This research investigates whether and how two fundamental environmental factors--harshness and unpredictability--interact in regulating child and adolescent development, informed by life-history theory and drawing on data from the National Institute of Child Health & Human Development Study of Early Child Care and Youth Development (N = 1,364). Early life harshness was operationalized as the typical level of family income-to-needs based on six repeated measurements across the first 4.5 years of life and early life unpredictability as random variation using the same family income measurements. Results revealed that children functioned most competently in the social and academic domain as kindergarteners when exposed to low environmental harshness and low unpredictability and least competently when they experienced high harshness and low unpredictability. The same interaction pattern emerged in adolescence in forecasting cognitive-academic competence and sexual behavior. Findings are discussed in terms of how reliable and unreliable environmental cues shape developmental trajectories.