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Title
Japanese:市場が無視するもの：アクティビスト介入の二段階構造と日本における株価反応の遅延 
English:What the Market Ignores: The Two-Stage Nature of Activist Interventions and Delayed Price Reactions in Japan 
Author
Japanese: 小間井 俊輔.  
English: Shunsuke Komai.  
Language English 
Journal/Book name
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English: 
Volume, Number, Page        
Published date May 23, 2026 
Publisher
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Official URL https://papers.ssrn.com/sol3/papers.cfm?abstract_id=6818058
 
Abstract We document a systematic failure of equity markets to incorporate publicly available activist shareholder information—a finding that directly challenges both the semi-strong form of the efficient markets hypothesis (Fama, 1970) and conventional assumptions in the activist intervention literature. Using a novel database of 973 public statements (2006–2026) and 2,490 large shareholding reports (2021–2026) filed by fifteen activist funds in Japan, we show that activist campaigns unfold in two structurally distinct stages with dramatically different market reactions. Pre-threshold public statements (below 5% ownership), which precede formal filings by a median of 3.1 years, generate cumulative abnormal returns statistically indistinguishable from zero and from randomly selected trading days. Yet first-time formal filings generate average CARs of +3.13% over three days—4.9 times the reaction to continuation filings (+0.64%)—despite the underlying activist intent having been publicly declared years earlier. Three standard explanations for this pattern—rational discounting of signal credibility, investor inattention, and institutional constraints—fail empirically. We propose the concept of 'Information Dormancy': public activist signals enter a dormant state upon release, informationally inert until a formal 5% disclosure threshold acts as a societal 'certification trigger' that coordinates investor response. This framework suggests that market efficiency failures can be structural and persistent, rooted in institutional architecture rather than behavioral biases, and may generalize beyond activist interventions to other settings where information certification thresholds shape price discovery.

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