The present study presents one of the first investigations of the effects of accomplice witnesses and jailhouse informants on jury decision-making. Across two experiments, participants read a trial transcript that included either a secondary confession from an accomplice witness, a jailhouse informant, a member of the community or a no confession control. In half of the experimental trial transcripts, the participants were made aware that the cooperating witness providing the secondary confession was given an incentive to testify. The results of both experiments revealed that information about the cooperating witness’ incentive (e.g., leniency or reward) did not affect participants’ verdict decisions. In Experiment 2, participant jurors appeared to commit the fundamental attribution error, as they attributed the motivation of the accomplice witness and jailhouse informant almost exclusively to personal factors as opposed to situational factors. Furthermore, both experiments revealed that mock jurors voted guilty significantly more often when there was a confession relative to a no confession control condition. The implications of the use of accomplice witness and jailhouse informant testimony are discussed.