During a research visit to Israel nearly two years ago, I interviewed leaders of various ethnic conflict resolution groups and the academics who evaluate them. When asked to describe their biggest challenge, all mentioned this puzzle: why doesn't the attitude and behavioral change generated in their conflict resolution groups "stick" with some individualsi They reported that nearly all individuals who completed their programs positively changed their attitudes and behaviors towards the ethnic outgroup, yet after a few months the effect was gone. This paper presents some of the results of the project that grew around that puzzle, with the goal of generating knowledge that will be used to design ethnic conflict resolution interventions with more lasting impact. In particular, it provides initial answers to the following question: What individual-level processes motivate individual acts of interethnic aggression, and how can these processes be alteredl After presenting new data on the widespread occurrence of acts of interethnic aggression between Arab-Israelis and Jewish-Israelis within Israel's pre-1967 borders and the municipality of Jerusalem, the paper builds on theories from scholars in philosophy (Buber, 2000), social psychology (Anderson and Bushman, 2002; Bandura, 1991, 1999; Leyens et al., 2001; Struch and Schwartz, 1989; Bar-Tal, 2007), Middle Eastern Studies (Lustick, 1980; Jamal, 2008a,b), and political science (Fearon and Laitin, 1996; Varshney, 2002)--and on findings from a year of interviews in Israel--to detail a new theory of interethnic aggression. It then presents the results of a test of this theory using original data from two interethnic aggression experiments recently conducted with National Science Foundation (NSF) and Fulbright funds in Israel--a large-n survey experiment and a laboratory aggression experiment--using Arab-Israeli and Jewish-Israeli subjects. The results of these experiments, the first in political science to gather actual individual-level interethnic aggression data, suggest a different approach to individual-level ethnic conflict resolution programs than that currently practiced.