FIGURE 1. Assortative, disassortative.

a, In assortative networks, well-connected nodes tend to join to other well-connected nodes, as in many social networks — here illustrated by friendship links in a school in the United States⁶. b, In disassortative networks, by contrast, well-connected nodes join to a much larger number of less-well-connected nodes. This is typical of biological networks; depicted here is the web of interactions between proteins in brewer's yeast, Saccharomyces cerevisiae⁷. Clauset and colleagues' hierarchical random graphs² provide an easy way to categorize such networks. (Part a reproduced with permission from ref. 6.)