For years it has been accepted that it is not possible to say how and when DNA came to be on a surface. Indeed, our published review of the scientific literature of the research on transfer and persistence of DNA (Meakin and Jamieson 2013[1]) showed that it is not possible to infer how or when DNA was deposited. Nothing has materially changed since as evidenced by several other more recently published reviews. Clearly, ‘don’t know’, didn’t suit some practitioners. This response in 2016 was intriguing;
“The BFF [Body Fluids Forum – [a group composed entirely of labs providing services to law enforcement in the UK and Ireland] believe that even in the presence of limited data sets and unpublished laboratory data forensic practitioners should where possible, objectively evaluate the available evidence or at a minimum communicate to the courts the scientific reasons why an evaluation was not possible.”[2]
This desire to assist the Court despite the lack of reliable data persists largely unchallenged. A number of labs have now adopted what is called Activity Level Reporting (ALR). In this approach, the ‘expert’ creates propositions that ostensibly relate to the action which caused the DNA to be on a surface. This procedure, however, does NOT tell the court, or anyone, the probability that the DNA was deposited in a particular way, contrary to what the court may believe or have asked.
The guidelines issued by the International Association of Forensic Genetics[3] state;
“Scientists must not give their opinion on what is the ‘most likely way of transfer’ (direct or indirect)…”
You might think that was the end of that. No. The ‘guideline’, continues:
“… as this would amount to giving an opinion on the activities and result in a prosecutor’s fallacy (i.e. give the probability that X is true,
.. we urge scientists not to give explanations in court e.g., "the most likely explanation for this DNA quantity is primary transfer". Instead, most importantly when the source of the DNA is uncontested, biological results should be assessed given activity level propositions”
Here's what an ALR statement might look like:
“In order to evaluate the findings, I have also assumed the DNA findings are accepted and I have considered the following propositions:
· [X] Defendant handled the item; or
· [Y] Defendant did not handle the item.
If the first proposition were true and defendant handled the shotgun, I would have a moderate expectation of finding his DNA on the item.
If the second proposition were true and defendant did not handle the item, I would have no expectation of finding his DNA on the item.
In my opinion, the finding supports the view that defendant handled the item, rather than he did not handle the item.”
Despite appearances, this is NOT a statement of the probability of X being true rather than Y (i.e. contact rather than indirect transfer).
The scientist is considering two propositions, not explanations[4]. A proposition is a statement, of one position or story. It does not need to be true. The use of ‘supports’ implies that the evidence, NOT the proposition, is more likely assuming the truth of both propositions in turn. That is why the ALR statement provides no information as to the truth of either proposition. The probability of the evidence according to the proposition may be high, yet the probability of the proposition very low. To give an example reductio ad absurdum: the evidence is that my Christmas tree has been decorated overnight. My child proposes that the fairies did it. The probability of the evidence if he is correct is 1 (100%). That, of course, is not the probability of the proposition.
If his is the only proposition available, absent any other information, then we know nothing about the probability of that proposition. If I propose that ‘it was Father Christmas’, then the probability of the evidence under that proposition is also 1. The probability of the evidence under each proposition is the same. Neither probability is the probability of the true cause.
Now let’s look at the ‘defence’ proposition from the handling example (assuming it is the defendant’s DNA but he claims that he did not touch the item). It then becomes necessary to assess two propositions:
1. The prosecution proposition that he had contact with the item and his DNA was transferred directly.
2. The defence proposition that he had no contact with the item but his DNA was transferred indirectly.
Recall that previously, the position was that we could not say how or when DNA came to be on an item. ALR requires that we know the probability of the defendant’s DNA being observed IF he touched it, but also the probability of the defendant’s DNA being observed IF he had not touched it. Yet there is no reasonable way that either of those could be known in most circumstances. It requires knowledge of the provenance of the item; people in contact with the item and any association between the defendant and those people; people in contact with those in contact with the item; places the item has been; who has been in those places who may have had some association with the defendant; and so on. This is why the guidelines are careful to explain that the scientist is assessing the propositions, not the explanations. And, to top it all, they are not even opining on which proposition is more likely – only under which assumed proposition the evidence is more likely.[5]
In principle, an expert cannot perform an ALR assessment in the absence of an alternative (defence) proposition[6]. A ‘no comment’ interview provides no alternative proposition from the defendant, yet this has not prevented some experts from creating their own to enable the ‘evaluative opinion’.
Even more bizarrely;
“Statements given activity level propositions aim to help address issues of indirect vs direct transfer, and the time of the activity, but it is important to avoid use of the word ‘transfer’ in propositions.”
If you are not already perplexed, you are not alone.
“Providing a list of possible explanations for the results may be relevant during the investigation phase, but not in court, as it does not allow an assessment of the value of evidence.”
Not in court? If Counsel posits a possible explanation (i.e. a defence proposition) for how DNA came to be on an item, the ISFG Guidelines are clear and simple:
“we urge scientists not to give explanations in court".
We believe that this is a seriously misguided attempt to be ‘helpful’ (usually to the prosecution) with little or no support other than ‘casework experience’ and limited, mostly irrelevant, published research. Absent an explanation from the defendant, it is inappropriate for the scientist to posit an alternative explanation or proposition. The approach used comes under the ‘Bayesian’ paradigm, albeit disguised, already disparaged by the Court of Criminal Appeal, yet is being admitted without serious challenge.
The Appeal Court in England and Wales have several times rejected this suggestion, as yet without hearing the specific challenge in this case which is that, without such a calculation about the other evidence, the LR has no relevant meaning:
“Of course, it is a matter for the jury how they set about their task, and it is no part of this court's function to prescribe the course which their deliberations should take. But consideration of this case along the lines indicated would in our judgment reflect a normal course for a properly instructed jury to adopt. It is the sort of task which juries perform every day, carefully and conscientiously, on the evidence, as they are sworn to do. We do not consider that they will be assisted in their task by reference to a very complex approach [Bayes Theorem] which they are unlikely to understand fully and even more unlikely to apply accurately, which we judge to be likely to confuse them and distract them from their consideration of the real questions on which they should seek to reach a unanimous conclusion. We are very clearly of opinion that in cases such as this, lacking special features absent here, expert evidence should not be admitted to induce juries to attach mathematical values to probabilities arising from non-scientific evidence adduced at the trial.”
“We turn therefore to the second of the three submissions which we recited, namely that the Bayesian approach is logically sound and approved by expert opinion. We would not for our part wish to take issue with that statement so long as it is applied to appropriate subject matter by persons competent to apply it. We have no reason to doubt, as is stated by a number of highly authoritative experts, that it is a sound and reliable methodological approach in some circumstances. We have, however, the gravest reservations about its use in jury trials in cases such as this.”[7]
“"It has been suggested that it may be appropriate for the statistician to expound to the jury a statistical approach to evaluating the likelihood that the defendant left the crime stain, using a formula which gives a numerical probability weighting to other pieces of evidence which bear on that question. This approach uses what is known as the Bayes Theorem. In the case of Adams (Denis ) [1996] 2 Cr App R 467 this Court deprecated this exercise in these terms at p482:
'To introduce Bayes Theorem, or any similar method, into a criminal trial plunges the jury into inappropriate and unnecessary realms of theory and complexity deflecting them from their proper task.'
We would strongly endorse that comment."[8]
We are unconvinced that sufficient data exists to support most, if not all, assertions of probabilities of transfer and happy to provide expert opinion to that extent as well as assess DNA profiles and evidence.
[1] Forensic Sci Int Genet. 2013 Jul;7(4):434-43. doi: 10.1016/j.fsigen.2013.03.013 .
[2] Forensic Science International: Genetics 21 (2016) 117–118
[3] Forensic Science International: Genetics, 44, 1-13. Article 102186. https://doi.org/10.1016/j.fsigen.2019.102186
[4] “We will consider the third principle in the subsequent sections, but would like to stress that propositions are different from explanations. ‘DNA could have been secondarily transferred’ is an example of explanation.” ISFG
[5] “A common misconception is to believe that scientists evaluate the propositions provided. However, the scientist’s report relates only to the value of the forensic results given that the propositions representing the views of the parties, as understood, are true. This rationale follows evaluation of the ‘probability of the evidence given the proposition’ principle embodied in the likelihood ratio”
[6] “Accordingly, a competing defence proposition is necessary. To reiterate, the scientist offers no opinion about whether a proposition such as “He handled the knife” or “He stabbed the victim” is true or not because this would fall foul of the prosecutor’s fallacy (probability of the hypothesis given the evidence). Rather, the scientist’s role is restricted to assigning the value of the DNA evidence in light of these propositions.”
[7] Denis John Adams , R v. [1997] EWCA Crim 2474 (16th October, 1997)
[8] [2010] EWCA Crim 2439