They are the geniuses of the pre-WarUnited States selected to enter in Vault 0 and be kept in cryogenic stasis rooms to survive through the years. After the Great War, they started acting as guards, technicians and citizens, like in any other vault. But due to budget cutbacks by the Department of Defense, several important systems were not included, like the backup system for the Calculator or radiation protection for the cryogenic stasis room. This made the Calculator corrupted and instead of releasing the robots to make the Wasteland safe for humanity, the Vault 0 robots began to exterminate all life, completing the so called "pacification protocol" and killing some of the Vault 0 dwellers in the process. Those who were not killed were all inside the cryogenic stasis room.
In 2198, after the detonation of Plutonius in front the Vault 0 door, the rest of the Vault 0 dwellers who were not already killed by the Calculator were exposed to the radiation of the nuclear bomb during their cryogenic stasis. The radiation caused all of them to suffer massive brain tissue damage, giving them severe mental retardation and dementia.