We investigate Australia’s largest online stock message board, HotCopper, by focusing on the impact of sophisticated fundamentals-based posters on market efficiency. In analyzing over 2.5 million posts related to 2500 ASX-listed stocks from 2008 to 2016, we find that these posters significantly influence stock prices, particularly for small, thinly traded stocks with weak fundamentals. Their predictions yield an annualized abnormal return of 15.29%, with posting activity positively correlated to stock returns. Our findings highlight the critical role of informed posters in disseminating value-relevant information on HotCopper and, in general, their contribution to market efficiency.
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