The mechanisms for variation in the primary and apparent sex ratios, from both theoretical and empirical perspectives, are reviewed. A series of experiments on the sex ratios and mode of sex determination in the apple snail Pomacea canaliculata (Lamarck, 1822) show that broods have highly variable sex ratios even though the sex ratios of populations are 1:1. I suggest that the mechanism responsible for this pattern is oligogenic sex determination, i.e., sex determination by a small number of genes. Two other molluscan groups, the protandric oysters of the genus Crassostrea Sacco, 1897, and mussels of the genus Mytilus Linnaeus, 1758 also show variable sex ratios. In both cases, the number of genes responsible for the variation appears to be small.