• Sociometric analysts in the US during the 1930s, whose work had roots in Gestalt psychology, aimed to investigate how feelings of well-being are related to the structure of people's social lives. This movement is most closely associated with Jacob Moreno, who devised the sociogram, a visual diagram of people's relationship networks in which individuals are represented as points and their connections to others as lines. Other major players in this research movement were Kurt Lewin, whose greatest legacy was his promotion of mathematical models of group relations, and Fritz Heider, who focused on people's perceptions about their relationships with others.
• Also in the 1930s, Harvard University researchers began focusing on cliques in social groups to identify cohesive subgroups (such as work, church, family, associations, and clubs) within social systems. This group was influenced by anthropologist Alfred Radcliffe-Brown, whose work focused on factory and community life in the US.
• A group of anthropologists in Manchester, England, also drew on the work of Radcliffe-Brown in the 1950s. John Barnes, a member of this group, is attributed with having coined the specific term "social networks" in 1954. His work with Elizabeth Bott drew on the sociometric approach, but focused on people's informal social relationships rather than those associated with institutions and associations. In addition, their work focused on conflict and change in these networks. Clyde Mitchell extended the traditional sociometric approach with insights from the mathematics of graph theory to better deal with observations that were gathered.
• A is connected to — say, sends email to — B, C, E, F, and G.
• A receives email from B, C, F, and G.
• E and F send email to each other.
• B,C, and D all send and receive email from each other.
• A doesn't receive email from E.
• centralized, decentralized (that is, multicentered), or distributed (centerless);
• hierarchical or horizontal;
• bounded or boundless;
• finite (with fixed limits on the number of nodes and ties);
• accessible or inaccessible;
• inclusive or exclusive;
• intensive (that is, few nodes linked by a multiplicity of dense, strong ties) or expansive (many nodes enabling reciprocal, multidirectional flows); or
• noninteractive (enabling only unidirectional flows).