Prior studies have examined effectuation and causation as alternative behavioural logics used by entrepreneurs to manage uncertainty, noting a number of antecedents of the tendency to rely on a given logic at different levels (environmental, organizational, and individual) of analysis. This study aims to broaden the understanding of individual-level antecedents by examining the role of the so-called dark side of the CEO personality on decision-making processes within small and medium-sized enterprises (SMEs). Specifically, we focus on three personality characteristics: narcissism, psychopathy, and Machiavellianism. The impact of this dark triad on the behavioural logic employed by the entrepreneur is argued to be moderated by the general level of uncertainty experienced by the firm. A set of hypotheses regarding these relationships are tested with a random sample of CEOs in Russian SMEs. The findings suggest that CEOs scoring high in psychopathy tend to adopt a causal logic, while Machiavellians rely on an effectual logic. The level of uncertainty shapes these relationships by weakening the links between psychopathy and causation and Machiavellianism and effectuation. Implications are drawn for theory and practice.