Rather than attempting to prove that they themselves actually relied on misstatements or omissions by the defendant, investors bringing 10b-5 claims frequently employ a fraud-on-the-market theory, whereby they can win a presumption of reliance by showing that they reasonably relied on the integrity of an efficient market for the security they purchased.  The success of class actions in particular generally depends on a fraud-on-the-market argument, because showing actual reliance by each individual plaintiff would tend to raise so many individual factual issues as to render the class uncertifiable under Rule 23.

There is a circuit split as to the showing that a plaintiff must make to support a fraud-on-the-market presumption for the purpose of class certification.  In The Archdiocese of Milwaukee Supporting Fund, Inc. v. Halliburton, the Fifth Circuit required “plaintiffs to establish loss causation in order to trigger the fraud-on-the-market presumption . . . at the class certification stage by a preponderance of all admissible evidence.”  597 F.3d 330, 335 (5th Cir. 2010).  The Seventh Circuit rejects that rule.  Schleicher v. Wendt, 618 F.3d 679, 685–86 (7th Cir. 2010).  On Friday the Supreme Court granted cert in the Fifth Circuit case to address the issue.

More documents at SCOTUSblog.  Also see 10b-5 Daily.  Following the links in the paragraph above, other than to the cases themselves, will give you a sense of how Spindle Law handles splits.