To celebrate the IMA’s 60th anniversary in 2024, a number of articles have been chosen by IMA members and MT readers, who introduce their favourite MT articles and explain why they are important to them. Here David Percy FIMA, MT editor 2015–2019, introduces …
Lies, Damned Lies and Expert Witnesses by Colin Aitken
Imagine a murder trial where nobody else committed the crime and an expert witness declared the chance of natural death as 1 in 73 million. The defendant is guilty, right? But no, Pr (death given innocence) ≈ 0 yet Pr (innocence given death) ≈ 1. Tragically, this person’s life was destroyed by a wrongful conviction that could have been avoided if the courts had studied this research paper.
Professor Aitken describes this prosecutor’s fallacy and the defender’s fallacy, and raises general awareness of the importance of presenting scientific evidence correctly in criminal trials. He explains that the likelihood ratio Pr (evidence given guilt) ÷ Pr (evidence given innocence) converts prior odds of guilt into posterior odds of guilt. He compares its logarithm, the weight of evidence, to the scales of justice and argues that a Bayesian approach is the only way to evaluate evidence.
The article also investigates DNA profiling in forensic science. Although methods have advanced considerably in the past 27 years, this introduction remains interesting and relevant. In essence, Professor Aitken’s research provides an early exposition of scientific evidence and expert witnesses, which subsequently helped to identify criminals and avoid wrongful convictions.
Download the article, Lies, Damned Lies and Expert Witnesses (pdf) scanned from the 1996 Mathematics Today, vol. 32, no. 5/6, pp. 76–80.
