I was just discussing sex changes in Genicanthus sp. with my friend Lemon in Singapore, he posted this on FB, which I couldn't say any better, "Genicanthus are protogynous hermaphrodites, and are born as females with the ability to sex change later on into males. Testosterone production suppresses female characteristics and allows the fish to develop male coloration and functioning sex organs. Large males in the wild enjoy a lek style mating system with multiple females within the vicinity. In aquarium, the same sexual stimulation is not easy to replicate, and large males often have difficulty maintaining their coloration... They often get stuck in a "male-female" transition zone. Public aquariums or large tanks with numerous females will be able to maintain the males beautifully."
In short they can bounce between male/female in smaller home aquariums, particularly without the opposite sex present.