People holding attitudes are a social information system that can be modelled as a bipartite social identity network, where people are bound into groups via jointly held attitudes; and attitudes into clusters when jointly held by people. Attitude-based groups and group-related attitudes thus form a recursive dynamic structure where attitude expressions simultaneously produce a dynamical social field and position people within it (described as interactionism in social psychology and duality in sociology). Attitudes function to: (1) produce affiliation; (2) define and differentiate social groups; and (3) position people. Dynamic fixing occurs when network structures stabilize to associate certain attitudes and identities, producing a compressible social information system where identity is readable from attitudes, and people are positioned by the attitudes they express. Attitudes are thus coupling points between the individual and the social, and a key interface between social identity and individual psychology.