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This study models heterogeneous, boundedly rational agents: newswatchers who cannot infer other agents’ information from prices, and momentum traders who extrapolate past price changes. Gradual information diffusion generates short-run underreaction and exploitable trend-following. Restricting momentum traders to simple univariate strategies forces long-horizon arbitrage to unwind into overreaction, unifying under- and overreaction with additional distinctive implications.",{"@graph":69,"@context":114},[70,84,105],{"@type":71,"itemListElement":72},"BreadcrumbList",[73,77,79,82],{"item":74,"name":75,"@type":76,"position":8},"https://docshare.wps.com","Home","ListItem",{"item":78,"name":9,"@type":76,"position":14},"https://docshare.wps.com/document/",{"item":80,"name":40,"@type":76,"position":81},"https://docshare.wps.com/document/research-report/",3,{"item":83,"name":65,"@type":76,"position":19},"https://docshare.wps.com/document/a-unified-theory-of-underreaction-momentum-trading-and-overreaction-in-asset-markets/639168/",{"url":83,"name":65,"@type":85,"image":86,"author":91,"headline":65,"publisher":94,"fileFormat":97,"inLanguage":63,"description":67,"dateModified":98,"datePublished":99,"encodingFormat":97,"isAccessibleForFree":100,"interactionStatistic":101},"DigitalDocument",{"url":87,"@type":88,"width":89,"height":90},"https://docshare.wps.com/thumbnails/a-unified-theory-of-underreaction-momentum-trading-and-overreaction-in-asset-markets/639168.png","ImageObject",300,407,{"name":92,"@type":93},"WPS_1790064749","Person",{"url":74,"name":95,"@type":96},"DocShare","Organization","application/pdf","2026-10-28","2026-10-22",true,{"@type":102,"interactionType":103,"userInteractionCount":81},"InteractionCounter",{"@type":104},"ViewAction",{"@type":106,"mainEntity":107},"FAQPage",[108],{"name":109,"@type":110,"acceptedAnswer":111},"What prediction pattern does the paper aim to unify?","Question",{"text":112,"@type":113},"It aims to unify continuation (momentum) in the short to medium run with reversals (fundamental reversion) in the long run.","Answer","https://schema.org",{"og:url":83,"og:type":116,"og:title":65,"og:site_name":95,"og:description":67},"article",{"robots":118,"canonical":83},"index,follow",{"doc_id":120,"site_id":62},639168,1792926054,{"code":4,"msg":5,"data":123},{"doc_id":120,"user_id":124,"nickname":92,"user_avatar":125,"doc_module":4,"category_id":39,"category_name":40,"doc_title":65,"doc_description":67,"doc_content":126,"file_id":127,"file_url":128,"file_type":129,"file_size":130,"view_count":81,"is_deleted":4,"is_public":8,"is_downloadable":8,"audit_status":8,"page_count":131,"language":132,"language_code":63,"site_id":62,"html_lang":63,"table_of_contents":133,"faqs":134,"seo_title":135,"seo_description":67,"update_tm":136,"read_time":137},3985747859154,"https://ap-avatar.wpscdn.com/davatar_276721f389ce27ea32af1340a28f341c","A Unified Theory of Underreaction, Momentum Trading, and Overreaction in Asset Markets  \nHARRISON HONG and JEREMY C. STEIN*  \nABSTRACT  \nWe model a market populated by two groups of boundedly rational agents: “newswatchers” and “momentum traders.” Each newswatcher observes some private information, but fails to extract other newswatchers’ information from prices. If information diffuses gradually across the population, prices underreact in the short run. The underreaction means that the momentum traders can profit by trendchasing. However, if they can only implement simple ~i.e., univariate! strategies, their attempts at arbitrage must inevitably lead to overreaction at long horizons.  \nIn addition to providing a unified account of under- and overreactions, the model generates several other distinctive implications.  \nOVER THE LAST SEVERAL YEARS, a large volume of empirical work has documented a variety of ways in which asset returns can be predicted based on publicly available information. Although different studies have used a host of different predictive variables, many of the results can be thought of as belonging to one of two broad categories of phenomena. On the one hand, returns appear to exhibit continuation, or momentum, in the short to medium run. On the other hand, there is also a tendency toward reversals, or fundamental reversion, in the long run.1  \nIt is becoming increasingly clear that traditional asset-pricing models—such as the capital asset pricing model ~CAPM! of Sharpe ~1964! and Lintner ~1965!, Ross’s ~1976! arbitrage pricing theory ~APT!, or Merton’s ~1973! intertemporal capital asset pricing model ~ICAPM!—have a hard time explaining the growing set of stylized facts. In the context of these models, all of the predictable patterns in asset returns, at both short and long horizons, must ultimately be traced to differences in loadings on economically meaningful risk factors. And there is little affirmative evidence at this point to suggest that this can be done.  \n* Stanford Business School and MIT Sloan School of Management and NBER. This research is supported by the National Science Foundation and the Finance Research Center at MIT. We are grateful to Denis Gromb, René Stulz, an anonymous referee, and seminar participants at MIT, Michigan, Wharton, Duke, UCLA, Berkeley, Stanford, and Illinois for helpful comments and suggestions. Thanks also to Melissa Cunniffe for help in preparing the manuscript.  \n1 We discuss this empirical work in detail and provide references in Section I below.  \n2144 The Journal of Finance  \nAs an alternative to these traditional models, many are turning to “behavioral” theories, where “behavioral” can be broadly construed as involving some departure from the classical assumptions of strict rationality and unlimited computational capacity on the part of investors. But the difficulty with this approach is that there are a potentially huge number of such departures that one might entertain, so it is hard to know where to start.  \nIn order to impose some discipline on the process, it is useful to articulate the criteria that a new theory should be expected to satisfy. There seems tobe broad agreement that to be successful, any candidate theory should, at a minimum: ~1! rest on assumptions about investor behavior that are either a priori plausible or consistent with casual observation; ~2! explain the existing evidence in a parsimonious and unified way; and ~3! make a number of further predictions that can be subject to “out-of sample” testing and that are ultimately validated. Fama ~1998! puts particular emphasis on the latter two criteria: “Following the standard scientific rule, market efficiency can only be replaced by a better model   The alternative has a daunting task. It must specify what it is about investor psychology that causes simultaneous underreaction to some types of events and overreaction to others   And the alternative must present well-defined hypotheses, themsel","cbCaieUEBhgSPTbL","https://ap.wps.com/l/cbCaieUEBhgSPTbL","pdf",309457,42,"English","# Abstract\n## Predictable return patterns: momentum and reversals\n## Limits of traditional asset-pricing models\n## Behavioral alternatives and selection criteria for new theories\n## Unified behavioral modeling via interacting heterogeneous agents\n## The two agent types: newswatchers and momentum traders","[{\"question\":\"What prediction pattern does the paper aim to unify?\",\"answer\":\"It aims to unify continuation (momentum) in the short to medium run with reversals (fundamental reversion) in the long run.\"}]","A Unified Theory of Underreaction, Momentum Trading, and Overreaction in Asset Markets | PDF",1792702116,106]