Causation and apportionment in psychiatric injury claims: the Scottish position

Scots law asks whether the defender's conduct caused, or materially contributed to, the pursuer's loss. For psychiatric injury, that legal test is answered by a clinical exercise: establishing what disorder the pursuer has, when it began, what factors were operating at the relevant time, and whether the index event is properly regarded as a cause of the disorder as opposed to merely the occasion for symptoms that would probably have appeared in any event. The expert does not decide the legal test, but the clinical reasoning that the expert sets out is what the sheriff or judge relies on to apply it.

This is more demanding than establishing that a disorder exists and that an event preceded it. Post hoc is not propter hoc: a depressive episode that follows a workplace incident by six weeks is not thereby caused by it, and a report that treats temporal sequence as sufficient evidence of causation will not withstand scrutiny. What is required is an account of the mechanism — why this event, given its nature and the individual's psychological make-up, plausibly produced this disorder — set against the alternative explanations the evidence discloses.

Many instructions do not involve a single, clean causal event. A pursuer may have a documented history of depressive episodes, a family history of anxiety disorder, ongoing relationship or financial difficulties, and then an accident. In that situation the psychiatric question is not simply whether the accident caused the current disorder, but whether it materially contributed to it — whether, but for the accident, the current presentation would probably not exist in its present form, extent or timing.

Addressing this properly requires the report to set out what is known of the pre-accident state: from GP records, not merely from the examinee's own account, since recollection of one's pre-accident mental health is unreliable and coloured by the current presentation. Where the records show previous episodes that resolved, with return to full function, that history does not preclude a finding that the accident caused a new or materially different episode; it does, however, bear on prognosis and on the proportion of ongoing symptoms properly attributable to the accident as against the underlying vulnerability.

Apportionment arises where the evidence supports a finding that part of the pursuer's current presentation is attributable to the index event and part would have occurred, or did occur, independently of it. This is most commonly encountered where there is a pre-existing psychiatric disorder that has been aggravated, or where a constitutional vulnerability has been activated earlier or more severely than it otherwise would have been.

Psychiatric opinion can properly address apportionment where the clinical picture allows a reasoned view — for example, where records document a stable, treated condition before the accident and a clear deterioration in severity and function afterwards, permitting an opinion that a given proportion of current impairment is due to the acceleration or aggravation caused by the accident. Where the evidence does not permit that degree of precision, the report should say so rather than manufacture a percentage that gives an impression of precision the clinical material cannot support. Courts are entitled to a reasoned range, or a statement that apportionment cannot reliably be quantified on the material available, rather than an unsupported figure.

Scots law does not permit a defender to escape liability because the pursuer had a pre-existing vulnerability that made the psychiatric consequences of the accident worse than they would have been in a person of ordinary robustness — the well-established principle that a wrongdoer takes the pursuer as found. This principle does not, however, remove the need for careful causation reasoning. The expert's task is to distinguish between a pursuer whose pre-existing vulnerability was activated or aggravated by the accident, to whom the eggshell skull principle applies in full, and a pursuer whose current presentation would have arisen at essentially the same time and to essentially the same extent regardless of the accident, where the accident is not a material cause at all. That distinction turns on the clinical detail, not on the label attached to the pursuer's pre-accident history.

Where causation or apportionment is likely to be contested, instructing agents assist the process considerably by providing complete pre-accident GP records, not a summary, and by putting the causation question to the expert explicitly rather than leaving it to be inferred from a general request for 'an opinion on the pursuer's psychiatric condition'. A precognition or supplementary report addressing apportionment specifically, once records are to hand, is often more useful than attempting to resolve the question in the first report before the full history is available.