The Number of Unanimous Witnesses who Identify a Suspect from a Lineup Influences Mock Jurors
During a criminal proceeding, jurors need to weigh up the presented evidence and determine a verdict. Research has shown that witness identification evidence is compelling to jurors, despite the fact that it can be unreliable. How reliable are the combined lineup decisions gathered from multiple witnesses? Generally, the more witnesses who identify the same person from a lineup, the more likely that person is guilty. But recent theoretical evidence suggests that a greater number of witnesses identifying the same person from a biased lineup can indicate that person is actually less likely to be guilty than if there were a smaller number of witnesses identifying that person (Gunn et al., 2016). As the number of agreeing witnesses increases, the more likely that agreement is caused by the lineup bias, rather than consistent witness memories of the crime. In this thesis, I examined how unanimity and lineup bias influenced jurors’ perceptions of guilt. Subjects who saw a biased lineup gave lower ratings of guilt compared to subjects that were shown a lineup that had no obvious bias. In addition, warning subjects that a lineup was biased led them to give lower guilt ratings than subjects who did not receive a warning. Subjects who were told there were two witnesses who identified the police suspect gave higher guilt ratings than subjects who were told there was one witness who identified the police suspect, but only when the lineup was clearly not biased. Subjects’ guilt ratings were not significantly greater in conditions with more than two unanimous witnesses identifying the police suspect. It seems subjects had a limit of certainty based on changes in witness numbers alone. We also found that the way in which witness numbers were presented to subjects influenced guilt ratings. When we presented witnesses coming forward in different groups and on different days, subjects shifted their guilt ratings upwards. When the number of witnesses decreased during the experiment, subjects did not decrease their guilt ratings to the same extent as those subjects in conditions in which the number of witnesses increased by the same magnitude. This finding is consistent with the literature on confirmation bias and the story model of juror decision-making—subjects likely formed an initial belief that the identified suspect was guilty and subsequent evidence was evaluated against that belief (Nickerson, 1988; Pennington & Hastie, 1993). The finding that presenting witnesses coming forward in separate groups increased subjects’ guilt ratings adds to the literature showing that jurors are influenced by irrelevant information presented to them during a proceeding. This research also demonstrates that future research should examine strength of evidence manipulations over multiple levels—rather than as dichotomous “strong” and “weak” extremes.