Principles for Assessment of Damages
51 There was no dispute about the principles that should be applied in assessment of damages, though counsel on both sides adverted to those principles in their submissions. Rather, the dispute was about what proper application of the principles required in the present case.
52 Watts v Rake [1960] HCA 58; (1960) 108 CLR 158 held that if a person who has a predisposition to a disability of a particular kind suffers an injury that is a cause of a disability of the same kind, in consequence of the negligence of a defendant, the plaintiff is entitled to recover damages for the full extent of the disability that has been shown to be caused by the accident, unless the defendant can disentangle the causes of the plaintiff's condition, and show that, to some particular extent, the accident was not a contributory cause. On the facts of that case, it was accepted that the plaintiff's pre-existing disability would have resulted in him becoming unfit for work 13 years after the accident, and hence the plaintiff was entitled to general damages assessed on the basis of the accident having caused him to lose 13 years of working life.
53 Purkess v Crittenden [1965] HCA 34; (1965) 114 CLR 164 makes clear that the onus on the defendant to disentangle the causes is an onus of adducing evidence, not an onus of proof. Barwick CJ, Kitto and Taylor JJ, at 168 said:
"… it is not enough for the defendant merely to suggest the existence of a progressive pre-existing condition in the plaintiff or a relationship between any such condition and the plaintiff's present incapacity. On the contrary … both the pre-existing condition and its future probable effects or its actual relationship to that incapacity must be the subject of evidence (ie either substantive evidence in the defendant's case or evidence extracted by cross examination in the plaintiff's case) which, if accepted, would establish with some reasonable measure of precision, what the pre-existing condition was and what its future effects, both as to their nature and their future development and progress, were likely to be. That being done, it is for the plaintiff upon the whole of the evidence to satisfy the tribunal of fact of the extent of the injury caused by the defendant's negligence."
54 Malec v JC Hutton Pty Ltd [1990] HCA 20; (1990) 169 CLR 638 requires prediction of future events to be assessed by reference to the probability of their occurring. It concerned a situation where the plaintiff contracted a disease in consequence of the defendant's negligence, and that disease was one that could cause both a depressive illness and a degenerative-like condition in the spine. As well, the plaintiff had a pre-existing back condition, the onset of which was itself likely to produce a "neurotic condition". Deane, Gaudron and McHugh JJ held, at 645 that:
"The plaintiff is entitled to damages for pain and suffering on the basis that his neurotic condition is the direct result of the defendant's negligence. Those damages must be reduced, however, to take account of the chance that factors, unconnected with the defendant's negligence, might have brought about the onset of a similar neurotic condition."
55 In Seltsam Pty Ltd v Ghaleb [2005] NSWCA 208; (2005) 3 DDCR 1 Ipp JA (with whom Mason P agreed) at [103] drew the following propositions from Malec:
"Therefore, according to Malec :
1. In the assessment of damages, the law takes account of hypothetical situations of the past, future effects of physical injury or degeneration, and the chance of future or hypothetical events occurring.
2. The court must form an estimate of the likelihood that the alleged hypothetical past situation would have occurred.
3. The court must form an estimate of the likelihood of the possibility of alleged future events occurring.
4. These matters require an evaluation of possibilities and are to be distinguished from events that are alleged to have actually occurred in the past, which must be proved on a balance of probabilities."
56 Ipp JA continued in Seltsam v Ghaleb at [104]-[107]:
"What was said in Watts v Rake and Purkess v Crittenden now has to be qualified by these principles (cf Commonwealth of Australia v Elliott [2004] NSWCA 360 at [81]). Malec has an important bearing, for example, on the way in which a court must determine whether a defendant has discharged the "disentangling" evidentiary burden on it of showing that part of the plaintiff's condition was traceable to causes other than the accident and that, had there been no accident, the plaintiff would have suffered disability from his pre-existing condition.
Where a defendant alleges that the plaintiff suffered from a pre-existing condition, the evidential onus as explained in Watts v Rake and Purkess v Crittenden remains on the defendant and must be discharged by it. Nevertheless, to the extent that the issues involve hypothetical situations of the past, future effects of physical injury or degeneration, and the chance of future or hypothetical events occurring, the exercise of "disentanglement" discussed in those cases is more easily achieved. That is because the court is required to evaluate possibilities in these situations - not proof on a balance of probabilities.
Without intending to give an exhaustive list of possibilities, it may be that, had the defendant's negligent act not occurred, a pre-existing condition might have given rise to the possibility that the plaintiff's enjoyment of life and ability to work would have been reduced and to a susceptibility to further injury; in addition, other causes entirely unrelated to the defendant's negligent act might have contributed to the plaintiff's ultimate condition.
Appropriate allowances must be made for these contingencies. A proper assessment of damages requires the making of a judgment as to the economic and other consequences which might have been caused by a worsening of a pre-existing condition, had the plaintiff not been injured by the defendant's negligence. A pre-existing condition proved to have possible ongoing harmful consequences (capable of reasonable definition) to the plaintiff, even without any negligent conduct on the part of the defendant, cannot be disregarded in arriving at proper compensation."