什麼是經濟週期

經濟活動比率（按就業、價格和生產的水平計量）週期性的波動。經濟學家長期以來一直爲繁榮時期之後爲什麼總是出現經濟危機（股市崩盤、銀行倒閉、失業等）而爭論不休。有人提出每八～十年爲一個週期，康得拉季耶夫則主張更長的週期。除了偶然的衝擊（戰爭、技術變化等等）之外，投資和消費是影響經濟活動水平的主要因素。增加投資，譬如建造工廠，除原有開支本身之外還要創造其他的收入，因爲建廠的工人們還得用掉他們的工資。相反地，如果消費需求增加了，爲了滿足需求最終還得建造新的工廠。最終經濟總會到達它容量的極致，這時候很少自由資本，沒有新的需求，因此程序又顛倒過來，結果產生緊縮。農業市場的自然波動、時尚風潮等心理因素以及貨幣供給（money supply）的變化，是投資或消費方面造成這些變化的起因。自第二次世界大戰以來，政府以貨幣政策做爲調節經濟週期的手段：在經濟呆滯時期起刺激作用以避免停滯性通貨膨脹或經濟蕭條，在經濟擴張時期起抑制作用。

business cycle

Periodic fluctuation in the rate of economic activity, as measured by levels of employment, prices, and production. Economists have long debated why periods of prosperity are eventually followed by economic crises (stock-market crashes, bankruptcies, unemployment, etc.). Some have identified recurring eight-to-ten-year cycles in market economies; longer cycles have also been proposed, notably by Nikolay Kondratev. Apart from random shocks to the economy, such as wars and technological changes, the main influences on the level of economic activity are investment and consumption. An increase in investment, as when a factory is built, leads to consumption because the workers employed to build the factory have wages to spend. Conversely, increases in consumer demand cause new factories to be built to satisfy the demand. Eventually the economy reaches its full capacity, and, with little free capital and no new demand, the process reverses itself and contraction ensues. Natural fluctuations in agricultural markets, psychological factors such as a bandwagon mentality, and changes in the money supply have all been proposed as explanations for initial changes in investment and consumption. Since World War II, government monetary policy has aimed at moderating the business cycle, preventing the extremes of inflation and depression by stimulating the economy in slack times and restraining it during expansions.