Dogs that couldn't escape a shock stopped trying to escape one they could

In 1967, Martin Seligman and Steven Maier found that dogs first exposed to inescapable shocks later failed to escape shocks they could easily have avoided — they simply lay down and endured them. Dogs without the earlier helpless experience escaped with ease. The finding launched the concept of "learned helplessness" and later reshaped how psychologists understand depression.

— Martin E. P. Seligman and Steven F. Maier, Failure to Escape Traumatic Shock — Journal of Experimental Psychology, 1967