Sex-changing societies or species

Some stories imagine societies in which people can change gender freely back and forth at will, or with some effort or particular phases. Another model imagines gender in phases, with little or no ability to switch back and forth.

Frequent Sex Changing

• Iain M. Banks' "Culture" universe (people can change gender at will, although it is not instantaneous)
• Ursula K. Le Guin, The Left Hand of Darkness - characters are neuter until they enter kemmer; then, they shift into either male or female mode - classic gender-exploration novel; the world is peopled with hermaphrodites who phase in and out of male or female genders ("kemmer"); in a later short story, Le Guin explored the differences in sexual experiences across the genders on this planet
• Tanith Lee
• Ian McDonald, "Some Strange Desire" (in The Best of Omni III) (a gender-switching race lives among us)
• John Varley's Eight Worlds universe (people can change gender pretty easily)
• The Breeds of Man by F. M. Busby (humans mutate into people with cyclical sex changes, basically every month or two)
• The X-Files episode "GenderBender" (episode 13, Season 1) (a species of aliens can change gender; most seem to change gender not that often, but one murderous member of the group changes gender after intercourse)
• Theodore Sturgeon's Venus Plus X - everybody has chosen to change sex/gender to neuter

Life Cycle Gender

Societies in which people choose or are fixed into one gender or another, often at adolescence; or a novel in which an individual is forced to choose one or another.

• Duchamp, L. Timmel. "Welcome, Kid, to the Real World" in Tales of the Unanticipated #16 (1996) [children are neuter until late adolescence, when they must choose between male and female]
• Gardner, James. Commitment Hour [children switch back and forth between male and female until 21, when they must choose one or the other; a rare discriminated-against few choose a third sex, described as neuter or hermaphroditic]
• Gentle, Mary. Golden Witchbreed (1983) [children are gender neuter]
• David Gerrold. Moonstar Odyssey (1977) (children are neuter until adolescence)
• Gilman, Carolyn Ives. Halfway Human [children are neuter until adolescence; then they became male, female or neuter]
• McIntyre, Vonda N. "Wings" in The Alien Condition, edited by Stephen Goldin (New York: Ballantine, 1973) [children are gender-neutral and choose at puberty]

• Simoun - anime; the world of Daikuuriku features people who are all female while young, and choose gender at 17

Note: Varley's first eight-worlds novel in which anyone can change gender, but it's really not supposed to happen until you pass through adolescence.