In the past 25 years, French Professor Denis Retaillé was involved in the supervision of 39 graduate students. In 2006, I became one of them after completing a joint dissertation between the University of Lausanne in Switzerland and the University of Rouen in France.
During my professional career, I have often wondered who else had the good fortune to be supervised by Denis Retaillé. I knew some of his graduate students personally, but it was hard to imagine the connections that could have bound us together, beyond the fact that we all had the same mentor.
Connecting the dots
To understand how we were connected, I used the electronic portal theses.fr, a treasure trove of information for people interested in mapping research networks, like me. The site lists the names of all doctoral candidates who have successfully defended their thesis at a French university and the composition of their committees. It also provides crucial information about the topic of their work, their discipline, and the name of the university awarding the PhD.
I used this information to reconstruct Denis Retaillé’s PhD network from 1999 to 2018. For each thesis, I linked the doctoral candidate and jury members to each other, creating peers of actors who participated in the same defense. After a few hours of data structuring, I identified 148 unique names and created 598 pairs of actors. The data is available here.
In the French academic system, the PhD committee is composed of a Directeur de thèse, who supervises the candidate, a Président, who leads the committee, and several committee members called rapporteurs and examinateurs. The thesis director is a member of the jury but does not take part in the final decision to award a doctorate, unlike the Président.
I simplified these categories and represented actors according to whether they were graduate students or committee members. Using a network program called ORA, I represented the width of the ties between actors in proportion to the number of times they were part of the same jury.
A fascinating ego network
The result is an ego network which focuses on a single individual and their direct connections to others, as can be seen below. Professor Retaillé occupies the center of the network, which represents a fraction of all the ties that connect academics together in the French academic system.
Professor Denis Retaillé’s PhD network, 1999-2018
The structure of the network reflects academic hierarchy. PhD students, in red, are often more peripheral than committee members, colored in white. This is because experienced researchers are much more likely to be connected to each other through the various committees in which Professor Retaillé was represented, compared to newly minted researchers.
Very few committee members form dense communities, however. Instead, each PhD candidate seems to be part of a small cluster of professors. These clusters are then tied to Professor Retaillé. This suggests that each committee is formed on an ad hoc basis, depending on the candidate’s topic and expert knowledge found at different universities.
This kind of structure that crosses university boundaries is known to enhance innovation in science. Innovation arises when researchers have the right density of pre-existing ties. If they don’t know each other, they will likely fail to work together, while if they know each other too well, they won’t be able to generate new ideas.
What’s next?
Visualizing a research network is the first step toward a more nuanced understanding of how science actually works. The next step will be to examine the structural importance of each node of the network and highlight who has the largest number of ties, who plays the role of broker, and who is connected to well-connected actors.
I also want to better understand the overall architecture of the network – its topology – to identify clusters of researchers who work more frequently together. Interpersonal relations within these communities is what ultimately produces new knowledge.
I am hoping that this formal approach to Professor Retaillé’s PhD network can help understand the long term trajectory and scientific impact of the “mobile space” concept, which he introduced 30 years ago. This concept developed to reflect the primacy of movement in the changing organization of space in Africa and beyond is far from mainstream in Geography today.
As Thomas Kuhn argued, new theories progress not only because they explain reality better than old ones, but also because they are supported by a collective capable of convincing people of their usefulness.
By tracing the connections between researchers who have used the concept of ‘mobile space’, I hope to show how it has spread in the scientific community, what obstacles it has encountered, and how it could reach a wider audience.
By Olivier Walther, 3/24/2025.