Different assumptions about information availability give rise to different types of market efficiency. A market in which prices reflect all public and privately available knowledge, including past and current information, is a strong-form efficient market. In such an efficient market, even corporate officers and other insiders cannot earn above-average, risk-adjusted profits from buying and selling stock; even their detailed, exclusive information already is reflected in current stock prices. Few markets can ever pass the test of strong-form efficiency. US laws prohibit insider trading, or trading based on important, nonpublic information. These laws reflect a public perception that it is unfair for someone with access to private information to use that position for their own profit. Remember that corporate officers should try to maximize shareholder wealth. Using inside information to benefit themselves at the expense of unknowing shareholders is a violation of the trust that should exist in the principal-agent relationship.