In the Known Space stories, Niven had created a number of technological devices (GP hull, stasis field, Ringworld material) which, combined with the "Teela Brown gene", made it very difficult to construct engaging stories beyond a certain date—the combination of factors made it tricky to produce any kind of creditable threat/problem without complex contrivances. Niven demonstrated this, to his own satisfaction, with "Safe at Any Speed" (1967). He used the setting for much less short fiction after 1968 and much less for novels after two published in 1980. Late in that decade, Niven invited other authors to participate in a series of shared-universe novels, with the Man–Kzin Wars as their setting. The first volume was published in 1988.
