Forensic DNA Phenotyping in Criminal Investigation: Casework Applications, Deterrence Theory, and the Case for Regulatory Reform

Background/Objectives: Forensic DNA phenotyping (FDP), the inference of externally visible characteristics from biological trace evidence, has become an operational investigative tool, yet its implications for criminal justice systems remain poorly understood outside specialist forensic circles. This study examines the scientific foundations, operational utility, and regulatory context of FDP, and situates its findings within a criminological framework that forensic science literature has largely overlooked. Methods: A narrative review of scientific and legal literature was conducted across six electronic databases, covering publications up to 2026. This was complemented by a structured analysis of publicly documented criminal investigations in which FDP was operationally applied, incorporating both successful outcomes and documented failure contexts drawn from peer-reviewed validation studies. Results: Pigmentation traits are predicted with substantial reliability by validated systems; accuracy declines markedly for complex traits and non-European populations. Casework evidence suggests that FDP can contribute to the reactivation of stalled investigations when combined with complementary methods but is insufficient for identification in isolation. Among the nine European jurisdictions examined, several lack explicit legal frameworks governing phenotyping deployment, with Spain serving as a representative example. Conclusions: The disconnect between forensic science and criminal justice practice is itself a governance problem. FDP has shown investigative potential and, in some documented cases, restorative value—particularly in cold cases—but its responsible deployment requires simultaneous progress in scientific validation, regulatory reform, and the professional training of legal practitioners who currently lack the knowledge to deploy or oversee it effectively.