Classifying a species as having either genotypic or environmental sex determination may oversimplify the processes that influence offspring sex. Using two independently gathered data sets on veiled chameleons (Chamaeleo calyptratus), we show that a hatchling's sex is affected by the interaction between egg mass and incubation temperature. In both studies, larger eggs produced daughters at lower temperatures and sons at higher temperatures; however, the relationships between incubation temperature, egg mass, and sex diverged markedly between the two studies. The shift in egg-size effects was seen between 25°C and 28°C in one study and between 28°C and 30°C in the other. The links between offspring size, sex, and incubation temperature are not an artifact of differential mortality; in one of the studies, egg survival rates were uniformly high. Other data on scincid and agamid lizards also do not support a simple dichotomy in sex-determining systems. Our results challenge simplistic classificatory schemes and call for a reanalysis of existing data sets to look for multifactorial as well as unifactorial effects on offspring sex.