Mark Bell’s article (2016) is a welcome contribution to the unavoidable task of evaluating a research program that exhibits the problems he identifies: the failure of most quantitative studies to offer strong explanations for proliferation patterns, and their inability to predict out-of-sample cases. His findings resonate with those of other proliferation experts. The existing quantitative literature, argues Bell, produces more tentative findings than scholars typically understand. We concur fully with the first part of the last sentence but believe that the broader community of experts typically does understand the serious limitations of most quantitative studies (and qualitative ones) on this topic. What are those limitations of quantitative studies according to Bell? First, there are many more distinctive explanations than cases. Second, most variables identified as significant determinants of proliferation “fail to provide robust explanations for existing patterns.” Third, studies question the robustness of each other’s quantitative findings. Fourth, they provide little sense of the hierarchy of importance of different explanations. Fifth, they offer “little predictive ability beyond what we can achieve with an extremely simple model” (which, incidentally but unstated in Bell’s piece, can be of a qualitative kind). Sixth, they are not transparent about those limitations. Seventh, they typically model the effects of variables as constant across time and space. Here Bell reiterates a point others have made: studies must control for the world-time under which nuclear weapons are developed or eschewed, such as pre- or post-NPT era (Solingen 2007). Because of all of the above and more, Bell concludes that weak correlations between proliferation and many variables in extant quantitative studies offer no proof whatsoever that those variables do not in fact cause or prevent proliferation. In other words, the absence of evidence is not evidence of absence, as is sometimes argued in court. Most of these shortcomings are well known, and some can afflict qualitative studies as well (for an extensive review, see Wan and Solingen 2015 and Solingen 2007)
