ABSTRACTSex‐determining mechanisms are broadly categorised as being based on either genetic or environmental factors. Vertebrate sex determination exhibits remarkable diversity but displays distinct phylogenetic patterns. While all eutherian mammals possessXYmale heterogamety and female heterogamety (ZW) is ubiquitous in birds, poikilothermic vertebrates (fish, amphibians and reptiles) exhibit multiple genetic sex‐determination (GSD) systems as well as environmental sex determination (ESD). Temperature is the factor controllingESDin reptiles and temperature‐dependent sex determination (TSD) in reptiles has become a focal point in the study of this phenomenon. Current patterns of climate change may cause detrimental skews in the population sex ratios of reptiles exhibitingTSD. Understanding the patterns of variation, both within and among populations and linking such patterns with the selection processes they are associated with, is the central challenge of research aimed at predicting the capacity of populations to adapt to novel conditions. Here we present a conceptual model that innovates by defining an individual reaction norm for sex determination as a range of incubation temperatures. By deconstructing individual reaction norms forTSDand revealing their underlying interacting elements, we offer a conceptual solution that explains how variation among individual reaction norms can be inferred from the pattern of population reaction norms. The model also links environmental variation with the different patterns ofTSDand describes the processes from which they may arise. Specific climate scenarios are singled out as eco‐evolutionary traps that may lead to demographic extinction or a transition to either male or female heterogameticGSD. We describe how the conceptual principles can be applied to interpretTSDdata and to explain the adaptive capacity ofTSDto climate change as well as its limits and the potential applications for conservation and management programs.