  Although it is well-known that some exponential family random graph model
(ERGM) families exhibit phase transitions (in which small parameter changes
lead to qualitative changes in graph structure), the behavior of other models
is still poorly understood. Recently, Krivitsky and Morris have reported a
previously unobserved phase transition in the edge/concurrent vertex family (a
simple starting point for models of sexual contact networks). Here, we examine
this phase transition, showing it to be a first order transition with respect
to an order parameter associated with the fraction of concurrent vertices. This
transition stems from weak cooperativity in the recruitment of vertices to the
concurrent phase, which may not be a desirable property in some applications.
