The common interest doctrine (CID), also known as the community-of-interest doctrine, is an exception to the general rule that attorney-client privilege (ACP) is waived when privileged information is shared with a third party. The CID allows attorneys representing different clients with the same or substantially similar legal interests to agree to (and do) share privileged information without waiving the ACP.
For the CID to apply, (1) there must generally be co-parties (that is, co-plaintiffs or co-defendants—but the CID may also apply to communications between parties and nonparties, and sometimes in nonlitigation matters), (2) the co-parties must be represented by separate counsel (the CID is different from the co-client (or joint-client) privilege, which applies when multiple clients hire the same attorney to represent them on a matter of common interest), and (3) the co-parties must share a common legal interest, not merely a common commercial interest. Courts are divided on whether interests must be legally identical or somewhat less than that, such as substantially similar. And, of course, there must be an agreement among attorneys to share information.
