How Accurate is DNA Evidence?

By Eva Thiel, MScReviewed by Megan Craig, M.Sc.

DNA evidence has revolutionized forensic science and continues to play a vital role in solving countless criminal cases worldwide. From murder scenes to your daily hit-and-runs, DNA material is routinely collected from low-level offenses and high-level cases daily (Bond, 2003). Its accuracy and reliability have always been celebrated, but it is also essential to critically evaluate the limitations and potential sources of error associated with DNA evidence analysis.   

Interpreting DNA Profiles

Interpreting complex DNA profiles can pose many challenges and introduce interpretation errors. A study conducted by Taylor et al. (2022) explored the impact of DNA mixture interpretation and the accuracy of the interpretation. DNA samples that are considered mixtures include DNA from multiple contributors, such as in cases of rape.

Such mixtures can be complicated to analyze and require specialized techniques and training to interpret the results. The ability to determine the contributors of each individual in a DNA mixture sample accurately remains a challenge, and its impact can be detrimental to case results while also impacting the reliability of DNA evidence in court.

Contamination and Sample Degradation

A significant factor affecting the accuracy and trustworthiness of DNA evidence is its high potential to be contaminated, such as during the collection, handling, and processing of the samples (Forsberg et al., 2020). To minimize the risk of contamination, personnel involved in the investigation and handling of the DNA samples need to adhere to rigorous protocols, complete proper training, and adhere to strict chain of custody procedures.

Contamination can occur in many forms, not only from individuals handling the samples but also from external sources. For example, crime scenes that are outdoors are exposed to nature, casual individuals, and the weather, possibly contaminating DNA evidence prior to crime investigators being able to preserve and collect the samples. Due to exposure to environmental conditions, a study by Dutta et al. (2022) emphasized the importance of assessing the degradation degree of DNA samples collected before analysis.

False Positives and False Negatives

False positives and false negatives in forensic DNA testing can be due to several factors, such as errors in sample handling or test interpretation (Thompson et al., 2003). A false positive occurs when a DNA profile is incorrectly determined to be a ‘match’, which leads to a false accusation of an innocent individual.

A false negative arises when a true match is deemed inconclusive or inaccurate, which can cause a guilty individual to walk free of any criminal charges. Some of the factors that contribute to the occurrence of false positives and false negatives include technical errors, sample contamination, and challenges in interpretation (Johnson et al., 2020). To reduce the risk of such errors, there needs to be continuous research to understand these factors and improve laboratory procedures and protocols in DNA sample handling and analysis.

Statistics and Analysis

The statistical analysis of DNA evidence plays a crucial role in interpreting the results. Estimating a match probability or likelihood ratio is a fundamental component of DNA profiling. However, the accuracy of these statistical calculations depends on the underlying assumptions and available population databases. An article by Bright et al. (2022) highlighted the need for comprehensive databases that accurately represent the relevant populations to avoid potential bias and inaccuracies in calculating match probabilities.

Expert Testimony and Admissibility in Court

After collecting, handling and analyzing the DNA samples, the results must be presented in court. This is when the accuracy and reliability of DNA evidence impact its admissibility and weight in court and in given expert testimony. Emphasis is placed on the importance of ensuring proper validation and peer-reviewed methods in Forensic DNA analysis to ensure this occurs (Roberts, 2020).

DNA analysis is subjected to scrutiny regarding its accuracy, reliability, and potential limitations, which is why legal systems require robust scientific evidence to support expert claims. To avoid misconceptions and to ensure fairness during trial, experts need to be able to effectively communicate the limitations of DNA evidence to the courts.

Undoubtedly, DNA evidence has revolutionized forensic science and continues to have a significant impact on criminal investigations. However, its accuracy is not infallible, and several factors can affect the reliability of DNA analysis. As mentioned in this article, factors such as interpretation challenges, sample contamination, and misconceptions in court can all contribute to the complexity surrounding DNA evidence.

Although there may be doubts surrounding the accuracy and reliability of DNA evidence, there is ongoing research and development to negate these doubts. In addition, implementing rigorous quality control measures, validation studies, and population databases can also improve the accuracy of statistical calculations. Finally, proper training and education of forensic scientists and effective communication of limitations by expert witnesses are vital in ensuring fair trials are conducted and proper use of DNA evidence occurs.

Further Reading

• All Forensics Content
• Collecting Evidence at a Crime Scene
• Using Forensics to Tackle Child Exploitation
• Forensic Investigations of Natural Disasters
• Forensic Identification in Mass Disasters