This article investigates the long-term effect of September 11, 2001 on the political behaviors of victims' families and neighbors. Relative to comparable individuals, family members and residential neighbors of victims have become-and have stayed-significantly more active in politics in the last 12 years, and they have become more Republican on account of the terrorist attacks. The method used to demonstrate these findings leverages the random nature of the terrorist attack to estimate a causal effect and exploits new techniques to link multiple, individual-level, governmental databases to measure behavioral change without relying on surveys or aggregate analysis.terrorism | political participation | voting | big data | matching S eptember 11, 2001's (hereafter 9/11) effect on the UnitedStates population has been a topic of intrigue across the social sciences for the last 12 years. Short-term and long-term changes stemming from the attacks may help us understand how common citizens react to the stress and threat of mass terrorism. Following 9/11, researchers found heightened levels of posttraumatic stress across the country (1), especially close to the New York attacks (2), as well as increases in individuals' trust in government (3) and conservative political attitudes (4). Heightened levels of stress and threat are linked to authoritarian political attitudes (5-7). Changes in political attitudes are consequential and measurable outcomes of the psychological effects of politicized violence.Estimating the specific and long-term effects of the attacks on the families and neighbors of victims aids our understanding of terrorism, but also sheds light on individuals who have substantial influence in the shaping of public policy. Changes in the law often come when policy equilibria are punctuated by sudden events (8). September 11th was one such event, resulting in policy shifts in domestic security and foreign relations. However, 9/11 is just one example: natural disasters, acts of gun violence, child abductions, even corporate scandals can serve as shocks that catalyze long-lasting policy change. Victims of politicized tragedies can play an important role in shaping policy change. For example, parents of children murdered in the 2012 Sandy Hook school shooting have become active in lobbying Congress to amend gun laws (for other examples, consult ref. 9). Families of 9/11 victims became similarly involved in politics following the attacks, primarily lobbying policymakers to advance anti-terrorism laws. (See, for example, the following organizations: Families of September 11th, 9/11 Families for a Safe and Strong America, Voices of September 11th, and September Eleventh Families for Peaceful Tomorrows.) Americans who lost loved ones in 9/11 are thus not only a population of interest for their connection to a major historical event, but their behavior holds lessons for understanding policy influence following system shocks.To estimate behavioral change among 9/11 victims' families and neighbors, I used a method of an...