From river systems to electrical grids, most networks exhibit some degree of hierarchy, but researchers have been slow to understand why. In a recent paper in PNAS, SFI External Professor Ricard Solé and colleagues at Pompeu Fabra University argue that much of the hierarchy in the world could have emerged at random.

The team began by constructing three measures of hierarchy: how clear the “chain of command” is, how prevalent circular chains are, and how high up the network those circles are. Using those measures, they identified four distinct types of hierarchy, corresponding to software and electronic circuits; language, metabolic, and neural networks; gene regulation networks; and food webs.

Simulations showed all but gene regulation networks and food webs shared similar structures with random networks, suggesting hierarchical networks can form without anything guiding them there.

Read the paper in PNAS (July 29, 2013, subscription required)