This presentation examines how keywords that are now commonly used to describe forced assimilation in settler colonial societies — such as the term “cultural genocide” to describe the Canadian residential school system — were first adopted by anthropologists in the context of debates concerning the genocide of Indigenous peoples in South America. The talk describes how, during the early 1970s, anthropologists, legal experts, political scientists, journalists and philosophers from Paraguay, Germany, Switzerland, the UK, and the US began to warn that the Indigenous peoples known as the Aché were subject to genocidal extermination at the hands of an extractive Paraguayan dictatorship. In addition to being subject to terrible acts of physical violence, critics of the Paraguayan government warned that the Aché were subject to a process of “deculturation” defined as a process of where loss of culture destroys self-esteem and the will to live – a phenomenon that some called “psychic death”. This account of Aché genocide from the 1970s, stands in stark contrast to descriptions from the 1950s of an adopted Aché girl, whose story figured prominently as an example of the positive benefits of “cultural change” in UNESCO campaigns against racism. Building on work by Marco Ramos and Ian Hacking, this paper will argue that what was at stake in this shift is the historical ontological question of how experts “make and mold” the reality of violence during a period in which Cold War development projects became increasingly linked with genocide.