Adaptive sex allocation, the unequal investment in the production of offspring of different sexes, has been observed across a wide range of species. Whilst equal investment in the sexes is well understood, skewed investment has proven to be harder to explain.  Although numerous hypotheses have been suggested, variously linking sex allocation decisions to environmental, behavioral, social, ecological, demographic and physiological factors, many fail to take account of the complex ways in which factors may interact. Furthermore, classic sex allocation hypotheses to date are over-simplified, often failing to account for the sophisticated allocation decisions made by species which produce multiple-offspring clutches, that exhibit high levels of parental care, or the potential constraints imposed by chromosomal sex determination. Consequently, sex allocation remains poorly understood. Here we use the combination of a 15-year field data set and a manipulative laboratory experiment to disentangle the effects of the complex interplay of environmental and maternal factors on sex allocation decisions in the spotted snow skink (Niveoscincus ocellatus), a viviparous lizard that exhibits temperature-dependent sex determination. We confirm a sex ratio response to gestation temperature and demonstrate that sex allocation decisions may result from a complex interplay of multiple factors that can reinforce or counteract each other’s effects. This highlights the need for a multi-factorial approach.