Structural insights into the inquisition register of Bologna (1291-1310)

Inquisition registers are an excellent source for studying the social organisation of medieval communities. But, when a historian has to keep track of hundreds of entries and names in a register, it can be difficult to fully imagine the nature of social organisation just by close reading. Did the investigated people form one or a few cohesive social bodies of “heretical movements” – communities – or are they better understood as many unrelated cases of small groups or individuals who drew the attention of the inquisitors separately? This snapshot discusses how visual network graphs can help historians study social relations on the basis of the registers of the Bologna inquisition.

28 Mar 2025 Tomáš Hampejs Katia Riccardo David Zbíral

Network visualisation of incriminations and religious transgressions in the Bologna inquisition register, 1291-1310
Source

The Acta S. Officii Bononie ab anno 1291 usque ad annum 1310 (“Acts of the Holy Office of Bologna from the year 1291 to the year 1310”) is an inquisitorial register – a set of records related to the activities of the inquisition in Bologna, Italy. It covers an early but already fully developed phase of the papal inquisition of heresy, which was established in 1231 under the pontificate of Gregory IX. It is a rich historical source containing references to more than two thousand people over twenty years and mainly to two cultures of religious dissidence – Cathars and Apostles.

It is difficult to fully grasp the social organisations of the population addressed by the source by close reading alone. Did the investigated people form one or few cohesive social bodies of “heretical movements” – communities – or are they better understood as many unrelated cases of small groups or individuals which were primarily linked by the attention of the inquisitors?

The volume of information is large – among the 922 documents, there are 609 depositions, i.e. interrogation records, which yield thousands of references to specific social relations between individuals. One of the most common ways individuals are introduced in the register is by incrimination, meaning the nomination of a specific person as a participant in an action or belief which potentially constituted a criminal offence in a heresy trial by the deposing person (the deponent).

Title page of the published edition  "Acta S. Officii Bononie ab anno 1291 usque ad annum",  Sources for the history of Italy 106, Rome, Italian Historical Institute for the Middle Ages Rome, 1982.
Incrimination network

Visualizing the chosen social relations as a network graph (model of the population as deposition-based incrimination relations) allows us to glimpse into the register’s population structure and beyond. The graph represents 663 individuals connected by 1074 directed dyadic relations: out of all deponents who testified in front of the inquisitor, 177 incriminated at least one person, leading to a total of 557 individuals incriminated by other deponents (not counting self-incriminations), while 5 deponents did not incriminate other individuals.

We can immediately see that inquisitors investigated 1) two bigger groups 🔍 of inter-incriminated individuals. If we assume that incrimination indicated at least social acquaintance, these two groups could be representations of the two existing dissident collectives. However, there are also 2)  many cases of isolated incriminations, where the inquisitors were either uninterested in the incriminated and/or their crime sociality (e.g. they did not follow up on the incriminated, did not try to question them) and/or they were isolated cases of reported transgressions.

The small disconnected subgraphs allow us to see two interesting network patterns - stars: a) when one individual incriminates many other people (out-star) 🔍, and b) when multiple witness deponents incriminate the same one person (in-star) 🔍

No description
Stages of investigation

In the register, it is possible to identify several stages of investigations separated by times of interrogation inactivity, at least as far as the official record goes. These stages can be understood as separate “campaigns” with different investigation aims and, most importantly, different leading inquisitors.

The legal history of the Bologna inquisition started in 1291 with the interrogation of just one Cathar – with the deposition of an old man, Onebene di Volta Mantovana. As he recollected his life, he incriminated 68 other persons 🔍. However, it took almost 10 years before a small number of people from Onebene's lists appeared in front of the inquisitor and revealed a few others.

As is visible from the many disconnected subgraphs in the 1299-1302 campaign, depositions often do not "snowball" from one to another. While the first, third and fourth stages form interconnected components of somewhat cohesive social bodies, the “green campaign” appears different — either the inquisitor followed a unique strategy or primarily encountered disconnected groups of people and their transgressions.

However, few individuals in the green campaign relate to the largest orange component 🔍. Some people investigated between 1299 and 1302 are also connected to the most socially dense investigation of a local religious dissidence – the Apostolic milieu (1303-1305). Is this indicative of a Cathar connection to the Apostles?

No description
Heterodoxies

The visualisation of three different categories of dissidence transgression reveals that the 1299-1302 campaign covers two different populations: there is a light green “Cathar component” originating from Onebene, and there are many small disconnected incriminations with no distinctive religious dissidence properties.

Overall, the religious dissidence affiliation type strongly fits the components and almost fully aligns with the stages – the apparent exception is 1) the presence of Other heterodoxy in the green campaign and 2) the “incrimination bridge” from the green stage to the orange Apostles’ milieu. Based on its scattering and with the highest presence of Other heterodoxy, the green campaign seems to have been the least categorically targeted and is clearly a transitory stage between the two milieus – all the more as the first few Apostles were incriminated in the 1299-1302 campaign 🔍.

No description

Cathar-affiliated are those who were reported to have been involved in meetings involving baptism by the laying-on of hands (consolamentum) or the ritual greeting of those who had received this baptism (reverencia, in verbal form also adoravit), or were termed to be believers, supporters, defenders or receivers of people so baptised (full members).

Apostle-affiliated are those who were termed “apostoli” (“Apostles”), “de secta apostolorum” (“from the sect of the Apostles”), “pauperes” (“poor ones”), “minimi” (“smallest ones”) or “de secta et societate Gerardi Segarelli, Dolcini de Novaria et suorum sequacium” (“from the sect and society of Gerard Segarelli, Dolcino of Novara and their followers”). In addition, we also counted those who engaged in meeting or assisting religious specialists so named.

Other-heterodoxy-affiliated flags those who did not belong to a two-tier structure of full members vs. supporters, characteristic of Cathars and Apostles, but who reportedly engaged in unorthodox ideas or behaviours, or expressed more widespread discontent concerning the corrupted state of the Church or its religious orders. A varied set of people fall under this category: for instance, there are low prelates denounced by their fellow brothers for not observing Catholic practices, more pronounced religious sceptics, as well as persons strongly adverse to the Pope or the inquisition.

📈

As the depositions are dated, we can explore the incrimination network in its precise chronology through an animated graph. For example, we can discover that the vast majority of depositions in the green campaign happened within the first year (1299). 

However, the deposition time granularity showcases probably more questions than answers. If we focus on the "first discovered Apostles" (0:59-1:01), we see that they appear in front of an inquisitor in November of 1299, in the first year of the predominantly "Cathar campaign",  years before the main Apostles investigation.  Who were they, and why there was no follow-up on them? 

For this, we need to go back to close reading. This illustrates how visualisation is a strong explorative tool - it can puzzle us more than inform, quickly confront our expectations and guide us to search within otherwise covert details.

For a more complex story about the social predictors of incrimination (gender, class, church affiliation and kinship) see our paper.


More articles

All articles

You are running an old browser version. We recommend updating your browser to its latest version.

More info