Who is the person who cares about and values social and cultural diversity? From a basic trait perspective, such a person might be curious—to want to learn about groups outside one’s own—and compassionate—to diversity as an opportunity for collaboration rather than competition. Despite curiosity and compassion’s clear mapping to Openness and Agreeableness within the Big Five, past research indicates that these traits are insufficient to account for the breadth of thoughts, feelings, and behaviors that arise in intergroup settings. In this dissertation we propose a novel personality trait that describes people’s appreciation, embrace of, and preference for social difference in their environment, which we call Openness to Other (O2), in order to better account for individual differences in valuing social diversity and navigating intergroup interactions.In Chapter 1 we described Openness to Other’s nomological network and demonstrate that it is independent of its parent traits, Agreeableness and Openness. We first develop and validate a brief self- report scale to measure Openness to Other, finding that participants can be ordered along it unidimensionally, respond reliably to it, and do not alter their responses to appear more socially desirable. We then demonstrate that O2 relates most strongly to Openness and Agreeableness, in both self- and peer-reports, but that it remains autonomous from each. In addition, we show that O2 was accurately perceived by knowledgeable peers at levels comparable to the other Big Five. We then examine the broad values that motivate people high and low on O2. We found that low-O2 individuals, like those low on Agreeableness or Openness, valued power and tradition. In contrast, those high on O2 valued equality and tolerance, whereas agreeable individuals valued helping and getting along with others and open individuals valued curiosity, creativity, and personal expression. In the domain of intergroup interactions, people high on O2 were described by peers as calm, seeking out such interactions, working to make sure these interactions go well, and seeking opportunities to learn about different cultures. All of these relationships were independent of Agreeableness and Openness. Finally, in the sociopolitical domain, high-O2 people held more positive views toward immigration in general and toward a central outgroup in the U.S. (Muslims) and, importantly, voted against an anti-immigrant presidential candidate (Donald J. Trump). All of these also held independently of Agreeableness and Openness. In Chapter 2 we applied O2 to an important life domain: racial homophily, or having friends and acquaintances who are the same race as oneself. We found that O2 consistently predicted cross-race friendship. Compared to Agreeableness and Openness, O2 had the stronger and only unique effect, suggesting it is the “active ingredient.” High-O2 individuals had an almost equal 1:1 ratio of same-to- different-race network members, whereas low-O2 individuals had 4:1 same-race. These results held for both college students and middle- aged adults, both friends and new acquaintances in the network, and both networks established before and at a diverse university. Finally, when moving to a more diverse environment, high-O2 individuals seemed to take advantage of the new environmental affordances, adding more different-race members to their networks. Overall, these studies advance understanding of person–environment transactions, showing how personality traits matter to the structure of people’s social networks.