Jaccard similarity values of binary variables.

The medieval inquisition of heresy strongly relied on depositions, where witnesses were expected to report on the crimes of others and oneself. The resulting patterns of incrimination could be influenced by various factors, including the characteristics of the underlying dissident social network; the investigators’ choices and biases; the trial circumstances, some of which must have exerted considerable pressure upon deponents; and the deponents’ decisions to protect some suspects more than others. This case study aimed at disentangling selected social factors of incrimination in the register of the inquisition in Bologna, 1291–1310. We used social network analysis and, more specifically, an Exponential Random Graph Model (ERGM) to assess the influence of four social predictors: gender, churchperson status, membership of the urban “middle class”, and kinship ties between incriminators and the incriminated. To increase the validity of our results, we controlled for various trial circumstances and structural parameters of the incrimination network. Our model corroborated a tendency towards female-to-female incrimination, while we did not find any positive or negative tendency towards male-to-male incrimination. We identified no effect of churchperson status on incriminating, while we found that among Cathars, members of the middle class were more likely to be incriminated than people without this status. Our model also corroborated the tendency to incriminate one’s kinship group. Overall, our study underlines the relevance, but also the non-trivial operation, of social and demographic predictors in medieval heresy trials.