DNA Evidence Can Be Wrong
DNA Frames Innocent Man
David Butler had just awakened when he heard a knock on the door. He opened it and was arrested by two policemen — for bludgeoning and strangling a sex worker to death. His DNA had been found on her polished fingernails and a sweater button. Police matched that DNA with a sample Butler had given after his mother’s house was robbed. Additional evidence pointed to his guilt: Butler was a taxi driver and a distinctive taxicab similar to his had been spotted near the victim shortly before the murder. Additionally, security cameras recorded photos of a man who looked very much like Butler.
The case against Butler was air-tight: DNA, camera footage, and a vehicle.
The catch? Butler was innocent.
So Why Was His DNA On the Victim’s Body if He Was Innocent?
Transfer DNA. As forensic techniques become more sensitive, they can detect smaller amounts of genetic material that may have been brought to the scene by a third party. Butler had a skin condition that led to his shedding more skin cells than most people. As a taxi driver, he came into daily contact with many people who sat in his cab and received change he had handed them for cab fare. These cells could then have been transferred from Butler’s passenger to the murder victim. The victim wore a type of nail polish that made cells more likely to adhere to it.
Butler’s defense team pointed out that DNA from other people was also on the victim and that Butler’s genetic material was only a partial match. His lawyers called other evidence into question as well. The security camera footage, it was later found, could not conclusively identify the defendant and he was not the only man to drive that type of taxi.
David Butler was one of the lucky ones. After being held without bail for eight months, a judge dismissed the charges.
The system worked for this defendant. The DNA analysis was accurate. Everyone told the truth. A judge examined the evidence and gave Butler the benefit of the doubt.
Other People Have Not Been as Lucky as David Butler
Unfortunately, other defendants have not been so lucky. Examples abound of serious mistakes made in forensic labs with some defendants being wrongfully sentenced decades of imprisonment and even death.
Clerks have typed the wrong names on files, leading to DNA test results being matched to the wrong person. Incompetent lab examiners using outdated procedures have claimed positive matches when, in fact, the samples were inconclusive. The situation in Broward County Florida crime labs was so bad that public defender Howard Finkelstein called it “junk science.”
Investigations of some labs have revealed multiple, unlabeled samples on the same work benches, leading to the possibility of cross-contamination, poor procedural safeguards, and supervisors who had no training in forensics.
Unwashed Petri Dish Gets Innocent Man Accused of Rape
Adam Scott’s DNA matched with a sperm sample taken from a rape victim in Manchester — a city Scott, who lived more than 200 miles away, had never visited. Non-DNA evidence subsequently cleared Scott. The mixup was due to a careless mistake in the lab, in which a plate used to analyze Scott’s DNA from a minor incident was accidentally reused in the rape case.
This incident occurred two weeks after the laboratory had been notified about possible contamination hazards. Scott was held for months even though cell phone records indicated that he was hundreds of miles away.
Unresolved Technical Issues Complicate DNA’s Use as Evidence
Other, more technical issues pertaining to DNA analysis remain unresolved. If DNA from several people is present in a sample, the mixture can sometimes, mistakenly, be read as one single profile. Furthermore, since only a few points in the genome are analyzed, somewhere between 13 and 20, depending on the lab, it is unlikely but not impossible for different people to share the same profile.
Incompetent Lab Personnel Regrettably Common
Worse yet, in some labs, investigations have uncovered cases of forensic examiners lying about their credentials, writing results for experiments that were never performed, and lying under oath in court. The point: a technology is only as good as the persons using it. Everyone is fallible. Some are dishonest.
Even Well-Trained, Honest Investigators Disagree Among Themselves
These problems cannot be fixed with tighter regulation. Even well-trained, honest investigators in government-accredited labs can come to different conclusions. In 2010, a reporter for New Scientists who now works for the BBC, sent the DNA evidence from a rape case to 17 different analysts all working for government-accredited labs. Of the seventeen, only one found that the suspect could not be excluded as the rapist. Four analysts determined that the results were inconclusive. Twelve maintained that the man could be excluded. The suspect had already been convicted and sentenced to prison before the 17 analysts reviewed the case.
What Should Be Done?
Given the importance of DNA in modern criminal investigations, how should the possibility of error be handled when such evidence is presented to juries?
A partial solution lies in changing the way DNA evidence is presented. The American Bar Association position, stated in JSTOR daily, claims “Telling a jury it is implausible that anyone besides the suspect would have the same DNA test results is seldom, if ever, justified,” the report states.
A better statistical procedure, this article argues, is comparing the likelihood that the DNA sample comes from the defendant to the likelihood that it comes from someone other than the defendant. If this comparison is expressed as a ratio, a value less than 1 would lean in the defense’s favor while a value greater than 1 supports the prosecution’s case. However, even if the ratio is greater than 1, jurors should be instructed that the DNA does not provide a definitive yes or no answer but only a probability that the defendant is in fact guilty.
More useful than technical discussions about ratios would be an honest attempt to impart to jurors the fact that science, no matter how cutting-edge, is only as good as the people using it. And all of us are fallible.
