We tested whether expert testimony concerning a biomechanism of psychopathy increases or decreases punishment. In a nationwide experiment, U.S. state trial judges (N = 181) read a hypothetical case (based on an actual case) where the convict was diagnosed with psychopathy. Evidence presented at sentencing in support of a biomechanical cause of the convict's psychopathy significantly reduced the extent to which psychopathy was rated as aggravating and significantly reduced sentencing (from 13.93 years to 12.83 years). Content analysis of judges' reasoning indicated that even though the majority of judges listed aggravating factors (86.7%), the biomechanical evidence increased the proportion of judges listing mitigating factors (from 29.7 to 47.8%). Our results contribute to the literature on how biological explanations of behavior figure into theories of culpability and punishment.