In a similar vein, some economists believe that persistently low bond yields are explained by a worldwide shortage of safe assets.

与此类似，一些经济学家认为持续的低收益率水平的存在可以解释为世界范围内的安全资产的短缺。

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While most believe an extension is likely, particularly given weakening economic growth and persistently high unemployment, nothing is certain.

FORBES: Second Quarter, 2012 Market Review & Outlook