Clarifying the ‘reliability’ continuum and testing its limits: biometric (fingerprint and DNA) expert evidence

This chapter revisits the basic concept of ‘evidential reliability’, which is conceptualised as a continuum rather than a binary standard. Areas of contested reliability may then be characterised as ‘liminal zones’ marking areas of transition where heightened institutional scrutiny may be required. The chapter presents a structured approach to the assessment of evidential reliability in terms of a tripartite conception of scientific validity, comprising foundational, applied and evaluative strands. Scientific evidence worthy of institutional reliance, or ‘critical trust’, should be demonstrably valid in all three senses of scientific validity. This novel analytical framework is further elucidated through detailed application to the two most prominent forms of biometric evidence, fingerprinting and DNA profiling. Attention is drawn to subjective factors and potential pressure points in disciplinary methods and working practices, where such evidence strays into a liminal zone of questionable reliability and should prompt intensified critical scrutiny. Priority areas for further policy development, with potential to enhance evidential reliability, include devising standardised formats and terminology for expert reports and refining interpretational protocols.