In any contribution case concerning a pair of tortfeasors there are two reasons why the causative potency of their actions can matter:

(1) because if their actions had no causative potency at all then there would be no tortious liability in the first place and so no contribution claim;

(2) because if we can say that relative to one another their actions were of unequal causative potency then that may be a reason to make an unequal apportionment for the purposes of the contribution claim.

Let's say for the sake of the argument that a victim sues D1 and D2 in tort for causing him the same damage, that the relevant causation test to establish D1 and D2's liability is 'but-for', and that this test is satisfied for both of them: but for D1's actions the harm would not have occurred, and but for D2's actions the harm would not have occurred.

Looking at (1) we can then say that the pre-requisites for a contribution claim between D1 and D2 are in place, and it makes no difference whether D1's actions were more causatively potent than D2's: they have both passed the threshold for liability; they are both liable to the victim for the same damage.

Now let's look at (2). Now it does matter if D1's actions were more causatively potent than D2's. Yes D1's and D2's actions are similar insofar as they have both passed the 'but-for' threshold. But this does not meant that they are necessarily the same. It does not follow from the fact that 'but-for' was the test used to establish liability to the victim that the court cannot use a different test to assess the relative causative potency of their actions for the purposes of a contribution claim.

An analogous fallacy can be found in Nolan v Merseyside CC (1982) 133 NLJ 616 where May LJ held at 621 that one breach of an absolute statutory duty could be neither more nor less blameworthy than another: the better view was taken by Tucker J in Rippon v Port of London Authority [1940] 1 KB 858 when he held at 866-7 that two parties were unequally blameworthy even though they were both strictly liable for breaches of statutory duty.

Suppose I'm on a pedestrian crossing and two negligently driven cars hit me simultaneously, one travelling at 60 mph and one travelling at 20 mph. I am badly injured. Both drivers have caused my injuries on a but-for test. But I would still argue (from my hospital bed - I just keep on going) that the actions of D1 driving the first car were a more potent cause of my injuries than the actions of D2 driving the second car.

Best wishes
Charles