Some science fiction writers have picked up on these issues, in "cross-gender" themes—e.g., Octavia E. Butler's "Bloodchild and Other Stories". Ursula K. Le Guin's novel "The Left Hand of Darkness" contains the sentence "The king was pregnant", and explores a society in which pregnancy can be experienced by anyone, since individuals are not sexually differentiated during most of their life and can become capable of inseminating or gestating at different times. Lois McMaster Bujold's "Ethan of Athos" features an all-male society in which men use artificial wombs, but experience many of the psychological effects of pregnancy (anticipation, anxiety, etc.). In Marge Piercy's feminist utopian novel "Woman on the Edge of Time", neither men nor women get pregnant, leaving that to artificial wombs, but both sexes may lactate and nurse the infant; the specifically female experiences of pregnancy and nursing were opened to men in the cause of gender equality. Larry Niven's 1969 essay "Man of Steel, Woman of Kleenex" ends with considering Superman as a carrier for his own baby, due to the difficulties a human female might encounter carrying a superpowered fetus.