Risk Assessment and Decision Analysis with Bayesian Networks

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13

Bayes and the Law

13.1 Introduction

The central idea of Bayes’ theorem⎯that we start with a prior belief

about the probability of an unknown hypothesis and revise our belief

about it once we see evidence⎯is also the central concept of the law.

Proper use of Bayesian reasoning has the potential to improve dra-

matically the efciency, transparency, and fairness of the criminal

justice system and the accuracy of its verdicts, by enabling the rele-

vance of evidence to be meaningfully evaluated and communicated.

Bayesian reasoning can help the expert in formulating accurate and

informative opinions; help the court in determining the admissibility

of evidence; help identify which cases should and should not be pur-

sued; and help lawyers explain, and jurors to evaluate, the weight of

evidence during a trial.

Although there is widespread disagreement among lawyers about

the kind of evidence to which Bayesian reasoning should be applied

and the manner in which it should be presented, there is an increasing

interest in its potential. The use of BNs is especially promising in the

law and is starting to be used to help lawyers understand the strength

of a case when there are multiple dependent and conicting pieces of

evidence.

In this chapter we explain why Bayes is a natural method for rea-

soning about legal evidence (Section 13.2). However, traditionally the

process of building BNs for legal arguments has been ad-hoc, with little

possibility for learning and process improvement. Hence, in Section 13.3

we show how the idioms approach of Chapter 7 enables us to build useful

(and even complex) legal arguments in a consistent and repeatable way

and Section 13.4 provides a comprehensive example of applying this idi-

oms based method. A recurring theme throughout is the extent to which

the BN approach helps avoid common probabilistic fallacies made in

legal arguments. In Section 13.5 we use BNs to explain and avoid some

additional legal fallacies.

Visit www.bayesianrisk.com for your free Bayesian network software and models in

this chapter

Misuse of probability in the law is

a continuing concern. Dozens of

widely documented cases in which

verdicts have been inuenced by

incorrect probabilistic reasoning,

such as the case of Sally Clark

who was wrongly convicted of the

murder of her two children, are

only the tip of the iceberg. Most

common fallacies of probabilis-

tic reasoning in the law are eas-

ily avoided by applying Bayes’

theorem.