Key takeaways:

• In Halliburton Co. v. Erica P. John Fund, the Supreme Court declined to overrule the “fraud on the market” presumption of reliance in Rule 10b-5 claims, but it held that a defendant may rebut that presumption at the class certification stage by introducing evidence that the alleged misrepresentation did not affect the stock price.
• The Halliburton decision will increase defendants’ focus on class certification as a substantial obstacle for putative securities class actions. It remains to be seen whether this new basis for defendants to attack class certification will lead to a substantial increase in the frequency of defense victories at that stage.