Basic rests on a specific causal account: professional investors rapidly assess public information, trade on it, and cause market prices to reflect it. A false public statement can therefore distort the price paid by investors who never personally encountered the statement. Hofman’s alleged tips, by contrast, were intentionally nonpublic and were said only to a few customers; they did not reach the professional investors whose trading ordinarily incorporates information into price.
The strongest version of market efficiency—that even nonpublic information automatically affects price—is empirically untenable. Public announcements regularly move stock prices, which would not occur if prices already reflected undisclosed information. Although market participants sometimes infer information from the identity or volume of trades, nothing indicated that Hofman’s customers’ identities or trading volume conveyed information about an impending acquisition to the market.
The district court could not avoid the causation inquiry merely because each side retained a reputable economist. At class certification, a judge must resolve factual and economic disputes relevant to Rule 23 rather than delegate the question to a future trial. If necessary, the court must hold an evidentiary hearing and choose between competing expert views.
The plaintiffs’ expert, Michael Barclay, relied on a demand-pull theory: Hofman’s tips supposedly increased demand and reduced supply, thereby raising Jefferson’s stock price. But securities investors seek risk-adjusted returns available through many substitute investments; in an efficient securities market, raw demand from uninformed purchasers does not ordinarily raise a particular stock’s price. Price changes instead generally reflect information. Barclay did not adequately reconcile his demand theory with the premise that Jefferson traded in an efficient market.
Barclay also failed to rule out other explanations for Jefferson’s price increase. Jefferson rose relative to a broad group of financial institutions, but not necessarily relative to comparable Midwestern financial intermediaries. Acquisitions involving similar Missouri banks and thrifts during the relevant period could themselves have increased the market’s assessment that Jefferson might be acquired. Without testing for these alternative causes, the plaintiffs could not reliably attribute the price movement to Hofman’s alleged tips.
Finally, the claimed prolonged effect of Hofman’s unsupported statements was inconsistent with the asserted market efficiency. If sophisticated investors closely followed Jefferson, an unexplained price rise would prompt investigation or short selling, and the failure of an imminent acquisition to occur would quickly expose the claim as false. The absence of such correction suggested either that the market was not efficient enough for Basic or that factors other than Hofman’s statements drove the stock price.