Graphs over time: densification laws, shrinking diameters and possible explanations
Jure LeskovecJon KleinbergChristos Faloutsos
Demonstrates that real-world networks densify over time and exhibit shrinking effective diameters, overturning long-held assumptions about network growth while proposing the Forest Fire Model to explain these dynamic phenomena.
The article examines how large real-world networks such as citation graphs, patent records, internet autonomous systems, and author-paper affiliations change as they grow. Conventional models assumed that the average number of connections per node stays roughly constant and that distances between nodes grow slowly, yet these assumptions had rarely been checked against long sequences of network snapshots. The work set out to measure actual growth patterns across multiple domains and to develop simple generative models that reproduce the observed trends.
The authors analyzed nine datasets spanning up to several decades, constructing successive snapshots and tracking the number of nodes and edges at each step. They also computed the effective diameter, defined as the distance within which 90 percent of reachable node pairs lie. Two new probabilistic models were introduced and tested through analysis and simulation: one based on a nested community hierarchy that controls the cost of cross-group links, and a second based on a recursive “forest fire” process in which new nodes attach by burning through existing edges.
The measurements reveal that every network follows a densification power law in which the number of edges grows as a power of the number of nodes, with the exponent lying between 1.08 and 1.68; as a result, average node degree steadily increases. At the same time, effective diameters shrink rather than expand. The Community Guided Attachment model produces the observed densification exponents from a single tunable parameter, while the Forest Fire model reproduces both densification and shrinking diameters together with heavy-tailed degree distributions.
These patterns matter because many practical tasks rely on realistic network models. Graph generators used for simulation and “what-if” planning can now incorporate densification and shrinking distances. Sampling algorithms for very large graphs can be validated against the same laws, and deviations from expected growth can serve as signals for anomalies such as fraud or attacks. Forecasts of future network size and connectivity become better grounded.
The findings support continued development of the Forest Fire model, including variants that allow multiple starting points or isolated nodes, and broader testing on additional network types. Further theoretical analysis is needed to explain why diameters converge and to quantify the range of parameters that produce realistic behavior. The main limitations are the restriction to nine datasets from four domains and the reliance on simulation for the more complex model; results should be treated as robust within the studied settings but require additional confirmation elsewhere.
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