Fragmentation of a longitudinal population-scale social network: Decreasing structural social cohesion in the Netherlands

Population-level dynamics of social cohesion and its underlying mechanisms remain difficult to study. In this paper, we propose a network approach to measure the evolution of social cohesion at the population scale and identify mechanisms driving the change. We use twelve annual snapshots (2010-2021) of a population-scale social network from the Netherlands linking all residents through family, household, work, school, and neighbor relations. Results show that over this period, social cohesion, quantified as average closure in the network, declines by more than 15%. We demonstrate that the decline is not due to changes in demographic composition, but to rewiring in individual ego networks. Statistical models confirm a decreasing overlap of social contexts and greater geographical mobility as drivers. Residential relocation, however, temporarily increases closure, suggesting that local cohesion-seeking behavior can yield global network fragmentation, with implications for policies related to housing, urban planning, and social integration.

💡 Research Summary

This paper presents a longitudinal, population‑scale analysis of social cohesion in the Netherlands using twelve annual snapshots (2010‑2021) of a multi‑layer network that links every resident through family, household, work, school, and neighbor ties. The authors first describe the macro‑level evolution of the network: the number of nodes (registered residents) steadily rises, and the total number of edges grows slightly, resulting in a virtually unchanged average degree over the twelve‑year period. Despite this stability in degree, two complementary measures of network closure—local clustering coefficient and “excess closure” (a metric that counts triangles formed by edges from different layers)—decline markedly, by roughly 7 % and 15.5 % respectively. This indicates that while people maintain roughly the same number of contacts, those contacts become less mutually connected, i.e., the network becomes less “triadic.”

To uncover the drivers of this decline, the authors decompose the year‑by‑year change in average excess closure into three components: (1) a group‑level term reflecting shifts in population composition (age groups, migrant background), (2) an individual‑level term capturing changes within demographic groups, and (3) an interaction term. Counter‑intuitively, demographic shifts such as aging and increasing migrant share contribute positively to closure, whereas the individual‑level term is strongly negative and outweighs the group contributions. Thus, the erosion of cohesion is not a by‑product of demographic change but stems from structural rewiring within individuals’ ego‑networks.

The paper then examines ego‑network trajectories. By applying k‑means clustering to normalized degree and excess‑closure time series for each individual, four degree patterns and four closure patterns emerge. The dominant degree pattern (≈50 % of the sample) is stable degree; the dominant closure pattern (≈86 % of the sample) shows decreasing excess closure. Importantly, many individuals experience a drop in closure even when their degree remains constant, confirming that the decline is not merely a degree effect. Cross‑tabulation of degree and closure clusters shows that the largest group consists of stable‑degree individuals whose ego‑networks become less tightly knit over time.

To pinpoint the mechanisms behind individual‑level closure loss, the authors estimate two‑way fixed‑effects OLS models where the dependent variable is the yearly percentage change in excess closure for each person. Baseline controls include degree and other structural metrics; subsequent models add variables for multiplexity (the share of alters connected via multiple layers) and geographic mobility (average distance to alters, share of alters in the same municipality). Results reveal two major drivers: (i) a decreasing share of multiplex edges, indicating reduced overlap among social contexts (family, school, work, etc.), and (ii) increasing geographic dispersion of alters, measured by longer average distances and fewer same‑municipality ties. Both factors exert significant negative effects on excess closure.

A striking “relocation paradox” emerges: residential moves are associated with a short‑term increase in closure, suggesting that newcomers initially form dense local ties, but this effect dissipates as geographic dispersion grows.

The authors interpret these findings in the context of modern societies where improved transport and digital communication reduce the cost of maintaining long‑distance ties, thereby weakening the structural importance of co‑located, overlapping social contexts. The paper concludes that contemporary Dutch society is becoming more socially fragmented at the structural level, not because of macro‑demographic trends but due to individual‑level reductions in multiplex relationships and heightened mobility.

Policy implications are discussed: urban and housing policies should foster local community building (e.g., shared spaces, neighborhood events), while education and labor policies could promote cross‑context interactions to preserve multiplex ties. Moreover, interventions aimed at mitigating the social isolation that may follow increased mobility are recommended.

Overall, the study provides robust, longitudinal evidence that structural social cohesion is declining in a high‑income European country, offering a methodological blueprint for similar analyses elsewhere and highlighting the need for policies that address the underlying drivers of network fragmentation.