A laboratory study of the factors of that influence the propensity of organisations to copy the strategic decisions of others, based around the BRANDMAPS simulation, investigated the effects of contingencies relating to the environment, the decision and the firm itself, in the absence of binding resource constraints and legal pressures. The proportion of isomorphic decisions diminished with industry age: isomorphic behaviour did not become institutionalised. It was positively associated with industry dynamism - isomorphism was possibly used for risk reduction. No association with industry concentration was found: field structuration did not affect isomorphic behaviour. Pressures to imitate their peers, as perceived by participants, appeared unrelated to firms' performance or socio-cultural characteristics other than team diversity. Isomorphic behaviour was unrelated to isomorphic pressures, suggesting that unconscious rationality in managerial behaviour. Knowledge of opponents' decisions was associated with isomorphic behaviour, indicating that isomorphism was not simply a response to the task environment. However, the association was positive for some decisions and negative for others, inconsistent with common explanations of isomorphism as deriving from a desire for legitimation. Rather, isomorphic behaviour results from managers making efficient use of available information on competitors' practices. Combinatorially complex bundles of decisions were not isomorphic between firms that had taken similar individual decisions. There was no evidence of an "iron cage" constraining them to become more similar. Isomorphism between firms is not necessarily linked to institutionalisation or stimulated by causes - notably legitimation - suggested by institutional theory. A model of isomorphic behaviour in firms is proposed, based on the perceived value of the practice being copied, the nature of the decision, contingent properties of the business environment and of the organisational field and the situation and internal properties of the firm. The implications for managerial praxis are explored.