Squamate reptiles possess two general modes of sex determination: (1) genotypic sex determination (GSD), where the sex of an individual is determined by sex chromosomes, i.e. by sex-specific differences in genotype; and (2) temperature-dependent sex determination (TSD), where sex chromosomes are absent and sex is determined by nongenetic factors. After gathering information about sex-determining mechanisms for more than 400 species, we employed comparative phylogenetic analyses to reconstruct the evolution of sex determination in Squamata. Our results suggest relative uniformity in sex-determining mechanisms in the majority of the squamate lineages. Well-documented variability is found only in dragon lizards (Agamidae) and geckos (Gekkota). Polarity of the sex-determining mechanisms in outgroups identified TSD as the ancestral mode for Squamata. After extensive review of the literature, we concluded that to date there is no known well-documented transition from GSD to TSD in reptiles, although transitions in the opposite direction are plentiful and well corroborated by cytogenetic evidence. We postulate that the evolution of sex-determining mechanisms in Squamata was probably restricted to the transitions from ancestral TSD to GSD. In other words, transitions were from the absence of sex chromosomes to the emergence of sex chromosomes, which have never disappeared and constitute an evolutionary trap. This evolutionary trap hypothesis could change the understanding of phylogenetic conservatism of sex-determining systems in many large clades such as butterflies, snakes, birds, and mammals. © 2009 The Linnean Society of London, Zoological Journal of the Linnean Society, 2009, 156, 168–183.