Sharp Force Trauma and Chop Mark Identification Bias: Experimental Evidence on the Effects of Bone Morphology, Cortical Thickness, and Ax Material

Sharp force trauma (SFT) is the main criterion used to identify chop mark butchery in zooarchaeology, yet its reliability as a diagnostic feature has not been systematically tested. Chop marks reflect both cutting and fracturing processes and exhibit characteristics of both sharp and blunt trauma. When present, SFT creates a distinct anthropogenic surface that is easily recognizable, whereas chops lacking SFT can resemble general fracture surfaces. This study investigates the potential bias within chop mark analysis by testing how bone type, cortical thickness, and ax material influence the presence and magnitude of SFT. Medium-sized mammal femora and cervical vertebrae were impacted using stone, copper, bronze, iron, and modern steel ax heads under controlled energy conditions using the Instron 9440 Drop Tower System. Results modeled with a Bayesian hurdle-lognormal framework show that bone morphology and tool material jointly determine SFT formation and depth. Iron and steel axes generated SFT more often and with greater depth than stone or copper axes, while femora produced significantly less visible SFT than vertebrae. Reliance on SFT as the main diagnostic criterion introduces material and mechanical bias, obscuring evidence of certain tool types and distorting interpretations of butchery behavior.