The purpose of this thesis was to assess the relationship between group rapport and nonverbal expressivity using three data sources: self-report, observer ratings, and test data. Assessing these constructs using multiple data sources enabled the construction of multitrait-multimethod matrices. These matrices allowed for a critical evaluation of the convergent and discriminant validity of the group rapport and nonverbal expressivity constructs. Participants (N = 162) were randomly assigned to small groups of 5-7 (24 groups total) and tasked with completing a puzzle activity in collaboration with their group members. Rapport has been colloquially defined as the “clicking, chemistry, and harmony” shared between interactants. After the activity, participants rated their rapport experience. Groups were filmed while completing the activity and objective raters assessed the groups on domains derived from the rapport (Tickle-Degnen & Rosenthal, 1987) and entitativity (Campbell, 1958) literatures. Group rapport has been theorized to be relevant for successful group collaboration in many applied contexts (e.g., business, health care, and engineering), therefore the primary outcome (test) measure of group rapport was whether groups successfully completed the puzzle activity before the other groups assigned to complete the same puzzle. It was expected that nonverbal expressivity (defined as the extent to which an individual uses their face, gestures, body, and voice to transmit emotion) would be associated with group rapport because expressive individuals are easier to accurately read and respond to compared to their unexpressive counterparts. Nonverbal expressivity had a weak relationship with group rapport, indicating that nonverbal expressivity may not be as important for effective group collaboration as it is for dyadic exchanges. In addition to the self-reports, observer ratings of group rapport and entitativity based on only ten-second segments (thin slices) of group behavior were associated with whether groups won the puzzle competition. Based on these findings, a development to group rapport theory is proposed that includes entitativity as a primary component of rapport in small groups. It is recommended that future investigations empirically test this supposition in addition to evaluating the utility of short segments of behavior (thin slices) to predict applied group outcomes.