We've known for a long time now that DNA evidence is foolproof - it's the best crime-fighting, conviction producing evidence that we have, right? Is DNA evidence reliable? Everyone knows it is...
Except, what do we know?
Most people know that, on television, in crime shows, and in media reports, DNA evidence gets convictions - you can't argue with DNA.
When we take a closer look, however, we find that DNA evidence is reliable when:
- The sample size is large enough for comparison;
- There is no mixture or a there is a sufficient sample size to differentiate between contributors;
- There is no contamination of the DNA sample;
- The analysts are unbiased, thorough, and competent; and
- The evidence is presented honestly and ethically.
Which means that much of the DNA evidence that is presented in SC courtrooms is not reliable.
How Does DNA Evidence Work?
For most people, it's like a magic black box - put the DNA sample in, you get the result back, and the lab worker reports the guilty verdict to the jury.
Is it really just that simple?
The truth is that most DNA work takes careful analysis of a large volume of data, and much of the work is subjective - because it is subjective, it is subject to error.
To understand how complex mixtures are analyzed-and how easily those analyses can go wrong-it may be helpful to recall a little bit of high-school biology: We share 99.9 percent of our genes with every other human on the planet. However, in specific locations along each strand of our DNA, the genetic code repeats itself in ways that vary from one individual to the next. Each of those variations, or alleles, is shared with a relatively small portion of the global population. The best way to determine whether a drop of blood belongs to a serial killer or to the president of the United States is to compare alleles at as many locations as possible.
Think of it this way: There are many thousands of paintings with blue backgrounds, but fewer with blue backgrounds and yellow flowers, and fewer still with blue backgrounds, yellow flowers, and a mounted knight in the foreground. When a forensic analyst compares alleles at 13 locations-the standard for most labs-the odds of two unrelated people matching at all of them are less than one in 1 billion.
With mixtures, the math gets a lot more complicated: The number of alleles in a sample doubles in the case of two contributors, and triples in the case of three. Now, rather than a painting, the DNA profile is like a stack of transparency films. The analyst must determine how many contributors are involved, and which alleles belong to whom. If the sample is very small or degraded-the two often go hand in hand-alleles might drop out in some locations, or appear to exist where they do not. Suddenly, we are dealing not so much with an objective science as an interpretive art.
Part of the problem is that it's not a magic black box. It's not even an objective process - in many cases, the analyst is making a number of "calls" that may or may not be right.
Is DNA Evidence Reliable Evidence in Criminal Cases?
We want to believe that there can be certainty when we sentence a person to live in a prison or even sentence them to die.
We want to believe that police, prosecutors, and judges are honest, ethical, and would not risk sending an innocent person to jail.
We want to believe that DNA evidence is the silver bullet that allows us to find the truth whenever it is present.
But, wanting to believe does not make something true. DNA evidence is reliable when it is used and analyzed properly, but it is not reliable when the analysts are biased, motivated by pressure to convict, and lack proper training and oversight.
Different Labs Reach Different Results
DNA labs usually refuse to conduct an analysis if another lab is also analyzing the same materials, and they refuse to conduct analyses in conjunction with other labs.
Why is that?
It's because, when different labs analyze the same DNA samples, they often reach different conclusions... is that an indication of reliability?
In a study published this year by the National Institute of Standards and Technology, researchers provided the same DNA mixture from a "mock crime scene," to over a hundred crime labs in the US and Canada.
The researchers also provided DNA samples from three individuals - suspects from the "mock bank robbery." Two suspects' DNA were part of the mixture, and the third suspect's DNA was not part of the mixture. The third suspect was "innocent" and should have been exonerated of the mock crime.
74 of the labs reported that the crime scene sample contained DNA from the third, innocent person...
Another unexpected result is that the crime labs calculated wildly varying statistics for their results:
One shocking result from the new N.I.S.T. study is that labs analyzing the same evidence calculated vastly different statistics. Among the 108 crime labs in the study, the match statistics varied over 100 trillion-fold. That's like the difference between soda change and the United States' gross domestic product. These statistics are important because they are used by juries to consider whether a DNA match is just coincidence.
The results from this study should be enough to put a moratorium on DNA evidence used in courtrooms - why isn't that happening?
Government Labs Hide Evidence of Faulty DNA Science
Crime labs fight hard to prevent analyses from multiple labs - if DNA is an exact science, what's wrong with getting confirmation?
The study linked to above was not published for years after its completion. When it was published, it included a disclaimer designed to prevent its use in the courtroom:
Given the adversarial nature of the legal system, and the possibility that some might attempt to misuse this article in legal arguments, we wish to emphasize that variation observed in DNA mixture interpretation cannot support any broad claims about "poor performance" across all laboratories involving all DNA mixtures examined in the past.
If the government wants to get it right, and not put innocent people into prisons, why wouldn't they:
1) Duplicate this study to determine whether this is really happening; and
2) Put whatever resources are necessary into identifying the problems and fixing them asap?
DNA Evidence has Become a Tool for Prosecutors to Win Convictions
"We have DNA evidence."
There's one statement that can predict how a trial is going to end - jurors who hear this will assume the defendant is guilty. Defendants and their defense lawyers who hear this will immediately begin thinking about a guilty plea to lessen the amount of time the defendant serves in prison - even when the defendant insists they are innocent...
The "CSI" Effect
Prosecutors who do not have DNA evidence will invariably complain to jurors about the "CSI effect."
"Everyone has watched Law and Order and other courtroom television shows. You've seen all the fancy technology that they use to get convictions on TV. This is real life, folks, and it's just not like that in real life. We can't always get DNA evidence to bring you in a case, and we must rely on good, old-fashioned witness testimony."
According to the Atlantic, a 2008 study published by a trial judge in Michigan found that:
Three-quarters of the jurors said they expected DNA evidence in rape cases, and nearly half said they expected it in murder or attempted-murder cases; 22 percent said they expected DNA evidence in every criminal case...They expect us to have the most advanced technology possible, and they expect it to look like it does on television.
Understanding this, prosecutors will always bring DNA evidence if they can get it, and government lab analysts have come up with a slew of misleading terms that they will present to jurors when they don't have a match, to make it sound like they have a match...
"Cannot Be Excluded"
In a 2010 study, two professors took the DNA evidence from a Georgia trial where the two state analysts testified that the defendant's DNA "could not be excluded" from the crime scene sample. The defendant was convicted.
They provided the samples to 17 analysts with an average experience of nine years in the field, but withheld details about the case to prevent unconscious bias. Only one of the analysts said they "could not exclude" the defendant's DNA. 12 said that they could exclude the defendant's DNA, and four said that their results were inconclusive.
When jurors hear that "the Defendant's DNA cannot be excluded," they will see that as "DNA evidence" of guilt - it's not. Prosecutors know that it's not, but they consistently present it to jurors as if it is.
Would it have made a difference in the Georgia trial if they had known that 12 analysts excluded the defendant's DNA as a contributor? As far as I know, that guy is still in prison...
Law Enforcement Analysts are not Impartial Scientists
When jurors hear a state analyst's credentials at trial - their training and experience in analyzing DNA and presenting their results - it usually sounds pretty impressive.
What jurors usually do not hear is the clear biases that government analysts have that, as in the example above, color the results of their tests.
State and local labs are often in the same building as police departments.
Police and prosecutors keep in close contact with them about their cases, and will often make sure that the lab analyst knows how horrible the case is - telling a person that "this defendant is a monster who hurts children" will almost certainly help get your case to the front of the list and get a positive result...
In North Carolina, state and local law-enforcement agencies operating crime labs are compensated $600 for DNA analysis that results in a conviction.
I mean, if you want to get convictions without regard to guilt or innocence, paying the crime lab for evidence that leads to convictions is a sure way to accomplish that.
If you are facing a felony trial with DNA evidence, you cannot rely on the state lab to provide impartial and fair results - you need a criminal defense lawyer who will find an independent lab to test all evidence in your case and present the results for you at trial whenever possible.