During their time in office, British governments’ styles of political judgement or bias in policymaking often become shorter in term and less intellectually coherent, sometimes in passive or coping ways, sometimes shifting toward imposition. This article offers an explanation, developing the neo-Durkheimian theory of institutional dynamics. Changing judgement style, it argues, is driven by changes in administrations’ informal institutional ordering of social organisation. “Isolation dynamics” are shifts in that ordering towards weakly cohesive but strongly constrained “isolate” forms. Increased isolate ordering is reflected in less cohesive but more constrained judgement style. Novel distinctions within isolate ordering explain key differences among administrations’ trajectories. Using extensive archival data, three British administrations between 1959 and 1974 are compared. The study finds that, among otherwise contrasting administrations, reinforcement or undermining in informal social organisation drove changes in styles of political judgement, as shown in their ways of framing policy problems, risks, time horizons, etc.