John Maynard Keynes' statement, "Once doubt begins, it spreads rapidly," refers to the idea that uncertainty or loss of confidence, especially in economic contexts, tends to increase once it is introduced. In financial markets, for example, if people start doubting the stability of a currency, a bank, or the economy as a whole, that doubt can lead to widespread panic or a cascade of adverse reactions, such as investors pulling out their money, stock prices plummeting, or a rush to sell assets.
Keynes believed that confidence and expectations play a central role in economic activity, particularly in times of instability. Once trust is shaken, it can lead to self-reinforcing cycles of fear and retreat, exacerbating economic downturns or crises. This idea is closely related to his views on "animal spirits," where psychological factors significantly influence financial decision-making. In Keynesian economics, restoring confidence is crucial for economic recovery because it can help break these negative cycles.