Diagnostic inversion at the suicide-homicide boundary: A bidirectional systematic review and forensic indicator crosswalk.

At the suicide-homicide boundary, the same physical finding may be interpreted in opposite directions depending on whether the initial hypothesis is homicide staged as suicide or suicide simulating homicide. This bidirectional systematic review synthesised two independently registered review arms: Arm A, homicides intentionally staged to resemble suicide; and Arm B, suicides that simulated or were initially suspected of homicide. PubMed/MEDLINE, Scopus and Web of Science were searched from inception to 3 April 2026 without language restrictions. Screening, extraction and quality appraisal were performed in duplicate, using PRISMA 2020, SWiM and JBI guidance. Fifty-eight studies were included: 12 in Arm A and 46 in Arm B, representing approximately 218 eligible cases. A harmonised Indicator Crosswalk identified 33 forensic indicators across 9 domains and classified them under a single-axis taxonomy of directional stability comprising five classes. The central finding was a group of five directionally unstable (Opposing) indicators: multiple gunshot wounds, multiple sharp-force wounds, atypical firearm entry location, range-of-fire/arm-reach compatibility and suspension-geometry plausibility. These findings are not merely non-specific; they may support opposite default interpretations across the two diagnostic directions. The Crosswalk therefore reframes equivocal manner-of-death assessment as an inversion-aware task: each apparently decisive finding should be tested against its opposite-direction meaning and interpreted only through convergence with companion scene, autopsy, toxicological, imaging, ballistic and contextual evidence. The Crosswalk is a hypothesis-generating, reasoning-support framework derived from case-based evidence, and not a validated diagnostic instrument.