Prior work has demonstrated that the sequential presentation of lineup members in eyewitness lineups can result in undesirable position effects. For example, some studies have shown that placing the suspect in later positions increases discriminability. However, the evidence for this late-position discriminability advantage is mixed and the processes by which the discriminability increase occurs are unclear. However, one theory in particular, diagnostic feature detection theory (DFDT) explicitly predicts a late-position discriminability increase. According to DFDT, because shared features across lineup members cannot be used as reliable recognition cues to guide identification, discounting these features from consideration improves recognition. In sequential lineups, when the suspect is in a later position, witnesses are exposed to more of these shared features and are expected to benefit from discounting. By contrast, when the suspect is in an earlier position, witnesses are exposed to fewer shared features, and hence do not have the same advantage under the assumptions of the DFDT framework. One reason for the mixed evidence across the literature might be due to variation in suspect-filler similarity relationship between lineup members across studies, which we expected would moderate late-position memory effects. With the above in mind, the primary goals of the present work were: (1) testing for position effects at different levels of suspect-filler (SF) similarity, which might help elucidate conflicting evidence from prior work, and (2) testing DFDT mechanisms by simultaneously manipulating both innocent-suspect-perpetrator (IS-P) similarity and SF similarity. We found no evidence for late-position increases in discriminability as predicted by DFDT; however, participants were more conservative in later positions, especially when SF similarity was low. Discriminability was most strongly influenced by IS-P similarity, and was maximized when both SF and IS-P similarity was low. Implications for theories of eyewitness memory, practical implications for policy recommendations, and future directions are discussed.