Tuesday, February 26, 2019

The Book of Why by Judea Pearl and Dana Mackenzie

Perhaps, after hearing rumours that artificial intelligence (AI) will replace judges and juries, you have picked up a book called The Book of Why – The New Science of Cause and Effect by Judea Pearl and Dana Mackenzie (2018). This sounds like just the thing to bring you up to speed on that topic.

The book is not really about replacing judges and juries, although it does deal with how to get computers to find causes by analysing data. Conventionally, unless a randomised controlled experiment has been conducted, data produced in an experiment says nothing about causes, only about correlations, although observational data, such as (for example) may be gathered in surveys and analysed by statisticians, can generate conditional probabilities from which causal effects are inferred. The main focus of the book is on how to avoid the need for randomised controlled experiments, and yet be able to say something about causes, or at least how to identify when there is no alternative to conducting a randomised controlled experiment to determine causal effects.

What is of potential interest to lawyers, it seems to me, is the diagrammatic representation of causal relationships that can arise in given circumstances. Can the kinds of diagrams described in this book give lawyers tools for analysis of issues in trials, where the fundamental question will be whether the defendant caused the relevant harm?

Here I have borrowed from Chapter 4 some of Pearl and Mackenzie’s examples of the use of causal diagrams. What I borrowed is the diagrams, and I have tried to apply them to the facts of cases I have imagined. Any mistakes are therefore mine, not the authors’. Decide for yourself whether this kind of approach may be of assistance in your work. There are, as the authors acknowledge, other ways of addressing whether cause has been established, but here is the causal diagram method, inasmuch as I currently understand it.

The authors claim the method is such fun that it can be thought of as a game.

The letters represent events or states of affairs (or, as one could say, variables, in which we may have varying degrees of belief), not people. I illustrate this in Game 1 for X and A, just so you get the hang of it.

Game 1


Defendant pulls trigger (X = this action by defendant)), gun fires (A = what the gun was caused to do), cartridge casing ejects (B), bullet hits victim (Y).

Here there is nothing to block the causal chain from X to Y. B is irrelevant to whether the firing of the gun caused the bullet to hit the victim.

Game 2


The prosecutor charges the defendant with participating in the manufacture of an illegal drug on two occasions, C and Y. The prosecutor's case is that the defendant had an appetite (A) for the illegal drug and that this appetite grew (X). On each occasion the appetite for the drug caused the defendant to collect (B and E) quantities of a substance that could be used to make the illicit drug, and his possession of this substance caused him to be accepted as a participant in the manufacturing process. However, the defendant explains (D) the accumulation of the substance by saying that the appetite is for that substance and that there was no purpose in accumulating it except to consume it.

Blocking (constriction) of a causal chain can occur if there is more than one possible cause for an event or a state of affairs. Here, there is a blockage in the prosecutor's causal chains A to C, X to Y, and B to E. B to E is blocked by the defendant’s explanation D. If the blockage is strong enough to maintain the causal direction D to B then C will be irrelevant to Y and A will be confined to enhancing X, which again will be irrelevant to Y because of the blocking of E. X to Y is blocked if D to E is of sufficient strength (ie if it carries sufficient credibility to raise a reasonable doubt about X to Y).

Game 3


This diagram can be taken in stages to reflect the course of a trial. The prosecutor alleges the defendant hated (X = the defendant's emotion) the victim, and this caused the defendant to kill (Y = what happened to) the victim. Hence the first step is the arrow from X to Y.

The defendant claims that it was another person who both (B) caused the defendant to hate the victim and killed the victim. There is now an arrow from B to Y. This creates a back-door cause, B to Y, of the victim’s death by the hand of that other person, which weakens the causal connection X to Y.

The prosecutor replies by adopting the defendant's allegation that the other person caused the defendant to hate the victim. Hence the arrow from B to X. The prosecutor argues that the causal chain B, X, Y strengthens the link X to Y.

The defendant adduces evidence of an alibi, that he is identified by security camera footage as being somewhere else at the time the victim was killed. This is represented by the arrow from X to A. The defendant hopes this will reduce the strength of the alleged causal link X to Y.

The prosecutor replies by challenging the identification of the defendant in the footage and says it really showed the other person who hated the victim. This is represented by the arrow from B to A.  

If the arrow B to A were to be reversed, because the defendant's alibi made it more likely that the other person who hated the victim was the murderer, there would be a causal chain A, B, Y, which the defendant would hope would reduce further the alleged link X to Y. The prosecutor's attack on the defendant's alibi was an attempt to block the suggestion that the other person was the murderer.

Game 4


Prosecutor alleges the defendant has large accumulated debts (X) which have caused the commission of the present fraud (Y), relying on propensity evidence: when the money problems began (A) they led to the defendant committing fraud (B), and contributing to that earlier fraud was a propensity for dishonesty (C), which also caused the presently alleged fraud (Y).

The proposed causal chain generated by the propensity evidence is blocked by C to B. This reflects the legal position that propensity alone is insufficient to prove current offending. There must be some evidence of current offending before the propensity evidence has anything to corroborate. If there is some such evidence, the admissibility of the propensity evidence will depend on the extent to which the prosecutor can reverse the causal link from C to B, for example by showing that previous commission of fraud (B) strengthened the defendant’s propensity for dishonesty. If the link then went from B to C, there would be no blockage in the causal chain X, A, B, C, Y.

Game 5

This is said to be a more realistic version of Game 4, with two new causal links B to X and X to Y. Using facts like those in Game 4, X to Y is the evidence necessary before propensity evidence has anything on which to operate (that is, to corroborate). B to X is the allegation that previous fraud (B) has caused present indebtedness (X), for example by resulting in reparation orders of a magnitude beyond the present means of the defendant.

Now the alleged causal link X to Y is supported by A, X, Y, and by A, B, X, Y, and by C, B, X, Y.

So, how secure is the position of judges and juries in coming to verdicts? Computers can't perform experiments, they can't use what the authors call "do-operators" to change things in the experimental environment. Where there are causal chains, data may be available to be used to generate conditional probabilities which AI can handle. But where a blocked chain cannot be ignored as being irrelevant, an experiment will be necessary to provide more data. A computer could not, therefore, play Game 2 because of the blocking effect of D (which would require an investigation of the likelihood that the defendant did consume the substance), or Game 3 because of B to A (unless it was able to compare the video image with the defendant, although there may be legal objections to that, or with the other person), or Game 4 because of C to B. Judges and juries surmount these difficulties with something ineffable called judgement.