Effects of Evidence Strength, Format, and Order on Police Judgments of Suspect Guilt

Abstract

Police encounter evidence when conducting criminal investigations. This evidence informs their judgments of suspect guilt and can affect investigative decisions. Yet, surprisingly little research examines how evidence affects police judgments of suspect guilt. This dissertation contains three papers to address this gap. Paper #1 is a theoretical review paper that proposed a unified definition of evidence strength based on competing hypotheses, as varying definitions of evidence strength could lead to inaccuracy when police use evidence to judge suspect guilt. Paper #2 is an empirical paper that recruited police officers (N = 209) to examine the effects of evidence strength format and evidence type on police guilt judgments. I assigned participants to a 3 (evidence strength format: LR vs. RMP vs. neutral) x 3 (evidence type: DNA vs. fingerprint vs. eyewitness identification) between-subjects factorial design and asked them to judge suspect guilt on two measures. Overall, participants were most accurate when encountering evidence strength in an LR format but only for DNA evidence. Paper #3 is an empirical paper that recruited police (N = 75) and laypeople (N = 636) to examine the effects of evidence order and social norms on police evaluations of evidence and judgments of suspect guilt. I randomly assigned participants to a 2 (evidence order: incriminating evidence first vs. incriminating evidence last) x 2 (social norms: efficiency vs. thoroughness) x 2 (type: DNA incriminating-eyewitness ambiguous vs. eyewitness incriminating-DNA ambiguous) between-subjects factorial design and asked them to evaluate the evidence and judge suspect guilt. Overall, social norms that prioritized a thorough investigation (vs. efficient) minimized biased evidence evaluations and guilt judgments. Thus, this dissertation expands upon previous psychology and law research to better understand how evidence influences police guilt judgments.