Forensics should be a scientific process. But critics have complained for decades evidence is not always appreciated under laboratory conditions, and empirical studies do not support the methods of analysis.
The consequences of faulty forensic evidence are severe. Forty-five percent of wrongful convictions which were later overturned due to DNA evidence turned out to be the result of inaccurate evidence. Advocacy groups like the Innocence Project argue that many forensic techniques, such as bite mark analysis, are unreliable and unscientific.
In 2016The President’s Council of Advisors on Science and Technology (PCAST) examined the scientific validity of commonly used forensic methods and found that most are not reliable.
In recent years, an increase in peer-reviewed studies has helped forensic medicine become more consistent and systematic. Here are five techniques that scientists continue to test and improve:
1. DNA from a single source / Top 5 forensic evidence used to solve a crime
At the crime scene, evidence technicians collect samples blood, hair, sperm or skin cells to create a DNA profile. In this process, DNA is chemically extracted from the sample and polymerase chain reaction is used to amplify the DNA segments. The resulting DNA fragments are measured and analyzed by a software program. Many states already have databases of DNA profiles of past arrestees, and the profile in question can be compared to others for a match.
PCAST believes that single-source DNA is a reliable forensic technique well supported by empirical research. However, the board also warned that the method could be subject to human error if samples are tainted in the field or mislabeled in the lab.
Read more: The problem with forensic science
2. Ballistic forensics / Top 5 forensic evidence used to solve a crime
The type of weapon used in a crime can be informative to police, prosecutors and judges. Ballistic analysis is becoming more sophisticated and the method is improving reliability. In 2022 study in Journal of Forensic Sciencesresearch team found that professional analysts had a high accuracy rate for correctly identifying bullets and shell casings.
The research team recruited 173 participants through professional organizations such as Association of Firearms and Instrument Examiners. The team sent a test pack to the participants containing 15 sets of cartridges and 15 sets of bullets. About 80 percent of participants made no errors in identifying either the bullets or the casings.
The research team found that most errors belonged to only a handful of participants. Thirteen participants were responsible for more than half of the errors, and six participants made 30 percent of the errors. These results are consistent with other firearms studies.
Read more: How science helps catch criminals with better ballistics
3. Blood Stain Analysis (BPA) / Top 5 forensic evidence used to solve a crime
At a suspected crime scene, the BPA may provide additional information to law enforcement. BPA, for example, can determine whether it is the result of a fatal gunshot suicide or murder. Or, if the accused claims self-defense, the BPA can show whether there really was a fight.
Testimony in the courtroom has accepted BPA for more than 150 years, but the PCAST report criticized the technique’s high error rate. Scientists responded with empirical studies to test levels of accuracy and identify consistent flaws in the process.
In a 2021 study in International Forensic Medicine, an interdisciplinary team tried to determine how often analysts are wrong or contradict each other. They employed 75 practicing analysts who reviewed 192 bloodstain patterns.
In samples with known causes, participants had an 11.2 percent error rate. They also contradict each other in 7.8% of cases. The authors concluded that the rate of error and controversy could be reduced if the BPA discipline streamlined its terminology and limited semantic disagreements, which they warned could have serious real-world consequences.
4. Fingerprint analysis
With this method, investigators identify a fingerprint or partial fingerprint left at a crime scene or on a murder weapon. They then compare the print to full kits made by suspects or stored in databases. The technique has been used in the US for more than a century, but only in the last 20 years have scientists begun to study the process.
2004 bombing in Madrid’s metro system prompted scientists to rethink the reliability of the method. Spanish investigators are struggling to identify a suspect. They approached international investigative groups, such as the FBI, and provided their evidence, including a latent fingerprint found on a bag of explosives.
The FBI determined that the print matched an Oregon attorney whose full set was registered due to his past military history. But at the time of the bombing, the lawyer said he did not have a valid US passport and had not left the country since the early 1990s. The FBI later had to apologize for the wrongful charge, and forensic scientists began to question the accuracy of the fingerprint analysis.
In the 2016 PCAST report, the board noted that the FBI conducted empirical studies after the bombing to improve reliability. The report concluded that the method had become “fundamentally valid” but warned that jurors should be informed that false positives do occur. In one study they cited, there was one error in 306 cases, but it was as high as 1 in 18 in another study.
5. DNA with multiple sources
At crime scenes, investigators may encounter more than one DNA profile. For example, there may be blood samples from the victim and the perpetrator. Or there may be evidence of “touch DNA,” meaning skin cells remain when multiple people touch the same object.
The PCAST report noted the growing interest in “touch DNA” but found that DNA from multiple sources was most reliable when limited to two profiles. The council warned that DNA analysis of “complex mixtures” was not yet groundbreaking and could lead to erroneous results.
Touch DNA, for example was used to condemn American student Amanda Knox for murdering her roommate in Italy in 2007. Knox was later acquitted.
Read more: How Indirect DNA Transfer Challenges Forensics and Overturning Wrongful Convictions