What value does a juror place on proof that a test – for gunshot residue, for latent prints, for DNA – produced no results? This article uses mock juries in a controlled setting, with careful alteration of variables, to discern how jurors respond.
What were the findings? In one sense, the obvious:
The first question we sought to answer with this experiment is whether people consider the probability of detection (under relevant hypotheses) when evaluating a negative test result. It appears that they do. Strength of case ratings and the proportion of participants voting to convict were both significantly lower in the 100 percent condition, where the probability of detection was high, than in the 0 percent condition, where the probability of detection was low.
The researchers went deeper. “The second question we sought to answer with this experiment was whether “contextualizing testimony” affects people’s reactions to negative evidence. The testimony we presented did not significantly affect the weight given to the negative evidence.”
The research proceeded further:
The third question we sought to answer with this experiment was how people evaluate negative evidence when there is a substantial chance of nondetection, as was the case in the 50 percent condition…
We found that regardless of the contextualizing testimony or the dependent variable (verdicts or strength of evidence ratings) there were no significant differences between the 50 percent condition and the 0 percent condition, which indicates that jurors gave little or no weight to the negative evidence in the 50 percent condition.
A second experiment delved further into the complexities of decision making informed by negative evidence. The overall conclusions included the following:
Negative evidence (the failure to find something) is indeed evidence that the thing sought was “absent” to the extent the probability of detection exceeds the probability of a false positive. The claim that such evidence has no value arises from a misapplication of the deductive logic of hypothesis testing in a legal context that requires inductive inferences.
For those who deal directly with forensic evidence, or who more generally concern themselves with juror decision-making, this research is foundational and of critical importance.