The father and the child’s mother were visiting her parents at their overseas home when the incident occurred. After enjoying a morning cup of coffee on the veranda, the mother and grandparents were taken gravely ill. There was no dispute that they had been poisoned with thallium, an odourless and tasteless metal that was once widely used in rat poison and pesticides. The grandfather died.

The couple separated soon afterwards when the mother became convinced that the father was a poisoner. After an initial fact-finding hearing, a judge decided that she was probably right in her suspicions, but that ruling was subsequently overturned by the Court of Appeal. A fresh fact-finding hearing was ordered.

In his defence, the father, a medical professional of good character, asserted that it was inherently improbable that he would have administered the poison. There were other possible explanations for what happened, including food or environmental contamination. He had no motive to poison anyone and the mother’s accusation, he argued, amounted to no more than speculation, inference and possibly fabrication.

Ruling on the matter, the judge gave anxious consideration to the evidence. The incident had taken place some years in the past and there was a risk of witnesses being affected by memory creep or confirmation bias. He found, however, that it was more likely than not that the father had introduced thallium into the coffee cups.

Only a finger pinch of the substance would have been needed to cause the maternal grandfather’s death and, but for fate, the maternal grandmother might well also have died. After the event, the father failed to take any steps to assist in their treatment or that of his partner and sought to divert attention away from the possibility of thallium poisoning. In the absence of any proven motive, only the father could know why he acted as he did.

The judge’s ruling was bound to have a major impact on underlying proceedings in which the father was seeking various orders to facilitate contact with his child, whom he had not seen for some years. Given her firm belief in his culpability for the poisonings, the mother was opposed to any such contact.