Why do some hawkish leaders become doves, and what determines whether these leaders' views affect dramatic change in a state's foreign policy? Structural and domestic political explanations of foreign policy change tend to overlook the importance of leaders in such change. Political psychologists offer important insight into how and why certain leaders are inclined to revise their beliefs. Two psychological factors in particular hold great promise for explaining leaders' foreign policy shifts: cognitive openness and cognitive complexity. Cognitively open leaders are receptive to new information and are thus more prone to changing their beliefs than cognitively closed leaders. Similarly, cognitively complex leaders recognize that distinct situations possess multiple dimensions, and so are more likely to engage in adaptive behavior than their cognitively simple counterparts. The primary case explored in this dissertation is that of Shimon Peres, who began his political career as a tough-minded hawk and, in mid-career, transformed into a leading dove. Peres is found to possess particularly high levels of cognitive openness and complexity, thus explaining why his dovish shift was more expansive and occurred sooner than did Yitzhak Rabin's dovish turn. Begin and Shamir, by contrast, are found to be more cognitively closed and simple than either Peres or Rabin, thus explaining why these hawks remained hawks despite having witnessed the same systemic-structural and domestic political events as Peres and Rabin. While cognitive structure is seen as the key causal variable for the leader's dovish turn, systemic-structural and domestic political factors serve as the permissive conditions and/or triggering events for this phenomenon. In the case of Peres, his cognitive structure also helps to explain why he was able to wield disproportionate influence on Israeli foreign policy during periods in which he was a secondary political actor. His high degree of cognitive openness enabled him to seek out domestic and international partners in an effort to build coalitions of support for his political agenda. His high degree of cognitive complexity facilitated an understanding of the intricate needs of the many political and bureaucratic actors with whom he dealt, enabling him to win over the necessary support for his agenda.