Surveys of plant breeding systems in angiosperm families have shown a significant association between monoecy and dioecy, and researchers have proposed that dioecy has tended to evolve from monoecy. We evaluated this hypothesis in the context of a phylogeny of 918 monocotyledons assembled from 19 published trees. Binary and multistate breeding system characters were mapped onto a set of composite trees, and alternative models of character change were compared using maximum likelihood. Over a range of tree topologies and optimizations, we found three to eight times as many changes from hermaphroditism to dioecy as we did from monoecy to dioecy. Also, the rate at which monoecy gave rise to dioecy was not significantly higher than the rate at which hermaphroditism gave rise to dioecy. Our analysis implies that the correlation of monoecy and dioecy in angiosperm families does not reflect a preponderance of changes from monoecy to dioecy. Instead, we postulate that the family-level correlation results from the clustering of breeding system changes in the underlying phylogeny. Our results suggest renewed attention to modeling the transition from hermaphroditism to dioecy, possibly involving transient intermediates such as gynodioecy.