3. Tests for causation
3.3 Over-determined causation
‘Over-determination’ means that there seem to be too many causes. Where this takes the form of ‘duplication’ two or more possible causes occur at the same time, and in ‘pre-emption’ the possible causes occur one after the other.57
54 Wright, ‘Pruning’ (n4) 1022. 55 Honoré (n39) 367.
56 Wright, ‘Causation’ (n2) 1792.
57 Hart and Honoré used different labels to describe the forms of over-determined causation, stating that ‘an event
may be causally overdetermined either because two conditions each sufficient though not necessary in the circumstances for its occurrence are present together (additional causes) or because they are so related that if one had not been present the other would have been (alternative causes)’ (Hart and Honoré (n7) xl). Wright’s terminology of ‘duplication’ and ‘pre-emption’ is adopted here for two reasons. First, given the emphasis on the importance of clear use of technical terminology, it is preferable to avoid the term ‘alternative causes’ which is sometimes used with a different meaning in the caselaw as a contrast with ‘cumulative causes’. Secondly, Wright’s account benefits from greater analytical clarity on this point because, as he observed, ‘Hart and Honoré also submerged and sometimes confused the critical distinction between duplicative and pre-emptive causation by constructing an overlapping typology of overdetermined cases’, so they did not always maintain a clear distinction between the two forms of over-determination (Wright, ‘The NESS Account’ (n3) 287).
75
3.3.1 But-for, NESS and pre-emption
Pre-emption describes a scenario where two acts could each have been sufficient to cause the harm, but one occurs before the other so the second one never gets to run its course. This is illustrated by the classic desert traveller scenario: A puts poison in the water keg of a desert traveller which is sufficient to kill him if he drinks it but before he drinks the water, B empties the keg, and the traveller dies of thirst. The but-for test leads to the conclusion that neither act was a cause of the traveller’s death: if he had not died of thirst he would have died from poisoning and vice versa. Effectively, the but-for test ‘produces the absurd result that the plaintiff’s injury was uncaused’.58
It has been suggested that the but-for test would identify B as the cause of the desert traveller’s death if the outcome was described more precisely as ‘death when and as it happened’ i.e. death through dehydration at that particular moment. This tactic of ‘detailing the injury’, Wright argues, is ‘nothing more than proof by tautology’ because ‘[t]he factors believed to be causally relevant…are incorporated into the description of the manner of occurrence of the injury…, and they are then demonstrated to be causally relevant because we cannot construct that precise description without them’.59 It is therefore clear that the but-for test cannot cope adequately with
pre-emptive causation.
The legal solution to this absurd outcome would be to impose liability on one or both parties despite the failure to satisfy the legal test for causation. Such a solution is driven by the instinctive knowledge that the death cannot be ‘uncaused’, one or both parties must be responsible for the death despite the difficulty of establishing a causal link. Thus reliance on the but-for test effectively turns the legal solution to pre-emption into a value judgment as to responsibility. But, as Hamer has argued, ‘the over-determination problem is one of fact and
58 Beever (n41) 331.
76
philosophy rather than value and policy’.60 This is also an unnecessarily complicated ‘solution’
because there is not really a ‘problem’ regarding the existence of a causal link and this can be seen by explaining and applying the NESS test.
In contrast to the but-for test, the NESS test asks whether the negligence was a ‘necessary element of a set of antecedent actual conditions that was sufficient for the occurrence of the consequence’.61 In the desert traveller scenario, by poisoning the water flask A created the
potential for a set to occur that would be sufficient to bring about the traveller’s death. However,
for the set of conditions to be complete, in other words for it to actually be sufficient, the traveller must drink the water containing the poison. Until he drinks the poison there is not an
actually occurring set of conditions sufficient to cause his death that contains A’s act of adding
poison to the water. Since B then emptied the water (and therefore also the poison) the traveller never actually drank the poison. This means that A’s act was prevented from becoming part of an actually occurring set of conditions that were together sufficient to cause the death, so A is not a cause of the harm.62 A set of conditions jointly sufficient to cause the death now exists
containing just B’s act of emptying the flask. This set of conditions runs its course so it is an
actually occurring set, so B is a cause.
3.3.2 But-for, NESS and duplication
The plurality of causes means that one outcome could be caused by a variety of sufficient sets and usually only one of these sets will actually have occurred, but over-determination in the form of duplication arises when more than one sufficient set of conditions actually occurs. Duplication, in its simplest form, is illustrated by the classic ‘double-hit hunters’ example: Hunter
A fires his gun and the bullet hits a walker. The walker dies and the bullet was sufficient to cause
60 David Hamer, ‘Chance would be a fine thing’: Proof of Causation and Quantum in an Unpredictable World’
(1999) 23 Melb U L Rev 557, 571.
61 Wright, ‘Causation’ (n2) 1790 (emphasis added). 62 See Wright, ‘Causation’ (n2) 1795.
77
his death. In the ordinary course of events, Hunter A would be considered to have caused the death because his bullet was sufficient to result in the death. However, Hunter A’s bullet was ‘duplicated’ by Hunter B simultaneously firing his gun and also hitting the walker with a bullet that was sufficient to cause the death.
If we apply the but-for test to determine whether Hunter A was a cause of the walker’s death then the answer is ‘no’. But-for Hunter A’s shot, the walker would still have died because Hunter
B’s bullet was sufficient to kill him, so Hunter A made no difference to the outcome and was not
a cause of the death. Likewise, but-for Hunter B’s shot, the walker would still have been killed by Hunter A so Hunter B made no difference to the outcome and is not a cause. Once again, the but-for test ‘produces the absurd result that the plaintiff’s injury was uncaused’.63
In contrast, the NESS test fully incorporates the plurality of causes and asks whether the negligence was necessary for the sufficiency of a sufficient set that actually occurred.64 It
therefore allows for the existence of more than one actually occurring set. If we subtract Hunter B’s shot we are left with a set of conditions containing Hunter A’s shot, along with wind speed and direction, the walker’s presence, the walker still being alive etc., that are sufficient to cause the walker’s death, and Hunter A’s shot is necessary for this set to be sufficient. Likewise if we subtract Hunter A’s shot we are left with a set of conditions containing Hunter B’s shot that is sufficient to cause the walker’s death and Hunter B’s shot is necessary for the sufficiency of this set. The plurality of causes, as identified by Mill, means that there are also many other possible sets that would hypothetically have been sufficient to cause the walker’s death had they occurred at that moment, for example a tree toppling onto him, but none of these other sets actually occurred. However, the fact that two sufficient sets independently and simultaneously have
actually occurred does not prevent either set from being a cause. Usually only one set will actually
occur, but on the rare occasion that two sets actually occur then both are causes.
63 Beever (n41) 331. 64 See text to (n61).
78
Whereas the but-for test told us that Hunter A made no difference to the outcome because the walker would have died anyway so Hunter A is not a cause, the NESS test tells us that in order for the walker’s death to have been prevented Hunter A’s shot would have to be absent so Hunter A is a cause, and that Hunter B’s shot would also have to be absent so Hunter B is also a cause.65 This
is a subtle yet significant shift in emphasis and one which better reflects the reality that given the plurality of causes it is possible, albeit rare, for two or more causes independently to occur at the same time.
It is clear that the NESS test, rather than the but-for test, accurately reflects and tests the qualities that were shown by Hume and Mill to characterise causes. The NESS test has therefore been described as ‘a real contribution to legal analysis. It provides an extremely helpful way of conceptualizing the nature of causal problems, and it offers a rational process for identifying causes in over-determined cause cases’.66 This means that on a theoretical level it would be
preferable to adopt the NESS test. It represents a subtle shift in emphasis from the but-for test which treats as a cause ‘something in the absence of which harm would not have come about’, to calling a cause ‘something such that in the prevailing conditions harm comes about’ under the NESS test.67 This may seem subtle but it is very significant because it accurately translates the
meaning of causation and the characteristics of causes into a workable test. In practice the NESS test will frequently collapse into the but-for test because most cases involve straightforward causal models, but NESS is to be preferred because it is able to tackle complex issues of causation in a way that the but-for test cannot. This choice is ‘not governed by policy considerations, but rather by how well each test corresponds with our intuitive concept of causation’ based on the philosophical explanation of causation.68
65 Stapleton ‘Choosing’ (n1) 438.
66 David A Fischer, ‘Insufficient Causes’ (2005) 94 Ky L J 277, 281. 67 Honoré (n39) 385.
79