Ton Derksen, professor of philosophy of science and cognitive science
Radboud University Nijmegen
Paper for University College London Seminar 20 March 2007.
Three reasoning instincts and the fabrication of facts: the case Lucia de B.
The nurse Lucia de B. was convicted by the Court of Appeal for 7 murders and 3 attempted murders in four hospitals in the Hague with a sentence of life imprisonment. Yet there was little evidence apart from the fact that at the hospital where it all started, quite a number of resuscitations happened during the shifts of this particular nurse. Then, after one more resuscitation it was felt that all this could not be just a coincidence.
Very early in the process a quantification was given to the uneasy feeling that proportionally there were too many resuscitations during the nurse’s shifts. The chance that such a coincidence could have happened by mere accident was calculated as 1 in 6 billion, and somewhat later by an “expert” as 1 in 342,000,000. With that number out in the open the general notion was that what had happened could not be just an accident. The nurse definitely had to be a serial killer, even if no one had any further evidence.
This notion was in fact the driving force in the whole judicial process. It coloured the perception of the hospital, the medical experts, the prosecution and both the court of first instance and the court of appeal. And it fabricated a whole series of incriminating facts which led to the inexorable conclusion.
In the first part of this paper I will document the driving force of the Incredible Coincidence, the disbelief of people that the spectacular coincidence was just a coincidence. I also examine the fabrication of the facts due to this disbelief. Peter Grünwald will expose the statistical elaboration of this idea as it is presented before the court. We shall detect that also in the statistical elaboration the facts have been manhandled. Careful attention to the facts would have yielded a very different outcome. Actually, when all the facts are in, the coincidence shrivels up drastically. Nothing particularly shocking remains. So there is every reason to think that the remaining coincidence is just that — a coincidence.
Throughout the process we see three reasoning instincts at work:
(1) the Smoke and Fire Instinct, which caused colleagues of the nurse to get worried about the many incidents during her shifts, and which later on caused the general public to be content with the verdict of the courts in spite of the shaky evidence;
(2) the Small Chance Instinct which makes us neglect small chances of ‘P’ and makes us automatically turn to the belief of ‘not P’; and
(3) our Instinct to Neglect Base Rates (in not-everyday situations).
These instincts are natural in the sense that they guide our cognitive housekeeping. They yield quick conclusions which usually are good enough from an evolutionary perspective. Unfortunately, in the case of Lucia de B. their influence was distastrous.
Next to this idea that it could not be just a coincidence, the court used two arguments in order to come up with independent evidence in at least two cases. In one case the prosecution and the experts concluded an acute digoxin poisoning. However, no one read the scientific literature carefully. A careful reading would have shown them that post-mortem redistribution could explain the allegedly high digoxin concentration in the blood, and that actually the concentrations found in the kidney and liver do not fit the hypothesis of an acute digxoin poisoning at all (they are way too small).
This is one refutation. Just to be sure, here are two more:
(1) the trend tables would have allowed a time period for the administration of the fatal dose of digoxin. The trend graphs (a more precise indication which the court did not use) show that it is impossible. (Two doctors examined the baby during that period).
(2) An acute digoxin poisoning leads to a contracted heart, but the heart was not contracted, a fact which the prosecution knew but did not deem necessary to share with the expert.
The other case was supposed to show that the nurse had a compulsion to murder. The alleged proof is a diary remark about giving in to an unspecified compulsionand the death of a patient during that day’s shift of the nurse. The court decided that the patient’s death was non-naturally and that the compulsion must be a compulsion to kill. All six medical doctors told the court that the death was natural and two psychiatrists stated that there was a satisfactory explanation for the word ‘compulsion’, yet the court overruled their unanimous judgement amd developed its own (amateur) medical story about the death. This is illustrative of the court’s treatment of experts. This court had a compusion to convict, and I assume that this compulsion is due to the bad statistics which determined its own conviction.
Ton Derksen was professor of philosophy of science and cognitive philosophy at the Universities of Nijmegen and Tilburg (The Netherlands). He got his D.Phil. from Magdalen College, Oxford for a dissertation on probability, chances and belief. He wrote extensively on the problem of rationality of science and relativism. During the last years his interests moved to issues in the adjacent area of psychology, aesthetics and the problem of conventionalism. Recently he published on linear perspective and depth perception in The British Journal of Aesthetics and The Journal of Philosophy. He plays the oboe and has his own Bach orchestra.