Social class is more than an economic divide; it engenders distinct cultural patterns. Building on tightness-looseness theory, we identify a critical yet overlooked dimension of within-society cultural variation across social class contexts: the strength of social norms. We propose that lower class contexts foster tighter cultures, characterized by stricter norms and lower tolerance for deviance, as a functional adaptation to chronic socioeconomic threat. Across five studies, we demonstrate that the lower class culture is tighter than the higher class one. Study 1 (57 countries) reveals that lower class individuals are less tolerant of norm violations and more inclined to punish them. Meta-analyses of global data over 52 years show that this divergence in tightness-looseness is associated with a broader trade-off between order and openness: The lower class emphasizes social order yet exhibits lower openness to new people, ideas, and change compared to the higher class (Study 2). Studies 3 and 4 identify the psychological mechanisms, showing that lower social class is associated with greater exposure to socioeconomic threat and heightened vigilance toward danger, which in turn are associated with greater tightness. These threat-based adaptations are further transmitted and reinforced through parental practices. Finally, Study 5 provides experimental evidence that lower class contexts causally increase the need for cultural tightness. By elucidating how social class shapes the experience, adherence, and enforcement of norms, we offer a new framework for understanding cultural differentiation within societies—revealing tightness-looseness as a product of structurally unequal threat exposure that is transmitted across generations and may itself perpetuate class inequality.