There have been several recent attempts to think about psychological kinds as homologies. Nevertheless, there are serious epistemic challenges for individuating homologous psychological kinds, or cognitive homologies. Some of these challenges are revealed when we look at competing claims of cognitive homology. This paper considers two competing homology claims that compare human anger with putative aggression systems of nonhuman animals. The competition between these hypotheses has been difficult to resolve in part because of what I call the boundary problem: boundaries (...) between instances of psychological kinds (e.g., anger and fear) cannot be directly observed. Thus, there are distinctive difficulties for individuating psychological kinds across lineages. I draw four conclusions from this case study: First, recent evidence from the neuroscience of fear suggests that one of the proposed homologies involves a straightforward conflation of anger and fear. Second, this conflation arises because of the boundary problem. Third, there is an implicit constraint on the operational criteria that is easy to overlook in the psychological case. In this case, ignoring the constraint is part of the problem. Fourth, this is a clear case in which knowledge of homology cannot be accumulated piecemeal. Identifying homologs of human anger requires identifying homologs of fear. (shrink)