DNA evidence is considered the gold standard of forensic science and is relied upon heavily to identify suspects or victims and secure convictions in criminal cases. However, DNA evidence is not infallible and errors can and do occur, leading to wrongful accusations, convictions, and incarcerations. Understanding the limitations and potential inaccuracies of DNA testing is important for proper analysis and use of DNA evidence in the justice system.
How reliable is DNA evidence?
In general, DNA profiling is considered highly reliable due to the extremely low probabilities of random matches for DNA profiles in the general population. The chances of two unrelated individuals having the same 13-loci DNA profile is estimated to be 1 in 1 billion or greater. This makes DNA evidence extremely powerful for identifying suspects when properly collected and analyzed.
However, the reliability of DNA evidence depends heavily on the quality of the sample, handling procedures, and laboratory analysis methods. Contamination, degradation, or mixing of samples can lead to inaccurate results. Technical errors in the laboratory procedures or interpretation of the DNA typing results can also lead to mistakes. Poorly designed quality control and assurance practices increase the risks of errors.
While the underlying DNA typing technology is scientifically robust, it is still subject to human error at many points in the collection, processing, interpretation, and testimony stages. High profile exonerations of wrongfully convicted individuals through DNA evidence highlight the potential fallibilities of DNA testing in the real world criminal justice system.
What are some common causes of wrong DNA matches?
Here are some of the ways errors can lead to inaccurate DNA matches and wrongful accusations or convictions:
- Contamination – Trace amounts of DNA from another source gets accidentally introduced into a sample during collection or in the laboratory.
- Degraded samples – Environmental exposure degrades the sample, obscuring results.
- Mixed samples – Multiple individuals’ DNA in one sample makes interpretation ambiguous.
- Lab errors – Mistakes occur during extraction, amplification, detection steps.
- Interpretation errors – Analyst makes subjective mistakes in interpreting the DNA profile.
- Statistical errors – Database match probabilities are miscalculated or misrepresented.
- Evidence mishandling – Lack of proper collection or chain of custody documentation.
- Cognitive biases – Circumstantial evidence influences DNA analyst’s subjective judgement.
While safeguards exist to minimize risks at each step, human error can never be fully eliminated and proficiency testing reveals that even the best labs have some error rates.
How often are DNA lab errors uncovered?
The true rate of DNA lab errors leading to wrong matches is unknown and difficult to study comprehensively. However, large scale audits and investigations of DNA labs following major exonerations have revealed alarming lapses and mistakes in many labs across the country.
For example, a 2005 investigation of the Houston Police Department DNA lab found serious contamination, procedural, and interpretational errors. Nearly 2500 cases were reviewed and reworked. This scandal led to an Texas-wide audit of 30,000 DNA cases, which found 180 errors including 2 wrong identifications. Half of the 180 cases involved serious issues like contamination or procedural errors, calling test results into question.
A 2010 investigation of the now-closed Department of Forensic Biology lab in New York City identified 26 cases where DNA results were overturned due to significant errors like unsupported statistical claims, incorrect mixture interpretations, and lab errors. Reviews of the Virginia and North Carolina state crime labs in 2010 and 2011 respectively uncovered hundreds of erroneous DNA test results, sample mix-ups, incorrect statistics, and inadequate procedures.
These audits demonstrate that even high volume DNA labs accredited by regulatory agencies are vulnerable to quality failures that compromise DNA identifications and statistical claims.
DNA exonerations of wrongfully convicted individuals also shed light on the problem of DNA errors. The Innocence Project has used DNA testing to help exonerate 375 wrongly convicted people to date, with erroneous forensic science being a contributing factor in nearly half the cases.
While DNA testing also helps identify mistakes and free the innocent, the original DNA testing errors highlight vulnerabilities in the system. Some common DNA exoneration cases include:
- Contamination leading to false positives.
- Improper statistical techniques overstating match probabilities.
- Failure to properly interpret complex mixed DNA sample results.
- Cognitive biases influencing DNA analysts to overstate results.
How many false positives from DNA labs?
It is difficult to pin down the exact frequency of DNA false positives, which occur when an error leads to an incorrect DNA match between an innocent person and a crime scene sample. However, based on lab scandals, audits, and exonerations, we know false positives do occur, sometimes at alarmingly high rates.
A recent study published in 2019 looked at DNA false positives by analyzing large amounts of casework data from a DNA lab. Out of 49,000 crime scene DNA samples interpreted, they estimated:
- 1 false positive due to contamination per 2400 cases
- 1 false positive due to technical errors per 18,000 cases
- 1 false positive due to cognitive issues per 2000 cases
This suggests very low but still clinically significant rates of false positives even in high quality labs. Since not all labs achieve the same quality standards, risks could be higher elsewhere.
How can DNA evidence be strengthened?
While DNA evidence remains highly reliable and useful for law enforcement when properly collected and analyzed, scientific expertise and technology alone are not enough to prevent all errors. However, steps can be taken to minimize risks and further strengthen the integrity of DNA evidence:
- Implement more rigorous accreditation, auditing, and quality assurance standards for labs.
- Use “blind” testing to check for cognitive and contextual biases.
- Standardize procedures and analysis approaches across labs.
- Improve training for proper evidence handling and chain of custody.
- Conduct routine and rigorous proficiency testing of analysts.
- Use computerized expert systems to aid in analysis and reduce human error risk.
- Exercise caution drawing absolute conclusions from very small amounts of DNA samples.
While the risks can never be completely eliminated, improvements to the overall system can help minimize DNA errors and strengthen the powerful tool of DNA evidence for justice.
DNA evidence provides highly reliable biometric data to identifiy individuals with an extremely low random match probability. However, it is still subject to potential human errors at critical points in the sampling, handling, analysis, interpretation, and testimony processes. Quality failures in forensic DNA labs have led to numerous errors, incorrect results, and wrongful convictions over the years. \
Ongoing lab audits and DNA exonerations of wrongfully convicted persons continue to uncover cases of contamination, analysis errors, flawed statistics, inadequate procedures, and cognitive biases affecting DNA testing results. Estimates suggest relatively low but still significant false positive rates even in high quality labs. Steps can be taken to strengthen standards, improve training, implement safeguards, and reduce risks of errors. While unlikely, the potential for DNA testing errors highlights needs for quality assurance measures and caution in overstating the infallibility of DNA evidence in the legal system.