Previous research has attributed the motivations of suicide combatants to religious fervor, political engagement, and organizational strategic goals, among others. The motivations of suicide combatants, however, are often related to local and familial dynamics; specifically, combatants' primary socialization on violent norms. To better understand suicide attacks, we use a data set of 2,923 individual fighter-level observations of combatants for the Islamic State. These data include individual demographic data and the fighter's role for the Islamic State; including if they volunteered to be a suicide fighter. Through these data, we test several extant theories of suicide combatant mobilization (i.e., ideational or economic incentives) compared to the effect of primary socialization. We find that if a combatant has a sibling that volunteered to be a suicide combatant their odds of volunteering to be a suicide combatant increase by over 1,000 times. This was the largest effect on mobilization compared to other theories.