Introduction:
This study assesses the effects of witness type (informant/police officer), jury instructions (no mention/mention witness' ulterior motives), and evidence type (drugs/cash) on mock jurors' verdicts and witness/defendant credibility ratings in a case of possession of crack cocaine with intent to distribute. Research has identified many possible ulterior motives that both police officer and civilian informant witnesses could have for testifying in trial (Thompson, 2012; e.g., maintaining public perception, monetary gain). In trial, jurors are instructed to consider a witness' potential vested interest in the trial outcome. However, this instruction is omitted when police witnesses testify (Johnson, 2017). This study analyzes whether including the same instruction for police officer witnesses would influence perceptions of witness/defendant credibility and ultimately trial outcomes.


Method, Results, and Conclusion:
Mock jurors (N = 355) read a trial transcript, provided verdicts, and completed defendant/witness credibility scales. With the police (vs. informant) witness, mock jurors had more confidence in the defendant's guilt and found the defendant to be more credible-- specifically when there were drugs in the defendant's system (vs. copious amounts of hidden cash). Regardless of witness type, witness credibility ratings were higher in the cash (vs. drug) evidence conditions but only when the jury instructions failed to tell participants to consider the witness' potential ulterior motives for testifying. Finally, the police officer (vs. informant) witness was seen as more credible when: (a) ulterior motive jury instructions were provided and (b) when presented with cash (vs. drug) evidence. These findings suggest that while the jury instructions may not impact juror perceptions in the way we expected, mock jurors perceive the credibility of police witnesses differently than civilian informants, which in turn can impact participants' perceptions of the defendant's guilt.