Nearly 30% of respondents to this week's poll think the behavioral biases of investors are the primary cause of investment bubbles, whereas 49% of those polled believe responsibility lies with central banks and their easy-money policies. Excessive leverage and government policies were less-favored responses (14% and 7%, respectively). Although investment bubbles are often induced by a confluence of factors, such behavioral concepts as herding and extrapolation are often at play when it comes to speculative bubbles. Similarly, moral hazards, a common unintentional consequence of government policy, alter investor behavior by lowering perceptions of risk and, in turn, driving up asset prices. An accommodative monetary policy of central banks is also often behind speculative bubbles; the real estate bubble of the last decade, fueled in large part by the ultra-low interest rate environment, is a prime example. Perhaps the 49% of poll respondents who see central bank policies as the primary cause of investment bubbles is in light of the meteoric rise in Japanese stocks in response to recent aggressive monetary easing in Japan. -- David T. Larrabee, CFA, Director, Corporate and Member Products, CFA Institute

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