Unpredictability, an aspect of early life adversity, creates significant disruptions for children and adolescents’ psychosocial development throughout their lifetime. Despite growing evidence highlighting the importance of predictable caregiving in fostering long-term healthy development, there is still limited understanding of proximal unpredictability within caregiver-child relationships. Given that both early childhood and adolescence are dynamic periods characterized by increasing self-regulation capacities and susceptibility to mental health disorders, this dissertation aimed to address three critical gaps in understanding caregiver unpredictability and its implications for child and youth adjustment: whether it is domain general or specific, the cascade between distal and proximal experiences of unpredictability and the developmental systems most influenced by caregiver unpredictability during childhood and adolescence.Study 1 examined the relations among family instability, maternal and paternal mood unpredictability, and youths' mental health problems in a sample of Mexican-origin families. Using random intercept cross-lagged models to disentangle between- and within-family differences, results showed that family instability was positively related to maternal and paternal mood unpredictability, and that maternal mood unpredictability was positively associated with youths' internalizing problems across adolescence. However, there was little evidence of reciprocal relations, suggesting a lack of transactional associations at the within-family level over time. Study 2 was designed to extend prior research on caregivers’ sensory signal unpredictability with infants, by (1) developing and validating an observational measure of caregiver affective and behavioral unpredictability in early childhood, and (2) exploring its implications for preschoolers’ development of biobehavioral self-regulation. Results demonstrated that caregiver unpredictability was related to only a subset of indices of biobehavioral regulation. At age four, caregiver affective and behavioral unpredictability was linked to greater respiratory sinus arrhythmia (RSA) suppression (i.e., decrease in RSA) during the Day/Night task, and to greater effortful control in a non-linear, inverted U-shape pattern. At age six, children whose caregivers exhibited greater sensory signal unpredictability had stronger RSA suppression during the Day/Night task. Conversely, children whose caregivers demonstrated lower and higher levels of affective and behavioral unpredictability had RSA augmentation relative to children whose caregivers were moderately unpredictable. However, neither sensory signals nor affective or behavioral unpredictability was associated with children's inhibitory control or behavioral adjustment. Together, these studies emphasize the importance of developing and validating methods to capture the dynamic and nuanced nature of caregiver unpredictability, as well as considering its domain specificity. This dissertation also raises questions about the appropriate temporal lens for examining caregiver unpredictability, the significance of developmental timing, and the impact of unique experiences and vulnerabilities among Mexican-origin families living in unpredictable environments. In conclusion, this research program provides valuable insights into the effects of unpredictability on early childhood development and adolescent mental health. Systematic empirical and translational progress in understanding unpredictability will enhance efforts to ensure continuity and stability in children's lives, particularly for families experiencing other environmental adversities that threaten children’s and adolescents’ well-being.

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