This study investigated state-level variation in the number of right-wing militia groups. We built on prior quantitative examinations by more carefully operationalizing theoretical concepts and testing alternative hypotheses offered to account for the etiology of social movements generally and of militias specifically. Using negative binomial regression, we found that, consistent with previous research, states with higher levels of farm job loss had more militia groups. However, in contrast to prior studies, we did not find gun culture, hospitable political climate, female empowerment, or general economic deprivation to be associated with the number of militia groups. We also found no relationship between the number of militia groups and levels of fundamentalist religious culture and minority empowerment. Our study was the first to test the effects of social disorganization on the number of militias in a state, and we found the two to be positively and significantly associated. We situate our results in the theoretical and empirical literature and outline two challenges for future research. First, scholars must operationalize variables carefully since different measures of the same theoretical concept may result in diverging conclusions. Second, since one major factor (i.e., social disorganization) found in this study to be associated with the number of militia groups in a state was examined here for the first time, it is important for researchers to continue to investigate new hypotheses.