In "Ringworld", it is revealed that the Puppeteer government meddled in human and Kzinti gene pools. They started a series of wars (the Man–Kzin Wars) between the warlike Kzinti and humans, and guaranteed that the Kzinti lose each time, not least by using a starseed lure to guide an Outsider ship into human space, introducing faster-than-light travel to humanity. This was a mechanism to cause rapid Kzinti evolution, since the most aggressive Kzinti would die in battle, leaving the more docile individuals to breed, eventually suppressing their racial instinct for aggression.