Reliability of Forensic Science

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Due to the popularity of crime shows, most people are familiar with forensic scientific techniques used in the investigation of crimes. In fact, many people believe that things like fingerprints, ballistics, and bite mark comparisons offer undeniable concrete evidence that can definitively tie someone to a crime. Other than having videotaped evidence such as in the Gaines case, forensics are needed to either prosecute or exonerate individuals of a crime. Forensic techniques include DNA analysis, fingerprinting, bite mark comparisons, ballistics, hair and fiber analysis, and burn patterns. How reliable are these methods, though? Despite widespread public faith in these methods, the validity of many of them has recently been called into question. Investigation into these ‘scientific’ techniques shows that some are certainly much more reliable than others. 


In 1984, Alec Jeffreys, a geneticist from Britain, discovered DNA printing, changing crime investigation forever. Since this discovery, this scientific tool has successfully identified perpetrators of crimes and even exonerated people who had been unjustly convicted.  DNA analysis is certainly one of the most reliable forms of forensic science, due to the fact that it underwent arduous scientific investigation and substantiation before becoming an acceptable forensic tool. In 2009, United States federal judge Harry T. Edwards said, “Among the biggest problems that we uncovered in the report is the absence of the application of scientific methodology to determine whether or not the discipline was valid and reliable as was done with DNA” (Jones). The analysis of DNA is the one form of forensic science that can be confidently depended upon to produce consistently reliable results. In fact, according to the co-founder of the Innocence Project, when they reviewed cases in which people who were convicted of a crime were later exonerated with DNA evidence, they found that the experts who testified on the prosecution’s side made a mistake during the investigation process or produced false evidence sixty percent of the time (Jones). Because it is entirely based in science, it is incredibly dependable.


Police have been using various body prints- fingers, feet, hands, etc.- for over a century to aid in the solving of crimes. Scientifically known as ‘friction ridge analysis’, this technique requires investigators to compare prints collected from a crime scene to known prints kept in the police database (Jones). Investigators examine the loops, ridges, and whirls in the prints that supposedly make them unique. Fingerprinting has gained a reputation for being solidly reliable and infallible (unlike faulty eye-witness recollections) under the assumption that examiners are able to determine which single source the print came from. It is widely believed, and perpetuated by television crime shows, that all that the police need to definitively link a suspect to a crime is one perfect print (Newman). However, in the last decade and a half or so, the dependability of this form of forensic science has been called into question after a number of crucial errors; in particular, in the example of Brandon Mayfield. During the investigation of the terrorist bombings in Madrid in 2004, which left nearly two hundred people dead, the FBI found a partial fingerprint on a bag containing detonators that was delivered to their central office (Schuster and Frieden). It was determined that the fingerprint matched that of Oregon lawyer Brandon Mayfield. Mayfield was immediately detained by police. Four separate examiners stated that his print matched the print from the detonator bag, including one that was hired by Mayfield’s own defense team (Jones). It was later discovered by Spanish officials, though, that the print actually belonged to Daoud Ouhnane, a suspect from Algeria. Mayfield was released and sued the government, a case that was settled for twelve million dollars. As a result, parameters for using fingerprints as a form of crime scene investigation were tightened and more fiercely regulated. Ken Moses, a veteran fingerprint examiner and one of the examiners who incorrectly matched Mayfield’s fingerprints to the suspect’s said that he knew his field was changing when there were suddenly a slew of new rules and protocols to follow (Jones). People’s faith in fingerprint analysis began to wane. The National Academies of Sciences stated there have been no scientifically reliable studies to prove that fingerprints are unique from person to person, which is something that is widely assumed. There are, however, studies that prove that contextual bias can affect examiners when comparing fingerprints. Jennifer Mnookin, a law professor at UCLA whose studies error rates in fingerprint comparisons are funded by the federal government, said, “Prior to Mayfield, there were some people in the fingerprint community who really were saying that something like Mayfield could never happen. And so part of the problem here really is about hubris or over-claiming. It’s about a field that didn’t seem to feel a need to recognize its limits.” (Jones). FBI fingerprint experts have since refused to testify in court that fingerprinting is definite as a result of the Mayfield case.

Bite Marks

Bit mark comparison is perhaps the most controversial method of forensic analysis. Like the doubt in validity surrounding fingerprinting, bite marks are often assumed to be totally unique for each person. Another characteristic they share is that such assumptions have never been adequately studied and scientifically scrutinized (“Bite Mark Evidence”). Attorneys that work with the Innocence Project state that bite mark comparisons have led to a several unfair convictions. Levon Brooke and Kennedy Brewer, for example, are two men who were wrongfully convicted for crimes they did not commit as a result of falsely matches bite marks. A significant number of people have been convicted of a crime based on bite-mark comparison and were sent to death row as a result; later, these cases were found to have been invalid due to unreliable testimony of odonatologists and forensic dentists (Jones). There just is not enough proof in the reliability of bite mark analysis for it to be seen as indisputable evidence. 

Firearms and Ballistics

Firearms, ballistic identification, and bullets are used in crime scene analysis whenever a weapon goes off while a crime is being committed. Forensic experts are called in to examine the weapon and ballistic factors and may later be called into court to testify to the weapon used, gunshot residue, or the direction the bullet came from or traveled. The National Academies of Sciences states that while ballistic science is not always exact, similar marking can be found on cartridges and bullets from the same weapon. However, they also recognize that no one has ever demonstrated completely ‘bullet-proof’ evidence that firearms produce unique and consistent tool marks. They remarked that there still needed to be a momentous amount of research to truly determine if only one firearms could produce the exact same unique marks every single time (Cork, et al. 3). One of the main problems that prevent firearms ballistics from being definitively reliable is the lack of a precise process in ballistics analysis. Ballistics examiners sometimes testify in court that two sets of tool marks have a “sufficient agreement” between them, but currently, there is no specific definition for what that actually means, as there is no solid proof that each firearms has totally unique tool marks (Jones). There are not even any specifications as to how much similarity there should be between two sets for the ‘sufficient agreement’ to be present; it is left up to the examiner.

Hair and Fibers

Hair and fibers can supposedly prove who was and was not somewhere at a certain time. However, the misidentification of these tiny pieces of evidence has resulted in hundreds of defendants across the United States in prison or on parole that may be owed exoneration (Hsu). Sometimes, such flaws FBI work has led to the execution of innocent men. A 1997 case in Texas left Benjamin Herbert Boyle a victim of wrongful conviction when a hair linked him to a crime he did not commit; Boyle was executed by the state before he could be exonerated (Hsu). In another case, John Norman Huffington is currently serving a life sentence in Maryland due to exculpatory findings by the Justice Department. Once his attorneys learned of this, they began seeking a retrial. The Department of Justice, though, felt they had been their legal obligations and were not required to inform defendants of possibly inaccurate crime scene analysis. The investigation into the case took nine years but the findings have never been made public. Even the information that examiners can often ‘match’ hairs and fibers incorrectly or that such analysis lacks specific lab protocols, the Justice Department asserted that the conviction should stand (Hsu). Several other cases exist in which people were wrongly convicted of crimes with hair and fiber evidence and served life sentences anyway. 

Burn Patterns

Fire investigators are called to the scene when a fire occurs to determine a cause for the fire. The National Academy of Sciences, though, stated that more research is required on the erraticism of burn patterns and the damage they cause in the presence of a number of accelerants before the analysis can be seen as totally reliable. Research has shown that several signs that are considered to be unique to arson can actually be caused by natural variables in accidental fires (Jones). The majority of the fire investigation community is firemen rather than forensic scientists; investigating fires requires a different skill set than those required in extinguishing them. Scientific expert Gerald Hurst reported that it is “frighteningly simple, frighteningly easy” to convince a jury that arson was committed (Jones). Death by Fire, a PBS documentary about the validity of fire forensics in crime scene investigation, called into question the conviction and execution of Cameron Todd Willingham for the arson-murder of his own children. According to the scientific expert who reviewed the case for the documentary noted markings that were traditionally interpreted as a pour pattern where an arsonist poured accelerant. The judge who reviewed the case noted the odd shape of the pattern and believed that accelerant had been poured in the shape of a pentagram. However, the documentary reveals that where the patterns spike in what was believed to be the points of the star, the fire was actually just followed traditional ventilation patterns to the five windows in the room (“Death by Fire”). The case report also stated that an accelerant was found in a wood sample taken from the porch leading into the house, but the experts in the documentary point out that the sample was taken from a spot on the porch near the gas grill. It was concluded by the documentary that the investigators did not definitively rule out accidental causes. This happened four days before Willingham’s scheduled execution and his attorney filed several requests for a stay of execution, but he was executed anyway. 


Forensic science is often assumed to produce indisputable results. These methods include DNA analysis, bodyprinting, comparison of bite marks, firearm ballistics, hair and fiber analysis, and burn patterns. Many would be surprised to learn, though, that the only one that can offer concrete results is the analysis of DNA. All other methods lack reliability and validity, despite their continued use in crime scene investigation.

Works Cited

“Bite Mark Evidence”. California Innocence Project. California Innocence Project, 2016. Web. 26 Jul. 2016. <>

Cork, Daniel L., et al. Ballistic Imaging. Washington D.C.: National Academies Press, 2008. Print

“Death by Fire”. Frontline. PBS. 19 Oct. 2010. Web. 26 July 2016  <>

Jones, Jonathan. “Forensic Tools: What’s Reliable and What’s Not-So-Scientific”.  Frontline. WGBH Educational Foundation, 17 Apr. 2012. Web. <>

Hsu, Spencer. “Convicted defendants left uninformed of forensic flaws found by Justice Dept”. The Washington Post. The Washington Post, 16 Apr. 2012. Web. 26 Jul. 2016 <>

Newman, Andy. “Fingerprinting’s Reliability Draws Growing Court Challenges”. The New York Times. The New York Times, 07 Apr. 2001. Web. 16 Jul. 2016 < court-challenges.html>

Schuster, Henry and Terry Frieden. “Lawyer wrongly arrested in bombings: ‘We lived in 1984’”. Cable News Network, 20 Nov. 2006. Web. 26 Jul. 2016 <>