Moties have a peculiar (absolutely unique, according to in-story knowledge) reproductive biology - They're ALL born male, grow up male, and some unspecified time after going through their equivalent of puberty, can impregnate a female. After some time (never specified in the novels) spent being male, they become female, at which point they can get pregnant and give birth. The process of giving birth stimulates certain cells in their reproductive tracts into becoming active, triggering a return to being male, lather, rinse, repeat until old age sets in and they die.
The catch: With a couple of unusual enough to be called "rare" exceptions that aren't relevant to this post, a female Motie *ABSOLUTELY MUST* become pregnant within a fairly short period of time (again, not specified by the novels, but from one case the reader gets to observe, can be safely assumed to be somewhere between "a few weeks" and "a couple of months") after becoming female, or she dies, generally quite unpleasantly, due to massive hormonal imbalances that result from not changing back to male, which, for an "unassisted" (not going to go into the assistance - too much to talk about, and like the exceptions mentioned, not relevant to this post, though it's one of the core ideas of the second novel in the set) Motie can only happen as a result of becoming pregnant and giving birth.
So they've got a cycle that goes pre-puberty infertile male -> puberty -> fertile male -> fertile female -> got pregnant in time -> gave birth -> fertile male -> fertile female, repeat indefinitely. Every time a motie shifts from male to female, she MUST get pregnant, or she WILL die pretty quick. This has continued for as long as anybody, motie or human, can determine, resulting in "The Cycles" - population grows, civilization develops, population goes beyond planetary carrying capacity, civilization collapses back to "the reinvention of the brick", repeat ad infinitum, with no amount of trying by the Moties succeeding in breaking the cycle, despite constant efforts through their entire known history.
Hence: Humans ain't Moties.
