When a method of risk assessment would endorse playing Russian roulette, something has gone badly wrong with its logic. Yet the current understanding of Briginshaw v Briginshaw means courts cannot properly account for the risks presented in just this sort of situation. In this article, I explain the Briginshaw principle by comparison to intuitive and mathematical models of decision-making under conditions of uncertainty. I show that, while Briginshaw itself left the High Court of Australia deeply divided about where the so-called principle was enlivened and its consequences, subsequent judicial consideration has partly resolved this confusion. However, these subsequent authorities depart from our models, because courts wrongly assume serious allegations are inherently unlikely, insufficiently account for the consequences of ‘false negatives’, and have contradictory attitudes towards economic consequences. More fundamentally, while no theory of decision-making can totally avoid risk, I show that the accepted interpretation of Briginshaw as a fixed standard of proof means courts cannot properly account for improbable but grave consequences. Adopting a variable standard of proof would resolve some of these issues, but current authority is inconsistent with this approach.
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