Like two rival siblings, the disciplines of personality and social psychology have common roots but an evolving and sometimes difficult relationship (Pettigrew and Cherry, 2012; Lanning, 2017). Both are diverse and have their own internal controversies, but, historically, the field has been divided according to two worldviews (Cloninger, 2020). Personality trait researchers favor a natural sciences approach, characterized by a search for general, nomothetic principles for understanding relationships between quantitative individual-difference variables (Boyle et al., 2008). They are sympathetic to biological explanations for trait variation, expressed in studies of evolutionary bases, behavior and molecular genetics, and neuroscience. By contrast, social constructivists are attuned to qualitative, idiographic studies of the ways in which people interact within a specific sociocultural milieu, with personality negotiated dynamically "between" as much as "within" people (Hampson, 1988). They also favor a humanistic over a natural-sciences orientation, which values efforts by psychologists to support individual flourishing and social justice (Cloninger, 2020). Neither worldview is monolithic. For example, on the trait side, variation in traits associated with the self has been attributed to motivational and cognitive factors rather than direct neurological influences (Ryan and Deci, 2017). Experimental social psychology lends itself to nomothetic theories, such as those focused on social cognition. Nevertheless, the tension between natural-science and humanistic perspectives (Cloninger, 2020) threatens the unity and integrity of the field. How can different networks of researchers get along with one another? There have been periods of direct competition: famously, Mischel's (1968) "situationist" critique of personality traits. Here, the clash of ideas was productive in leading to high-quality studies of cross-situational consistency that supported an interactionist model of personality (Funder, 2006). However, competition also risks generating empty rhetoric without scientific progress. In fact, much of the time, trait researchers and social psychologists co-exist but largely ignore one another's work, potentially missing opportunities for exchange of ideas. Happily, recent years have seen increased cooperation and theories that integrate the two fields. Often, such efforts require unpacking of the logically distinct elements of each perspective. For example, social-cognitive theory points toward psychological attributes, such as the self-schema that can be stable over time without being a direct expression of brain functioning. The grand challenge for personality and social psychology is to define the research problems for which competition, coexistence , and cooperation are appropriate. In this article, I emphasize prospects for cooperation: personality and social psychologists have much to learn from one another. However, while debate should always be collegial, cooperation may not be the answer for some topics. F...