Criminal law in the United States values conceptual definitiveness in its quest for resolution. But the work of open-ended humanization required of sentencing mitigation advocates in American death penalty cases defies this call for definitiveness. Even as formal legal processes seek to limit the knowledge that can be brought into the courtroom, new theoretical approaches that justify more understanding and fact-finding can help attain tangible goals of defense advocacy. This article provides an ethnographic account of how capital defense practitioners in the United States engage with anthropological theories of culture as a behind-the-scenes advocacy strategy that succeeds by exploiting the anti-definitiveness inherent in culture. I develop the concept of ‘subtension’ as an analytical trope to help elucidate these processes.