Academic Abstract Manhood is a precarious social status. Under perceived gender identity threat, men are disproportionately likely to enact certain stereotype-consistent responses such as aggression to maintain their gender status. Yet less is known regarding individual variation in men’s threat responsiveness—that is, the psychological conditions under which one’s masculine identity is more or less “fragile.” We propose a novel model of masculine identity whereby masculine norm expectancy generates discrepancy within the self to the extent that rigid norms are internalized as obligational (actual-ought discrepancy) versus aspirational (actual-ideal discrepancy), which predict extrinsic versus intrinsic motivations to reduce these discrepancies, respectively. Under threat, then, extrinsic motivations predict externalized responses (e.g., aggression), and intrinsic motivations elicit internalized responses (e.g., anxiety, shame, self-harm). We also consider the conditions under which masculinity may be less fragile—for example, in contexts with less rigid expectations and among men who reject expectations—as pathways to mitigate adverse masculinity threat-related outcomes. Public Abstract In many cultures, men prove their manhood by engaging in behaviors that harm themselves and others (e.g., violence, sexism, homophobia), particularly people from marginalized groups. Yet less is known about why some men are more likely than others to enact these masculinity-proving behaviors. The goal of our model is to specify certain conditions under which masculinities become “fragile” and elicit these responses when under threat. We start by describing the rigid expectations men experience—for example, that they are strong and tough. We propose that these expectations cause men to experience different forms of discrepancy within themselves that produce corresponding motivations to reduce these discrepancies. Under threat, motivations driven by others’ expectations elicit outward attempts to restore masculine status (e.g., aggression), whereas motivations driven by self-ideals cause internalized responses (e.g., shame, self-harm). We conclude by discussing how to reduce these discrepancies, such as mitigating the rigidity of and encouraging men’s resistance to masculinity expectations.