This dissertation explores how social closure produces intergroup inequality in the context of markets, families, and personal networks. Understood as exclusionary action, closure encompasses all forms of preferential or discriminatory interactions and transactions among groups or categorically bounded individuals that accrue or secure benefits to one group or category by means of excluding others, both intentionally and unintentionally. The study investigates closure in different contexts using agent-based simulation (ABM) and exponential random graph models (ERGM). First, closure in labor markets is studied as practiced by professional groups in markets. Second, closure is further explored when carried out by parents who follow different strategies to allocate resources among siblings, thereby producing skill inequality both within and across generations. And third, exclusionary action is analyzed in processes of friendship formation that lead to the segregation of personal networks into clusters of individuals sharing either positive or negative attributes.