In the Metazoa there are two common sexual systems; dioecy and simultaneous hermaphroditism with outcrossing.  These sexual systems are often characteristic of whole phyla and classes and have been very stable over evolutionarily time and a great deal of environmental change.  Other sexual systems such as sequential hermaphroditism, gynodioecy, androdioecy, etc, occur more rarely and are not evolutionarily stable in the sense that they are characteristic of major clades such as phyla and classes.  .  In angiosperms the evolutionary paths from dioecy to simultaneous hermaphroditism are well understood and typically involve either gynodiocey or androdioecy.  This is not the case in animals. Gynodioecy is extremely rare in animals. Although androdioecy has evolved repeatedly in animals, the hermaphrodites do not outcross as males, but either self-fertilize or act as females to males, except in barnacles.  In barnacles androdioecy can be a transitional state between dioecy and outcrossing simultaneous hermaphroditism.  Here I suggest that phenotypic plasticity in gender, based on social cues, including sequential hermaphroditism and environmental sex determination (ESD) may be a common evolutionary path between outcrossing simultaneous hermaphroditism and dioecy in animals. Recent work has demonstrated that simultaneous hermaphroditism often involves changes in sex allocation as function of age, size and social environment.  These changes in sex allocation can blur the distinction between simultaneous and sequential hermaphroditism.  Similarly, with ESD, a single genotype may develop as either a male, female or hermaphrodite depending on social and/or other environmental cues.  The distinction between sequential hermaphroditism and ESD has again become very blurry.  A plausible evolutionary path between simultaneous hermaphroditism and dioecy might start with simultaneous hermaphrodites which alter sex allocation according to social conditions evolving to sequential hermaphrodites and then these sequential hermaphrodites evolving an earlier and earlier sex change in response to social cues until there is effectively ESD of separate sexes.  This could then evolve into genetically determined dioecy which is unresponsive to early social cues.