Unnecessary and Insufficient Factual Causes

Abstract Law recognizes a necessary (i.e., but-for) factor as a factual cause. However, it is a common misconception that the but-for test is the exclusive test of factual causation. Longstanding case law reveals that a factor may be a factual cause of an outcome, even if it was neither necessary nor sufficient for it. This was recently unanimously confirmed by the Supreme Court of the United Kingdom in Financial Conduct Authority v. Arch Insurance (UK) Ltd (2021). In the law, there seems to be a uniform concept of what it means to be a factual cause, one that is wider than necessity and includes unnecessary and insufficient factors which are recognized as factual causes across a wide variety of legal contexts. This uniform concept of factual cause can be captured in a forensically straightforward way in an “extended but-for test”. Appreciation of unnecessary and insufficient factual causes, which are common, provides a more coherent frame in which to view the outcomes of certain earlier cases. It also illuminates the importance for clarity of legal reasoning of separating the context-independent factual cause issue from context-dependent analytical stages such as the scope of the rule and remedy, including loss quantification.