通过考察各国经济演变的历史可以发现,货币政策的效应可以用来作为判定一国经济是否走出危机或是否即将发生危机的判据.在其他条件不变的情况下,如果一国经济的常规调控措施(如货币政策)失效或效应严重衰减,则该国经济有发生危机的可能或仍未走出危机.本文以美国为例进行的实证研究,结果表明,非金融危机期间,特别是在经济扩张时期,美国货币政策基本有效;在次贷危机爆发的前兆期(2004~2007年),美国货币政策效应严重衰减乃至失效;从次贷危机发生至2009年6月期间,美国经济对关联储所采取的货币政策不敏感,货币政策效应起伏变化,有时还表现为严重的“反效应”.研究认为,2009年6月前后次贷危机“毒素”基本释放完毕,美国经济有走出危机的前兆,但经济体的活力并未得到较好恢复.“,”A criterion of determining whether the economy is out of or impending the crisis can be found after in - depth study of the economic evolution history of the countries around the world, especially the market economy countries, i. e. under the condition of other things being unchanged, the country has still not been out of the crisis or may be going to have one if a country economy's general regulation measure, such as monetary policy, is out of function or decays severely. Taking the United States' empirical results as an example, which proves the truth of this criterion.