效率市場假說認為人是理性的，所以股價將反應所有的相關資訊，即使股價偏離基本價值，隨著時間的經過，資訊的取得越來越公開訊息，投資人將藉由學習而正確解讀訊息，因此股價必定會回歸基本價值，所以價格的偏離是短期的現象，要打敗市場獲得長期超額報酬是不可能的。
但隨著研究發現越來越多效率學派理論所謂的異常現象，有些甚至是定期性的存在，也讓效率市場假說開始遭到質疑，這時行為財務學著試著將”人”這個重要因素導入研究中，以心理學為基礎，加上社會及其他科學觀點，試圖了解這些異常現象。Kahneman and Tverskey (1979)提出的展望理論(prospect theory) 說明人們在面對不確定性下的決策模型，解釋了傳統理論無法解釋的部分，另外雖然個人在決策時會做詳盡的分析，但還是會犯認知的偏誤(cognitive bias)，也就是常以經驗法則或直覺作依據，反應在投資行為上，則出現了過度反應或反應不足，或是本研究探討的再購行為，投資者喜歡再次購買之前賺錢的股票，趨避之前賠錢的股票，而這個現象也有學者以實證的方式證明這個行為的存在。本研究另外也加入投資者在面對不確定的決策環境下常見的從眾行為與投資者的風險偏好來觀察這三個變數彼此的關係，並以實驗設計法求證。
根據本實驗結果發現受測者的風險偏好程度對從眾行為與再購行為並無明顯的顯著關係，但從眾行為對再購行為會有顯著的影響。

Efficient market hypothesis holds that people are rational, so the stock price will respond to all the relevant information. Even the stock price deviates from its basic value, as time pass it will back to its fundamental price by reacting real information. Hence, it is impossible to beat the market and get long-term excess return because the price deviation is a short-term phenomenon. But as more and more anomalies which the school of efficient market called are found, EMH began to be questioned. Then the school of financial behavior try to introduce an important factor “person” to the research. Trying to understand these anomalies based on the views of psychology, sociology and science. Prospect theory proposed by Kahneman and Tverskey (1979) can explained some parts of uncertain decision model, which Traditional theory can't. People always commit cognitive bias in investment behavior such as overreaction or repurchase behavior, although they make a detail analysis when decision-making. The existence of this phenomenon are proved by some scholars in empirical way. In this research, we use empirical way to observe the mutual relationship between investors’ herding behavior and risk preference under an uncertain decision-making environment these three variables. According to the results of this study, we found observer’s risk preference have no significant impact on herd behavior and repurchase behavior, but herd behavior has a significant on repurchase behavior.

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