The paper introduces the Adaptive Calibration Model (ACM), an evolutionary-developmental theory that explains individual differences in stress responsivity. The stress response system (SRS) has three main biological functions: coordinating the organism's allostatic response, encoding and filtering environmental information, and regulating physiology and behavior. The ACM posits that individual differences in stress responsivity are primarily due to conditional adaptation, where the SRS adjusts its calibration based on developmental experiences and environmental cues. This adaptation is guided by life history theory, sexual selection, and parental investment theory, and it results in adaptive patterns of responsivity and behavior. The model describes four prototypical patterns of stress responsivity and discusses the emergence of sex differences. The ACM extends the theory of biological sensitivity to context (BSC) and provides a framework for understanding individual differences in stress responsivity across life stages.The paper introduces the Adaptive Calibration Model (ACM), an evolutionary-developmental theory that explains individual differences in stress responsivity. The stress response system (SRS) has three main biological functions: coordinating the organism's allostatic response, encoding and filtering environmental information, and regulating physiology and behavior. The ACM posits that individual differences in stress responsivity are primarily due to conditional adaptation, where the SRS adjusts its calibration based on developmental experiences and environmental cues. This adaptation is guided by life history theory, sexual selection, and parental investment theory, and it results in adaptive patterns of responsivity and behavior. The model describes four prototypical patterns of stress responsivity and discusses the emergence of sex differences. The ACM extends the theory of biological sensitivity to context (BSC) and provides a framework for understanding individual differences in stress responsivity across life stages.