Introducing the “Prosecutor’s Fallacy”: The Simple Mistake Lawyers (and the Media) Commonly Make When ‘Interpreting’ DNA Results
Can You Identify the Issue in This Statement?
“It is 100 million times more likely if Mr X is a contributor to the DNA profile than if another unknown individual from the Australian population is.”
This is an example of what is known as the Prosecutor’s Fallacy, also known as transposing the conditional—a logical error that occurs when DNA evidence is misinterpreted.
Understanding the Prosecutor’s Fallacy
The Prosecutor’s Fallacy leads to misunderstandings about the statistical relevance of information and has led to a number of high-profile convictions throughout the world that have since been overturned, including R v Keir (NSW Court of Criminal Appeal, 2002).
To an untrained ear, the above statement suggests overwhelming odds that Mr X is guilty. This may not be correct, as there are a myriad of ways the DNA may have been contributed that have nothing to do with the case, and which are not taken into consideration within the above statement.
The scientifically correct wording for this evidence is:
“The DNA profile is 100 million times more likely if Mr X is a contributor to the DNA profile than if another unknown individual from the Australian population is.”
Why Does This Happen?
The problem stems from the abbreviated reports provided to the courts, which do not fully explain the evidence, as well as the confusing statistical language being used in court. This is essentially a miscommunication of evidence from either expert witnesses (who assume a certain level of scientific understanding) or the prosecution (who misinterpret the DNA evidence due to lack of statistical training).
As biologists, we are always evaluating the probability of the evidence and not the probability that the individual is the source or has contributed to the DNA sample.
Scientifically, it is a matter of confusion between two conditional probabilities:
- The probability of the evidence (the DNA profile) given the propositions
- The probability of the propositions given the evidence (the DNA profile)
However, at its core, it is a human error of miscommunication.
How Do You Avoid the Prosecutor’s Fallacy?
Unfortunately, there is no easy way to avoid the Prosecutor’s Fallacy, as DNA reports are bound by the usage of strict scientific language.
Ultimately, the quality of your interpretation rests on the clarity of communication from your expert witness. Judges, juries, and lawyers are not trained in statistical interpretation. The chances of misunderstanding are high, and therefore the possibility of a miscarriage of justice is unacceptable.
There is no true way to avoid the Prosecutor’s Fallacy outside of using an Expert Witness who is a clear communicator as well as a skilled forensic analyst.
If a case you are involved with is utilising DNA reports and you have struggled to interpret the difference in language in the example above, please reach out for a confidential discussion.
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