Three studies supported a model whereby associations between ideologies that share roots in biological determinism and outcomes that reinforce inequality (based on gender, race, or class) were mediated by system justification beliefs (SJB). Outcomes included support for Donald Trump over Hillary Clinton as president (Study 1), justifying police brutality (Study 2), and support for a White House budget that slashed the social safety net to endow the wealthy with tax cuts (Study 3). These findings provoke a vital question: How do people deem unequal systems worthy of defense? Each study compared social Darwinism, social dominance orientation (SDO), and biological essentialism. We expected social Darwinism to account for the most variance in SJB because it provides both the rationale for social hierarchies (natural selection) and defends them as required for human welfare. This prediction was supported in each study. Implications for the psychology of legitimacy are discussed.