Proving Causation: Probability versus Belief

One of the frequently assumed major differences between civil law and common law systems is the standard of persuasion applied by each in civil (non-criminal) cases. In most civil law jurisdictions, it is commonly assumed that the standard of persuasion is the same for criminal and civil proceedings. The plaintiff in a civil case, as well as the prosecutor in a criminal case, must provide sufficient proof to convince the trier of fact of the truth of the facts at issue on the particular occasion. Although it is recognised that absolute certainty is impossible to achieve, the required degree of belief is often expressed in terms of a virtual certainty, or at least a very high probability. However, a mere statistical probability, no matter how high, will not suffice in the absence of the required conviction or belief in the truth of the facts at issue. In common law jurisdictions, there is an explicit distinction between the standard of persuasion in criminal and civil proceedings. In criminal proceedings, the standard is very high: the prosecutor must prove the defendant’s guilt ‘beyond a reasonable doubt’. In civil proceedings, however, the plaintiff generally only needs to prove his case by the much lower standard of a ‘preponderance of the evidence’ (the usual formulation in the United States) or a ‘balance of probability’ (the usual formulation in the British Commonwealth and Scandinavia). Both standards are often interpreted by academics, and sometimes by judges, as merely requiring a 50 per cent probability. For both standards, as so interpreted, it would be better to employ the term ‘standard of proof’ rather than ‘standard of persuasion’, since the latter implies an element of conviction or belief that is lacking when all that is involved is a class-based statistical probability. I have argued that the supposed major differences between the standards of persuasion in common law and civil law jurisdictions are greatly overstated. I retrace that argument in section II of this paper. I conclude that, in general, the common law as well as the civil law continues to view the applicable standards of persuasion in civil as well as criminal actions as requiring the formation of a belief in the truth of the facts at issue in the particular case, rather than a mere class-based statistical probability, and that concrete ‘particularistic evidence’ specific to the particular case is necessary to support such a belief. Although the available evidence is slim, I also conclude that in the civil law as well as the common law the strength of the required belief is lower in civil actions than in criminal actions. In section IV of this paper, I consider various types of problematic causal situations that courts around the world have struggled to deal with in recent decades. Clear recognition of the issues in these cases and their proper resolution has often been hindered by an unanalysed assumption that the standard of persuasion in civil actions is a mere statistical probability standard, which however is not consistently applied, since doing so would generate significant problems and paradoxes. The considerable confusion that now exists could and should be greatly reduced simply by replacing the highly misleading phrases ‘more likely than not’ and ‘balance of probability’, and even the less misleading phrase ‘preponderance of the evidence’, with ‘a minimal belief’.