What is sustaining the divide between nature and nurture, even though sciences like epigenetics have been challenging it for at least two decades? Evelyn Fox Keller asked this question and considered it a logical problem rooted in terminological confusion within the sciences. In this article, we propose a complementary diagnosis of the problem: the nature-nurture divide is (re-)mobilized when society faces questions of inclusion and solidarity. With examples stemming from the fields of insurance and health care, immigration policy and epigenetics, we demonstrate how the nature-nurture divide is performed through techniques of classification for a politics of solidarity. We identify a common operation to these different examples that we coin...