Prologue to the special section: Network multidimensionality in the digital age

There is a long and distinguished history of research on communication and other types of social networks. Some trace the intellectual foundations of this tradition to the work of John Stuart Mills and Herbert Spencer in the 19th century (Mattelart, 2000/1996), and others to the pioneering empirical work of Jacob Moreno in the early 20th century (Wasserman & Faust, 1994). Much has been learned about a wide variety of social networks, such as the use of mobile telephones in rural Africa (Castells et al., 2007), the spread of sexually transmitted disease among high school students (Bearman et al., 2004), and the development of transactive memories in work teams (Palazzolo et al., 2006), to name just a few. Interestingly, almost all of the published research has explored a single set of objects and a single set of relational links that connect them. In network parlance, these are called unidimensional networks, or equivalently, unimodal, uniplex networks. And yet, multiple types of objects can be tied together into a single network, such as a set of people (one type of object) who attend a number of different social events (a different type of object) (Davis et al., 1941). Likewise, the same set of objects can have multiple relations as reflected in the differences between formal (authority) and informal (social) communication relations that are typical in organizational networks (Krackhardt & Hanson, 1993). The "multiple types of objects network with single relations" model is called a multimodal, uniplex network, and the "single set of objects with multiple relations" model is called a unimodal, multiplex network. Both would be considered to be partial multidimensional networks because they contain only multiple sets of objects or multiple sets of nodes, but not both. Of course, it is possible to construct multidimensional networks that have two or more relations defined on two or more different types of objects, that is, partial multidimensional networks that are both multimodal and multiplex. Typically, this type of partial multidimensional network has relations only between different types of objects. Finally, a fully multidimensional network is one that includes multiple types of relations both among the same types of nodes and between different types of nodes. Thus, a fully multidimensional network has multiple types of connections among all possible types of entities.