slow, they may reduce employment levels and save money rather than invest. Such expectations can create a self-reinforcing downward cycle, bringing about or worsening a recession. Consumer confidence is one measure used to evaluate economic sentiment. The term animal spirits has been used to describe the psychological factors underlying economic activity. Keynes, in his The General Theory of Employment, Interest and Money, was the first economist to claim that such emotional mindsets significantly affect the economy. Economist Robert J. Shiller wrote that the term "refers also to the sense of trust we have in each other, our sense of fairness in economic dealings, and our sense of the extent of corruption and bad faith. When animal spirits are on ebb, consumers do not want to spend and businesses do not want to make capital expenditures or hire people." Behavioral economics has also explained many psychological biases that may trigger a recession including the availability heuristic, the money illusion, and normalcy bias. Balance sheet recession Excessive levels of indebtedness or the bursting of a real estate or financial asset price bubble can cause what is called a "balance sheet recession". This occurs when large numbers of consumers or corporations pay down debt (i.e., save) rather than spend or invest, which slows the economy. The term balance sheet derives from an accounting identity that holds that assets must always equal the sum of liabilities plus equity. If asset prices fall below the value of the debt incurred to purchase them, then the equity must be negative, meaning the consumer or corporation is insolvent. Economist Paul Krugman wrote in 2014 that "the best working hypothesis seems to be that the financial crisis was only one manifestation of a broader problem of excessive debt—that it was a so-called "balance sheet recession". In Krugman's view, such crises require debt reduction strategies combined with higher government spending to offset declines from the private sector as it pays down its debt. For example, economist Richard Koo wrote that Japan's "Great Recession" that began in 1990 was a "balance sheet recession". It was triggered by a collapse in land and stock prices, which caused Japanese firms to have negative equity, meaning their assets were worth less than their liabilities. Despite zero interest rates and expansion of the money supply to encourage borrowing, Japanese corporations in aggregate opted to pay down their debts from their own business earnings rather