This paper investigates the psychological mechanisms that underpin Qumran sectarian dualism and its construction of in-group/out-group boundaries. Specifically, evidence from experimental and developmental psychology and cognitive anthropology is used to argue that Serek ha-Yaḥad and the Two Spirits Treatise (1QS 3:13–4:26) reflect a deeply-engrained psychological essentialism wherein non-group members are conceptualized as having inherently different biological essences. This essentialist tendency is easily extended to the social domain in what scholars call the “naturalization” of social groups. After reviewing this literature, the paper examines the Serek and Treatise’s use of kinship terms, the word “spirit,” and language denoting human nature and living species, in order to demonstrate that essentialist intuitions about outsiders provide a foundation for the sect’s dualistic worldview. Importantly, the essentialist thinking in these texts is also firmly grounded in and channeled through the intertextual interpretation of scripture, drawing heavily on the rich creation vocabulary in Genesis 1–3.