Bowen family systems theory (BFST) identifies differentiation of self as a crucial characteristic that relates to one's individual and relational maturity. Bowen theorizes that an individual's level of differentiation typically remains static over time and that individuals select and pair in relationships with others who have similar levels of differentiation. This study aimed to test the hypotheses of BFST by using components of differentiation of self, emotional reactivity, and emotional cutoff, in dyadic structural equation modeling. Specifically, a longitudinal dyadic confirmatory factor analysis and structural equation longitudinal panel model were two analyses utilized to statistically test the dyadic nonindependence and stability of emotional reactivity and emotional cutoff across time. Results indicated mixed support for Bowen's assumptions regarding similarity (nonindependence) and stability. That is, small levels of nonindependence and significant stability paths across time were found. Theoretical implications and considerations for future research are discussed.