The Risk of Oral Opinion – and How Defence Expert Reporting Can Prevent Ambush
- What does this DNA evidence mean in the context of the alleged events?
Source-level evidence vs activity-level questions
- whose DNA was detected
- how strong the comparison statistic is
- how the DNA was deposited
- when it may have been deposited
- whether indirect transfer is plausible
- whether the findings support one alleged activity over another
Where the risk emerges: oral opinion at trial
- “What does the presence of this DNA indicate?”
- “Is this consistent with the prosecution case?”
- “Would you expect DNA to be present if the accused handled the item?”
- goes beyond the written report
- addresses activity-level issues for the first time
- implicitly supports the prosecution version of events
Why this is procedurally dangerous
- it has not been disclosed in advance
- it has not been subjected to expert peer review
- it has not been carefully framed or limited
- it arrives with the authority of “expert evidence”
- the defence has not led its own expert evidence
- the limitations of DNA inference have not been clearly articulated scientifically
- alternative explanations have not been placed scientifically before the court
The limits of cross-examination alone
- offered an opinion
- linked the DNA to the alleged activity
- framed the evidence in prosecution-friendly terms
How defence expert reporting changes the terrain
- distinguish clearly between source-level findings and activity-level propositions
- explain what the DNA results can and cannot support
- identify assumptions required to move from “DNA present” to “DNA deposited during the offence”
- address transfer, persistence, and contamination realistically
- Head off broad speculative excursions on the stand
- set scientific boundaries around permissible inference
Framing the narrative early
- the court is alerted to the limits of the DNA evidence
- the prosecution expert’s latitude to opine orally is narrowed
- activity-level assertions can be tested against a written scientific framework
- the expert’s role is appropriately confined
From reaction to strategy
- anticipate where oral opinion may emerge
- shape admissibility and fairness arguments
- prepare focused cross-examination
- prevent overstatement before it occurs
A practical observation
- proportional advice
- ensure that any litigating of the evidence before trial is done with full understanding
- identification of real forensic risks
- informed decisions about whether a full report is required