"We study the relationship between past returns on a company's stock and the level of investment in that stock by the participants in that company's 401(k) plan. Using data on 94,191 plan participants, we analyze several different decision points: the initial fraction of savings allocated to company stock, the changes in this fraction, and the reallocations of portfolio holdings across different asset classes. Like Benartzi (2001), we find that high past returns on company stock induce participants to allocate more of their contributions to company stock. We also find, however, that high returns on company stock have the opposite effect on reallocations of portfolio holdings, with high returns leading to shifts away from company stock and into other forms of equity. Overall, for company stock decisions, participants in our sample appear to be momentum investors when making contribution decisions and contrarian investors when making trading decisions"--National Bureau of Economic Research web site.