PROJECT SUMMARY/ABSTRACT Theory of mind (ToM) is the ability to perceive and understand mental states in others a nd represents one operationalization of the RDoC construct of Perception and Understanding of Others. ToM is an early emerging mechanism in the development of borderline personality disorder (BPD); individuals with BPD tend to misattribute mental states to others, spurring a cycle of anger dysregulation, interpersonal dysfunction, and suicidal behavior. Aligned with the 2020 NIMH strategic plan, to identify malleable targets for prevention of BPD, it is necessary to study the unfolding of this critical mechanism prior to the onset of symptomatology. The current proposal aims to do so in a high-risk mother-child sample in which mothers have BPD and offspring are 36-48 months old, a critical time for ToM development. Further, this study will elucidate the specific behaviors that mothers with BPD engage in that influence the development of ToM during the preschool period as well as child behaviors involved in this process. ToM undergoes rapid development during the preschool age and is highly susceptible to environmental influence, particularly interactions with caregivers. This proposal details a comprehensive training plan that will enable me to address these questions while building on previous experiences in important ways. First, expanding on my experience studying ToM and related processes in adolescence, I will focus on the developmental trajectory of this construct during the preschool age. Second, I will obtain training in measurement and interpretation of heart rate and RSA as physiological markers of preschooler social engagement with mothers. Third, building on my knowledge of global behavioral coding of mother-adolescent interactions, I will master the use of micro-analytic coding to obtain continuous time-series data of mother and child behavior within interactions. Further, I will learn novel analytic methods for this unique data based on dynamic systems modeling. The specific aims of this study are to 1) Determine whether two forms of maladaptive maternal behaviors--hostility and inaccurate mental state attribution--are observed more often among mothers with BPD compared to those with no psychiatric disorder; 2) Evaluate how preschooler social engagement with mothers (i.e., approach and withdrawal behaviors as well as heart rate and RSA) is influenced by these maternal behaviors; and 3) Test the effect of identified maternal behaviors on the trajectory of ToM development over the course of one year in preschoolers. The central hypothesis of this proposal is that maternal BPD is associated with maladaptive behaviors directed toward their children. These behaviors lead to a pattern of dysfunctional social engagement by preschoolers (decreased approach and increased withdrawal behavior toward mother during as well as heart rate deceleration and RSA withdrawal), which further inhibits their ToM development during this critica...