美國的內線交易犯罪是各聯邦法院透過個案建立在證券詐欺下的概念。聯邦最高法院在一九八〇年代，恐內線處罰範圍過大，乃提出信賴關係理論以界定內部人範圍。 然由於信賴關係理論過於狹隘，聯邦最高法院續於一九九七年做出United States v. O’Hagan案判決，提出私取理論，擴大內線交易行為人的定義。由於聯邦最高法院並未推翻先前內部人必須建立於信賴關係的理論，又主張「未向消息來源揭露，而擅用內部訊息進行證券交易」的私取理論，使得日後聯邦證管會以及各級法院必須界定：何等人員對消息來源者負有揭露交易計畫的義務，否則無從界定內部人範圍。 在O’Hagan案後，聯邦證管會以及各巡迴上訴法院提出各種見解，試圖匡定內線交易罪的適用。本文即以O’Hagan案為中心，討論O’Hagan案前後，各聯邦法院如何界定內部人、並論述各種見解所面臨的困難與挑戰。

The law against trading in security markets with the benefit of material, nonpublic information has been developed case-by-case un-der the Article 10b of the Securities Exchange Act of 1934. The Act defines insider trading as security fraud. However, who is “eligible” to be a fraudster/insider in insider trading has been a controversial issue. The U.S. Supreme Court in Chiarella applied a fiduciary theory to de-fine insiders. Then in 1997, in O’Hagan, the Court applied misap-propriation theory, holding that if an individual breaches a duty within a pre-existing relationship of trust or confidence with the source of the inside information, he is to be considered an insider. The Court, on the one hand, adopted a broader definition of insiders; on the other hand, it emphasized that O’Hagan did not overrule the aforementioned fiduciary theory in Chiarella. Therefore, after O’Hagan, the SEC and lower courts tried to clarify the matter by as-serting that in situations in which a "pre-existing relationship" exists, a fiduciary relationship is established between a tipper and a tipee. However, that issue is, as yet, unsettled despite a decade of argument. This article focuses on the tension between these two different theories. By analyzing recent cases in the lower courts, this article seeks to determine how the theories have been applied and inter-preted after O’Hagan, and how the courts and SEC have responded to the challenges posed by security fraud.