Keynes blamed economic booms and busts partly on raw human mood

John Maynard Keynes argued that much investment and business activity isn't driven by careful calculation of expected profit at all, but by 'animal spirits': a spontaneous, largely emotional urge to act. When confidence collapses, he wrote, investment can seize up even though the underlying numbers haven't changed, which is partly why recessions spiral beyond what rational calculation would predict. The term is now shorthand for the psychological, non-rational drivers of markets.

— John Maynard Keynes, The General Theory of Employment, Interest and Money, Chapter 12 — 1936