Existing literature has found no evidence of market-timing ability by mutual funds using tests based on fund returns. This paper proposes alternative market-timing tests based on observed fund holdings. The holdings-based measures are shown to be more powerful than the return-based measures, and are not subject to "articial timing" bias. Applying the holdings-based tests, we nd strong evidence of mutual fund timing ability. Our findings also suggest that market-timing funds tend to have higher returns and trade more actively. Furthermore, they seem to have market-timing information beyond those common return-predictive economic variables documented in the academic studies. Finally, we quantify the potential economic value of market-timing as a contingent claim. The magnitude of the estimated values indicates that market-timing is potentially an important investment strategy deserving more academic attention.

禁反言规则是体现保险法的最大诚信原则的重要制度设计之一，但我国保险立法中禁反言规则厥如。本文首先简要回顾禁反言规则在英美法中历史沿革，然后定义了保险之禁反言规则的概念以及四项构成要件，最后从保险合同三个环节和保险人与保险代理人两个角度对保险之禁反言规则的运作和限制予以阐释，希冀对完善我国保险立法有所裨益。