UCL psychologist David Lagnado on the psychological processes used when people make judgments based on complex bodies of interrelated evidence.

In everyday life, as well as more specialised contexts such as legal or medical decision making, people make judgments based on complex bodies of interrelated evidence. What psychological processes do they use, and how do these relate to formal methods of evidence evaluation? This talk will assess the applicability of Bayesian networks, both as a normative and descriptive model for evidential reasoning. We will outline a novel framework for evidential reasoning based on causal idioms (Fenton, Lagnado & Neil, 2011). These idioms can be combined and reused to capture complex bodies of evidence. This approach is applied to witness and alibi testimony. We show how the framework captures critical aspects of witness reliability, and the potential interrelations between witness reliabilities and other hypotheses and evidence in a legal case. We also report several empirical studies which suggest that people’s intuitive inferences fit well with the qualitative aspects of the framework.