Evaluating Negative Forensic Evidence: When Do Jurors Treat Absence of Evidence as Evidence of Absence?

• Author: William C. Thompson,Nicholas Scurich,Rachel Dioso‐Villa,Brenda Velazquez
• Date: 01 September 2017
• DOI: http://doi.org/10.1111/jels.12156
• Published date: 01 September 2017

Evaluating Negative Forensic Evidence:

When Do Jurors Treat Absence of

Evidence as Evidence of Absence?

William C. Thompson, Nicholas Scurich,* Rachel Dioso-Villa, and

Brenda Velazquez

Two jury simulation experiments tested participants’ sensitivity to variations in the

probative value of a piece of negative forensic evidence: failure to find gunshot residue

(GSR) on a defendant alleged to have fired a gun. Experiment 1 found that if no GSR was

detected, juries (N5115) of undergraduates were appropriately less likely to convict a

criminal defendant when the probability of detecting GSR was high than when it was low.

Participants were unaffected by contextualizing expert testimony that emphasized either

the value of the negative evidence for making inductive inferences or that the negative

evidence was inconclusive for making deductions. Experiment 2 used a sample of venire

jurors (N5420) and manipulated the probability of detecting GSR (0 percent, 50 percent,

60 percent, 90 percent, or 100 percent) given that a gun was fired. Consistent with the first

experiment, venire jurors were more likely to convict when the probability of detection was

0 percent or 50 percent than when it was 100 percent, but verdicts did not differ between

the middle groups. This pattern of results suggests that jurors may evaluate negative

evidence according to a fairly crude metric---giving it no weight if the probability of

detection is zero, a great deal of weight if the probability of detection is 100 percent, and

moderate weight if the probability of detection is somewhere in between.

Sherlock Holmes: [Consider] the curious incident of the dog in the night-time.

Inspector Gregory: The dog did nothing in the night-time.

Holmes: That was the curious incident.

A. Conan Doyle’s, The Silver Blaze

*Address correspondence to Nicholas Scurich, University of California, Irvine, 4213 Social & Behavioral Sciences

Gateway, Irvine, CA 92697-7085; email: nscurich@uci.edu. Thompson is Professor, Department of Criminology,

Law and Society, University of California, Irvine; Scurich is Associate Professor, Department of Psychology and

Social Behavior and Department of Criminology, Law and Society, University of California, Irvine; Dioso-Villa is

Senior Lecturer, School of Criminology and Criminal Justice, Griffith University, Queensland, Australia;

Velazquez is Director of Planning and Analysis for the Office of the Bronx District Attorney, Bronx, NY.

The research reported here was supported by National Science Foundation Grant No. SES-0617672 and by

the Center for Statistics and Applications in Forensic Evidence (CSAFE). The first author thanks the Isaac New-

ton Institute for Mathematical Sciences for its hospitality during the program Probability and Statistics in Foren-

sic Science, which was supported by EPSRC Grant No. EP/K032208/1. The authors also thank Mr. Alan Carlson,

Chief Executive Officer and Jury Commissioner of the Superior Court of Orange County, California, and his

staff, for their assistance in completing this research, and Nicci Bowman Fowler for assistance in data collection.

569

Journal of Empirical Legal Studies

Volume 14, Issue 3, 569–591, September 2017

I. Introduction

Does the absence of a defendant’s fingerprint on an alleged murder weapon mean he

never touched the weapon? Does the absence of detectable drug metabolites in a per-

son’s blood mean the person was drug-free? Questions like these concern the value of

negative evidence. Negative evidence may arise from the failure of an investigation to pro-

duce findings consistent with a particular hypothesis about what occurred. It may also

arise from the nonoccurrence of an event that was likely to occur if a particular hypoth-

esis is true. Negative evidence is generally admissible in jury trials when it is relevant to

a pertinent issue in a case, although courts may exclude it if they find it more prejudi-

cial than probative (e.g., United States v. Hoffman 1992).

In this article we will discuss two issues related to negative evidence. One issue is

the probative value of such evidence: When and to what extent does the failure to find

evidence of an event help prove that the event did not occur? The second issue is the

effect of such evidence in jury trials: When and to what extent do jurors treat negative

evidence as probative? In distinguishing the two issues we are, obviously, acknowledging

a possibility that jurors will assess negative evidence incorrectly, giving such evidence

more (or less) weight than it deserves.

It is important to distinguish negative evidence from missing evidence. Suppose that a

defendant is accused of shooting a victim with a handgun. If the police fail to test for the

presence of gunshot residue (GSR) on the defendant, then we will say that GSR evidence is

missing. If the police test for GSR and do not find it, we will say the evidence is negative.

According to some commentators, the CSI television series has conditioned jurors to expect

forensic science evidence in virtually every case (Podlas 2006; Shelton 2009; Shelton et al.

2006; Tyler 2006). There has been much discussion of whether this “CSI effect” exists and

whether it has made jurors reluctant to convict when forensic evidence is missing (Tyler 2006;

Cole & Dioso-Villa 2007; Schweitzer & Saks 2007). Our focus here, however, is not on cases

in which the parties have failed to conduct an inquiry or examination into a pertinent issue,

but on cases in which such an inquiry has been conducted and the findings were negative.

Negative evidence, as we use that term here, is also distinguishable from evidence

designed to explain or “contextualize” negative or missing findings. According to one legal

scholar, concern about “CSI infection within the jury” has led many prosecutors to pursue a

strategy of “defensive prosecution” in which they present expert testimony to explain “why

certain types of tests were not done,” and why other tests were negative, in order to “curb

unrealistic expectations and misinformation regarding forensic science and its capabilities”

(Lawson 2009). Some commentators have referred to this explanatory testimony as negative

evidence (Lawson 2009:168; Cole & Dioso-Villa 2011). To avoid confusion, we will reserve the

term negative evidence for evidence of nonoccurrence or nondetection, such as the failure of

a toxicology test to find evidence of drugs. Following Jenkins and Schuller (2007), we will

use the term contextualizing evidence to describe expert testimony designed to provide an

explanatory context for negative or missing evidence.

How do jurors respond to negative evidence? We know that people find it difficult

to draw inferences from the absence of information (Einhorn & Hogarth 1978; Ward &

570 Thompson et al.