Mistaken eyewitness identification is responsible for the majority of wrongful convictions in the United States (Wells et al., 1998; Warden et al., 2001). In one analysis of 86 cases of known wrongful conviction, multiple eyewitnesses testified in 14 of the 46 cases involving eyewitness testimony (Warden et al., 2001). Why would multiple eyewitnesses identify the same wrong person? Past research has demonstrated that memory is not only affected by cognitive variables, but social variables as well, especially in those situations in which the memory is not certain or poorly encoded (Wather et al., 2002). Researchers have shown that two eyewitnesses who discuss the crime can affect each other's memories for the event (e.g., Gabbert et al., 2003). However, very few studies have examined the effect of co-witness information on an eyewitness's identification choices. In a pilot study, we demonstrated that participants who learned the lineup decision of a co-witness (i.e., to choose from or reject the lineup) were influenced by the co-witness's decision when choosing from the lineup. Using social and cognitive psychological theory to inform our research, this project consists of two studies designed to explore when and why social information influences an eyewitness's lineup choice. In both studies, an eyewitness will view a crime with a confederate acting as a co-witness. In the first study, we will explore whether the strength (or perceived strength) of an eyewitness's memory compared to the memory of the co-witness affects whether an eyewitness is likely to conform to the decision of the co-witness. In the second study, we will explore the role of the co-witness's confidence (and the eyewitness's beliefs about the co-witness's confidence) in whether an eyewitness will be influenced by co-witness lineup decision information. Through these studies, we will be able to ascertain when and why co-witness information will affect an eyewitness's identification decision.