8.1 Ratio decidendi and obiter dicta

The first factor that influences how persuasive a rule is whether the rule was key to the outcome, or whether it was just a speculative comment by the judge. If the rule was key, then it can be binding on later courts, whereas speculative comments are only persuasive and do not need to be followed. It is therefore necessary to distinguish between them. The two types of rules also tend to be called by their Latin names.

Figure 9 Ratio decidendi and obiter dicta

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Three panel cartoon entitled ‘Never ask a lawyer for directions!’ with a woman on a bicycle asking for directions from a lawyer wearing a suit. First panel shows the cyclist asking the lawyer for directions to the park. The second panel, illustrating ratio decidendi, shows the lawyer giving her a straight answer to what she wants to know by saying: ‘Straight on and turn right.’ The third and final panel, illustrating obiter dicta, shows the lawyer speculating on a different question that the cyclist has not asked, saying: ‘… but if you’d like to see the duck pond, turn left then third right.’ The cyclist looks bemused.

Figure 9 Ratio decidendi and obiter dicta

The binding type of rule is one that actually decides the case. This is known in Latin as the ‘ratio decidendi’, or reason for the decision. Sometimes it is shortened just to ‘ratio’. All cases must have a ratio because it is the rule that the judge applies to decide the case and also the judge must explain this in their reasons. For example, in the circumstances of the case of MacLennan v MacLennan,^{Footnote 4} if the judge had decided that the partner who became pregnant by artificial insemination had not committed adultery, then the ratio might be something like: ‘a married woman who becomes pregnant through self-impregnation does not commit adultery’. The ratio is obviously the most important part of a case.

There is no easy way to identify what part of a case is the ratio (Holland and Webb, 2019). Court reporters such as those who work for the ICLR will try and identify the ratio in the summary of the case, but the reporters do not always get the ratio right. Judges themselves rarely try to highlight the ratio in their own decisions, but they may use the phrase when discussing what part of a previous case might be the ratio. As a result, spotting the ratio is a skill that comes with practice and familiarity with case reports.

The second type of rule is the one that is not binding. Judges sometimes like to offer comments about hypothetical and related circumstances that they do not have to decide. They do this to explain their thinking and check that the rules that they are applying make sense. This is also known by its Latin name of ‘obiter dictum’ meaning an incidental comment. Sometimes this is also shortened to just ‘obiter’. Rules that are obiter are not binding on later judges, but they may be persuasive. For example, in the case of MacLennan v MacLennan, when giving reasons, the judge might have said something like: ‘Of course, and hypothetically speaking, it might be the case that a married man might offer his sperm to somebody other than his wife for reproductive purposes and without having intercourse. While these are not the facts before the court today, my view is that those circumstances would not amount to adultery either’. If there was a later case that happened to fit those facts, then the earlier judge’s comments would not be binding, but they would be influential. Exactly how influential depends on how senior the court is. Obiter from a judge in the Supreme Court carries a lot more weight than obiter from an adjudicator in the Traffic Penalty Tribunal.

The second factor that determines how influential a rule in a case is, is how senior the court is. Or to put it in comedy terms of the ‘Class sketch’ (see Figure 10 overpage), if a court looks down on other courts, it is more influential than a court that looks up to other courts.