criminologist is all smiles with a sheet (or a tablet) in his hands and goes to his boss to show them the result, after making the analysis and the comparison to the database within a half-hour. But then you have to hurry, because the case should be closed within the day and this proof will be only one of the many which will make the characters come closer to the solution, though without being the solution itself.

Even in the Detective Eric Shaw Trilogy, I used fingerprints and DNA. In “The Mentor” I talk about a case of tampering of fingerprints, which the protagonist uses to push a suspect to confess. In “Syndrome”, however, the fingerprints on a murder weapon will be an important element in the identification of the culprit. I cannot go into details, you must read the book to learn more (it isn’t available yet in English, sorry!), but I can say that I explained how the fingerprints were collected and that there are different ways to do that according to when they were created.

The first, which is also the most common one in fiction, is when you brush a dark powder on a surface to reveal any latent fingerprints (like in the photo above with Eric Delko from “CSI: Miami”).

In reality (and also in my books) this powder is called silver-grey and is actually used for this purpose to highlight the papillary lines, those that have been imprinted on smooth surfaces mostly due to the presence of sebaceous material on the fingertips (in “Syndrome” I took yet another small licence, indicating a surface on which it is generally not easy to detect fingerprints, but I was deliberately vague, suggesting that it was really smooth or assuming that criminologists, as usual, were lucky). This method is used only if the prints are relatively recent, but, if they date back to more than two or three days, they may have dried up, so it is necessary to use a chemical called ninhydrin, which reacts with the amino acids present in the sweat, generating a visible colouring.

Also, DNA makes its appearance in my series (on blood stains in “Syndrome”) and again, in the best tradition of fiction, it is used mostly for reshuffling the cards and complicating the job of the protagonists.

But, if nothing else, I can say I gave a touch of realism by showing that they had to wait at least a few days before getting the result of its analysis.