From similar others to the spread of disease
Networks are everywhere: on the internet, on social media, and even in train timetables. They are also the pathways through which epidemics spread. And homophily – the tendency to connect with people who are similar to you – is an important factor. In the paper Homophily Within and Across Groups, Stegehuis and her team show how differences in homophily influence epidemic dynamics.
What is homophily?
The research used datasets from Facebook, the music platform Last.fm, call records, and face-to-face interactions at a scientific conference tracked by sensors. Stegehuis explains: “We wanted to know whether people cluster mostly with those who are similar to them, and whether that differs between pairs and larger groups. To study this, we developed a model that shows how homophily changes with group size.”
Clara Stegehuis
What the researchers found
In one-on-one Facebook connections, people mostly sought out peers, while in larger groups age diversity increased. A similar pattern was found for sex: same-sex connections were common in pairs but more mixed in groups. For status (student or employee) there was no clear trend.
Differences between online and offline networks
The type of network mattered greatly. In dating networks, one-on-one interactions were almost always between opposite sexes, while in other networks this was not the case. Homophily also differed between online and in-person interactions.
Why it matters for epidemic control
The way people cluster – in small or larger groups, vaccinated or unvaccinated – has a strong effect on how fast and how far an epidemic spreads. Homophily can raise or lower the threshold for spreading, meaning that targeted vaccination or distancing measures may have very different effects depending on group structure.
The same model can also be used to study how misinformation spreads within and across groups.
January 2026 update: the publication by Clara Stegehuis on this topic has now been published in the scientific journal Nature Communications.
