With the reliability of eyewitness testimony increasingly falling out of favor, and DNA evidence trumping it all, or so it seems, we’re facing a reckoning over what types of evidence really matter — and which types of evidence are so weak that they could potentially destroy the lives of a wrongfully convicted person and their families.
While every case is truly unique, there’s no denying that certain types of evidence will hold more weight in a courtroom than others.
But as science advances, the more “reliable” types of evidence—from fingerprints to hair left at the scene of the crime—are coming under scrutiny by leading scientists and associations.
For example, convictions based on hair may represent the next round of exonerations of wrongfully convicted inmates throughout the United States.
The following list is not a 100 percent undeniable ranking, but here’s our best shot at determining which types of evidence are most likely to send the right person to jail and the ones that make you stay awake at night … wondering if they really got the right one.
An unscientific ranking of different types of evidence
We have to give advanced DNA technology props for shaking things up and getting innocent people out of jail, with the Innocence Project leading the way.
While we’re ranking it at the top here, it’s not indisputable. But let’s start with the positives.
If the proper methods of DNA analysis are used, like the RFLP method or Restriction fragment length polymorphism if you prefer to use the elongated term, you can trace blood or bodily fluids like semen to a suspect with a very high rate of probability.
Other analysis methods may not be as accurate.
For example, the PCR method—polymerase chain reaction or other serological techniques—can determine with a lesser probability that the suspect is indeed guilty. These less costly methods effectively narrow down the possibility of a person belonging to a certain population group.
But, as with anything else, there can be flaws, as the Atlantic pointed out in a 2016 article—even with more advanced DNA analysis. For one, technicians who are not as highly trained to discern discrepancies can make mistakes.
Also, mistakes can be made in collecting DNA evidence as it is transported from the scene of the crime to the laboratory. Extreme care must be taken to ensure that it is collected with the right tools and preserved under the right conditions, and the process is not always executed to perfection. Human error can compromise the integrity of the most valid DNA evidence.
Yet, with expert analysis, DNA still ranks at the top of the list for determining a person’s guilt in a crime. We just can’t call it 100 percent infallible.
A crime scene investigator should consider themselves pretty lucky to nab some solid fingerprint evidence at a crime scene. Right?
Next to DNA evidence, latent fingerprints supposedly can identify with nearly 100 percent accuracy that a suspect was at the scene of a crime. As has long been proclaimed, a person’s fingerprints are unique only to them.
It’s become such a standard part of crime-solving that when a person is booked into the criminal justice system, a photograph and fingerprinting is a standard part of the process. That data is then entered into a national database run by the FBI, the Integrated Automated Fingerprint Identification System (IAFIS).
Though run through a computerized system today, the use of fingerprints to catch criminals is hardly new. The use of it as evidence dates back to 1910 when Thomas Jennings, an ex-convict who had just been paroled, was accused of the murder of a Southside Chicago man who had chased him after he suspected Jennings was the man who broke into his house.
During Jennings’ escape, he jumped over a freshly painted stairwell—leaving an undeniable set of fingerprints. His murder trial led to the first conviction based on fingerprint evidence, and it would go on to be used ever since.
However, just like with DNA, nothing is 100 percent accurate when it comes to the use of fingerprints as evidence, as the American Association for the Advancement of Science noted in a 2017 report—following similar findings by several other organizations.
Also, as with the gathering of other types of evidence, there can be flaws in the process—including the inability of crime scene investigators to always get full prints from the crime scene.
According to the AAAS report, “We have concluded that latent print examiners should avoid claiming that they can associate a latent print with a single source and should particularly avoid claiming or implying that they can do so infallibly, with 100% accuracy.”
So, there it is. Fingerprints are near the top of the list, but even they’re not a guarantee.
A single strand of hair found at a crime scene also ranks as an important piece of evidence since it falls in the category of DNA analysis.
Recent advances in DNA analysis —during the past 20 years—have made hair strands a more reliable form of evidence, but only if the root sheath is attached.
Using the PCR technique, a forensic scientist can determine if the suspect has genetic ties to a certain percentage of the population. However, this method is effective only if the root sheath remains intact. Without the root, a hair fragment can only be used to draw similarities to the suspect’s hair—i.e., the hair is similar to that of the suspect. And that’s where things can fall really, really short on the reliability scale.
Researchers have found that the use of hair as evidence has been majorly flawed in the past, leading to more discoveries of people who have been wrongfully convicted, including a Wisconsin man who had served 29 years in prison before his conviction was overturned.
In these cases, the problem was linked to faulty techniques that were implemented before 2000, when the FBI started routinely using mitochondrial DNA testing. According to a 2015 FBI report, microscopic hair analysis contained errors in at least 90 percent of the cases that had been analyzed before 2000.
The body has a lot to tell. That’s why broken fingernails—likely left behind after a struggle between the perpetrator and the victim—represent another top-ranking piece of evidence.
Fingernails can reveal unique striations that, if matched to those of the natural fingernails of the suspect, can lead to a reliable piece of evidence that places them at the scene of the crime.
If a firearm was discharged during an assault, crime scene investigators have an invaluable set of evidence—from bullets and casings to gun residue.
Any bullets and casings found at the scene of the crime will be sent to a crime lab, where forensic technicians can determine if they are a match for a gun found in the possession of the suspect.
If the gun was discarded, investigators can use a combination of fingerprints and a trace of the gun’s history to determine if there is a previous link to the suspect. Technicians can also determine if the firearm was used in a previous crime.
The evidence doesn’t end there. A ballistics examination, which includes a study of projectiles, motion, dynamics and angles, can paint a picture of what happened during the course of a crime. It can provide details as to whether the cause of death was by suicide or homicide. If it was an act of homicide, they can deduce where the shooter was standing at the crime scene. Even in the absence of bullets, marks left on a surface can give technicians enough information to identify the type of weapon used in the crime.
Gun powder residue also can be used to tie a person to a crime. When a gun is discharged, gunpowder particles automatically land on the skin and clothing of the shooter. These unique particles can then be examined against the skin and clothing of a suspect. Advanced techniques in analyzing gunpowder residue—Scanning Electron Microscopy (SEM)—have made this type of evidence much more reliable than former modes of analysis conducted 30 years ago.
When arriving at a crime scene, investigators will carefully assess the area for the presence of shoe prints. These can lead to the identification of suspects in the case. Technicians and investigators have a combination of options in capturing the prints, including photographing them, lifting them from a hard surface in a similar fashion as they would a fingerprint, and by making a cast if the footprint was left in soil, for example.
Using this information, investigators can determine if the size of the imprint matches that of a suspect. They also can determine if the type of shoe is similar to any of those owned by the suspect.
If a shoe print is hard to identify the pattern can be entered into the BCA Footwear Reference Collection database, which includes hundreds of shoe print impressions that have been collected through casework.
Tire tracks, similar to shoe prints, can leave a distinct impression that gives clues to the type of vehicle and/or type of tires on the vehicle that were at the scene of a crime.
The investigators may compare the tire tracks to those in the Tire Tread Design Guides®, an annual collection of technical tire information designed to assist investigators and others seeking this type of information.
As to whether it is a reliable form of evidence to actually convict someone is up for debate. Some critics argue that it is subjective since thousands of people can have the same type of tires on their vehicles.
However, advocates of the validity of tire impressions, point out that tires are worn down in unique ways by their users, allowing them to be used in a case with more confidence.
Other types of trace evidence
In addition to hair and gunshot residue, there are other types of trace evidence that can be useful in a crime investigation, including fibers, and glass and paint fragments. These can be left by the perpetrator or either picked up on their clothes or skin during the criminal act.
For example, fibers gathered from the crime scene that matches the color and type of clothing found in a suspect’s possession in their home or vehicle can be used to support the probability that the suspect was at the scene of the crime.
However, unlike DNA evidence, this type of evidence is not entirely unique since others can possess the same type of mass-produced clothing.
Sometimes a criminal will have a history with his victim, writing letters that may have revealed escalating tensions, or will taunt police and journalists with cryptic notes as they search for clues in a case.
While writing messages by hand is becoming a lost art in favor of digital forms of communication, handwriting analysis has been used to help solve crimes.
Experts in handwriting analysis can detect patterns in signatures, block lettering or misspellings that show consistencies or inconsistencies between two or more handwriting samples. The idea behind this analysis is that every person has a unique way of writing.
The reliability of this type of evidence has been debated among some experts who consider it illegitimate and others who advocate for it.
And, again, advanced technology is coming into the picture to change the narrative.
Forensic Information System for Handwriting (FISH) has been used to prove that handwriting can be approached from a scientific point of view. With this computerized system, examiners can scan documents to digitize the process—removing human error from the equation.
Bite marks as evidence is at the bottom of the list for numerous reasons. But first, let’s review the traditional way they have been used in court cases.
If the perpetrator left behind bite marks on the victim during an assault, crime scene or forensic investigators would take photographs and make a cast of the tooth impression to compare them to the suspect in the case.
In these cases, a forensic odontologist, who specializes in identifying remains and linking bite marks to an individual, would be consulted for further expertise. In a court case, this specialist may be called on to testify to the validity of their findings.
However, this form of identifying evidence has come under scrutiny or been entirely dismissed in the past 10 to 15 years.
In fact, the Innocence Project declared in a report that “Bitemarks Should Never Be Used in Criminal Trials.” The organization cited a case in which 19 marks were found on a three-year-old murder victim. A medical examiner believed that they were bite marks and called on a forensic odontologist to examine the marks on the body.
That forensic odontologist, Dr. Michael West, concluded that the marks came from teeth and that they were “indeed and without a doubt” marks made by the accused. That testimony heavily weighed into the death row conviction of Kennedy Brewer, who spent 15 years in jail before he was released.
The Innocence Project isn’t the only organization that has been questioning the reliability of bite marks.
In 2009, the National Academy of Sciences (NAS) released a report highlighting a high rate of errors with bite mark analysis and other forensic disciplines. They also said that it had not been scientifically validated.
One of the major findings was that a bite mark in skin is not the same as leaving an impression in plaster. Since skin is elastic, it can’t preserve teeth marks in the same way for examination. According to the NAS report, skin deforms bite marks created in a lab so much and with so many variances that it can not be used as reliable evidence to identify a suspect.
The American Board of Forensic Odontology also released a study that found many of its members could not conclusively determine whether the sets of bite marks from case files were actually bite marks.
Ranking the evidence—an evolving equation
With increased scrutiny, further research and advancements in technology, the methods and types of evidence used to convict criminals will undoubtedly continue to evolve. And, if recent exonerations of innocent people is a sign of things to come, the justice system may get better and better in going beyond a reasonable doubt in catching the ones who really did it.