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Although prior research has examined the relationship between genetic relatedness and helping behavior (Burnstein, Crandall, and Kitayama, 1994), less is known about its role in aggressive responses to insults ( Fitzgerald and Ketterer, 2011 ). Drawing on inclusive fitness theory ( Hamilton, 1964 ) and the Kinship, Acceptance, and Rejection Model of Altruism and Aggression (KARMAA; Webster, 2008 ; Webster et al., 2012 ), we designed a 2 (participant gender) × 2 (target gender) × 2 (insult: status vs. reproductive) × 3 (relatedness: stranger vs. cousin vs. sibling) between-person experiment in which 489 participants (a) read vignettes in which a stranger, cousin, or sibling was insulted and (b) reported their emotional reaction and retaliation likelihood (six-item α= .91) in response to the insult. Consistent with theory and prior research, men were significantly more aggressive than women, and people were significantly more aggressive responding to insults against kin than non-kin. These findings support theoretically-derived, dynamic, and domain-specific links among insults, gender, relatedness, and aggression.