Enduring rivalries are fruitfully understood as forms of inefficient equilibria. Parties should be cooperating to maximize joint gains, but something prevents them from overcoming costly conflict. These sorts of equilibria may result in large scale violence, but violence may only occasionally punctuate what is otherwise a durable circumstance of conflict. In this way, "enduring rivalries" and long civil wars are not the same thing. One could imagine an enduring rivalry marked by a number of short bouts of violence, for example. To the extent that this is a reasonable characterization of some political circumstances---and I think it is---the challenge is to identify such equilibria when they exist (or have existed).
Enter Derouen and Bercovitch:
[Enduring internal rivalries] denote internal conflicts between a government and an insurgency with at least 10 years of armed conflict in which there are at least 25 deaths – regardless of whether or not these years are consecutive.
They implement this definition by classifying and sometimes lumping together observations from the PRIO armed conflict dataset. They identify 60 enduring rivalries.
Looking at their list, I found the selection of cases to be rough but intuitively reasonable if we are to limit ourselves to situations that have at some point resulted in major violence. But even with this major restriction on cases, the assignment of start and end dates often seemed arbitrary. There also seemed to be some arbitrariness in the decision to define enduring rivalries in terms of factions in some cases and conflicting social groups ("Palestinian insurgents") in others. Perhaps the latter issue is not too important, but it may be indicative of general inconsistencies in the way the original data has been constructed.
More generally, it is dissatisfying to have to use a coding rule that relies on the pre-existence of insurgent groups and a threshold of violence to demarcate cases. This definition would thus exclude years of rivalry that certainly have major socio-economic consequences but precede bouts of violence. In addition, I am not sure what kind of general rule could be used to assign end dates to the more expansive notion of an enduring rivalry that I prefer. If Derouen and Bercovitch are not interested in having us consider such an expansive notion of enduring rivalry, then I am not sure how their effort makes progress over the many civil war duration studies that are already out there.
I was also surprised that the authors chose to study the effects of these enduring rivalries. Bracketing the enormous endogeneity problems here, substantively, do we really need to demonstrate that protracted conflicts are bad or "a distinct class of civil wars"? How about focusing attention on the causes of these enduring rivalries!