Dembski's Explanatory Filter Delivers a False Positive
Andrea Bottaro has shown by example the irrelevance of William Dembski’s explanatory filter to archaeology. In this contribution, I want to show how in the real world the explanatory filter can lead to a false positive.
Dembski’s vaunted explanatory filter is no more than a flow chart designed to distinguish events of low probability. If the probability of an event is low enough and if Dembski can discern a pattern, then he concludes that the event must have been the product of design. Dembski admits that the explanatory filter may produce a false negative (fail to infer design where design exists) but claims it will never produce a false positive (infer design where none exists). In this article, I will give a real example wherein the explanatory filter could have yielded a false positive.
According to an article in Science [Quinn Eastman, “Crib Death Exoneration Could Usher in New Gene Tests,” Science, 20 June 2003, p. 1858], a British woman lost three babies to sudden infant death syndrome (SIDS or crib death) within four years. The Crown Prosecution Service applied the explanatory filter as follows: One death is tragic; two deaths are suspicious; three deaths are murder. The woman was prosecuted.
According to the BBC, the rate of SIDS in England and Wales is less than 0.5 death per 1000 live births. In effect, prosecutors reasoned that the probability of three SIDS deaths was 0.00053, or approximately 10-10. They concluded that this probability was so small that a design inference was warranted, and the woman was charged.
What the prosecutors did not know or ignored was that SIDS may be a genetic disease that runs in families. Indeed, the woman’s grandmother testified that three of her children died of unexplained causes before the ages of 6 weeks (in the 1940’s, before SIDS was recognized). A geneticist further testified that SIDS could run in families and suggested two possible mechanisms. The jury used this information - what Dembski calls side information - and acquitted the woman.
In short, the prosecutors applied the explanatory filter and arguably got a false positive, whereas the jury applied side information and drew the opposite conclusion. In the case that Bottaro describes, the archaeologists likewise applied side information and entirely ignored the explanatory filter. The forensic archaeologist, Gary Hurd, argues further that only the side information is relevant [“The Explanatory Filter, Archaeology, and Forensics,” Chapter 8 of Matt Young and Taner Edis, eds., Why Intelligent Design Fails: A Scientific Critique of The New Creationism, Rutgers University Press, New Brunswick, 2004].
The archaeologists and the jury used a probabilistic argument, in a way. Each bit of side information - the wear pattern on the shells, the presence of pigment - strengthened the argument that the shells were beads. The archaeologists would not have been able to prove design without concentrating on the side information. The explanatory filter, a diagnosis of exclusion, was wholly irrelevant or, in the SIDS case, possibly misleading.
Now, I can’t prove that the prosecutors were wrong - only that the filter may well have produced a false positive. In this regard, my example is no different from a favorite example of Dembski’s: the county clerk who “randomly” gave 40 Democrats in 41 elections the top ballot position [No Free Lunch, pp. 55-58]. Dembski has adduced no hard evidence to prove that the clerk cheated; all he can say is that the clerk most probably cheated, that is, that the 40 Democrats were most probably the result of design.
In the same way, I can’t say that the explanatory filter gave a false positive; all I can say is that it may well have given a false positive. But Dembski claims it is immune to false positives. It is not, precisely because you cannot know when you have all the side information, and often the side information is crucial. (Yes, I know, you can take the side information and reevaluate the probabilities to draw the negative inference. But that’s not the point, is it? The point is that Dembski claims the filter to be immune to false positives, and it can’t be, because you never know whether you have all the relevant side information. [See also John S. Wilkins and Wesley R. Elsberry, “The Advantage of Theft over Toil: The Design Inference and Arguing from Ignorance,” Biology and Philosophy, 36, 711-724, 2001.])
Bottaro’s archaeologists inferred design because they knew something about the designers and what they do with beads. The explanatory filter was irrelevant. If it cannot be applied to a simple case such as identifying beads or ferreting out a murderer, how can it be useful for identifying the artifacts of a designer whose habits and intentions are wholly unknown to us?