Larger ants generally develop into queens, while smaller ants develop into workers. Environmental factors, such as caregiver behavior, temperature and food availability, can determine caste by influencing how large larvae grow. However, it has been unclear whether these environmental factors can influence caste independently of body size. This study finds that size and caste are developmentally tightly coupled. Genes determine the size threshold for becoming a queen, which means that genetics not only influence size—they change what a given size means for the caste development of an individual.