The Adaptive Calibration Model (ACM) is an evolutionary-developmental theory of individual differences in the functioning of the stress response system (SRS). The SRS has three main biological functions: coordinating the organism's allostatic response to challenges, encoding and filtering environmental information, and regulating physiology and behavior in fitness-relevant areas. Individual differences in stress responsivity are largely the result of conditional adaptation, the evolved ability to modify developmental trajectory to match environmental conditions. The ACM extends the theory of biological sensitivity to context (BSC) and provides an integrative framework for future research. It proposes that individual differences in stress responsivity are adaptive mechanisms, not pathological. The SRS acts as an integrative mechanism, mediating the development of alternative life history (LH) strategies. The ACM draws on life history theory, sexual selection, and developmental plasticity to model the development of stress responsivity across life stages. It identifies four prototypical responsivity patterns and discusses the emergence of sex differences. The ACM emphasizes conditional adaptation, where environmental cues guide developmental pathways. It also highlights the role of genetic and environmental factors in shaping LH strategies, and the importance of developmental switch points in coordinating physiological and behavioral changes. The ACM provides a framework for understanding how individual differences in stress responsivity are shaped by evolutionary and developmental processes.The Adaptive Calibration Model (ACM) is an evolutionary-developmental theory of individual differences in the functioning of the stress response system (SRS). The SRS has three main biological functions: coordinating the organism's allostatic response to challenges, encoding and filtering environmental information, and regulating physiology and behavior in fitness-relevant areas. Individual differences in stress responsivity are largely the result of conditional adaptation, the evolved ability to modify developmental trajectory to match environmental conditions. The ACM extends the theory of biological sensitivity to context (BSC) and provides an integrative framework for future research. It proposes that individual differences in stress responsivity are adaptive mechanisms, not pathological. The SRS acts as an integrative mechanism, mediating the development of alternative life history (LH) strategies. The ACM draws on life history theory, sexual selection, and developmental plasticity to model the development of stress responsivity across life stages. It identifies four prototypical responsivity patterns and discusses the emergence of sex differences. The ACM emphasizes conditional adaptation, where environmental cues guide developmental pathways. It also highlights the role of genetic and environmental factors in shaping LH strategies, and the importance of developmental switch points in coordinating physiological and behavioral changes. The ACM provides a framework for understanding how individual differences in stress responsivity are shaped by evolutionary and developmental processes.