Curt Paul Richter (February 20, 1894 – December 21, 1988) was an American biologist, psychobiologist and geneticist who made important contributions in the field of circadian rhythms.
Due to Richter's lack of experience with biology, Castle advised that he drop the course, so he switched to psychology instead, studying under E. B. Holt and Robert Yerkes.
[1][2][3] Richter induced need states in experimental animals by depriving them of substances essential to survival, or manipulating their hormone levels.
But if rats had repeated experiences of rescue (or of being held briefly and then freed) they "do not die", and "show no signs of giving up".
As described in the paper: Support for the assumption that the sudden death phenomenon depends largely on emotional reactions to restraint or immersion comes from the observation that after elimination of the hopelessness the rats do not die.