Bodies of knowledge and robes of expertise: expert evidence about drugs, gangs and human trafficking

This article discusses a type of expert evidence in which a witness, often a police officer or other state official, informs the court about patterns of criminal activity and in some cases draws inferences as to the defendant’s involvement in such activity. Their claim to expertise rests not on academic knowledge or scientific research but on some combination of personal experience and information from colleagues and informants which is considered to comprise part of the ‘body of knowledge’ in their field. While we accept that such witnesses may have genuine expertise, the reliability of their evidence is as much in need of critical scrutiny as that of scientific experts. We examine in particular the evidence of police officers who infer gang membership from rap lyrics and videos, and the decisions of the Single Competent Authority identifying victims of human trafficking, which have recently been found admissible as hearsay evidence of expert opinion.