What Is the Privilege Diversity Rule?
A plaintiff sues in federal court in Los Angeles, but the lawsuit is only there because the parties live in different states. The defendant subpoenas the plaintiff’s physician. Does the federal court apply the (nonexistent) federal physician-patient privilege, or California’s? Getting this wrong costs points on nearly every privilege-related essay.
The privilege diversity rule, defined: under FRE 501, state law governs privilege in a civil case with respect to any claim or defense for which state law supplies the rule of decision — an application of the Erie doctrine that treats privilege as substantive law, not mere procedure.
The Common Structure of Every Privilege
Before reaching the diversity question, remember that every testimonial privilege shares the same three-element skeleton:
- A confidential communication — made with a reasonable expectation of privacy.
- Within a defined protected relationship — attorney-client, physician-patient, psychotherapist-patient, spousal, clergy-penitent, counselor-victim.
- For the purpose the privilege protects — legal advice, medical treatment, spiritual counsel, and so on.
Privileges are construed narrowly because they sacrifice truth-seeking for a competing social value: the party asserting a privilege carries the burden of proving it applies, and courts read exceptions broadly. Presence of a third party — other than someone necessary to the relationship’s purpose, like a translator or a nurse — generally destroys confidentiality. An eavesdropper who overhears surreptitiously usually does not.
FRE 501: Federal Common Law, Except in Diversity
FRE 501 sets the baseline: federal common law of privilege governs in federal court — except in diversity cases, where state law controls any privilege bearing on a claim or defense for which state law supplies the rule of decision.
This is a direct application of Erie: privilege rules affect who wins, not just how the trial is run, so treating them as substantive and applying state law in diversity cases keeps state and federal outcomes consistent. In a federal-question case (say, a civil rights claim under 42 U.S.C. § 1983), federal common law of privilege governs instead, even in a California federal courtroom.
| Case Type | Governing Privilege Law |
|---|---|
| Diversity case, California-law claim, filed in federal court | California privilege law (CEC) |
| Federal-question case, filed in federal court | Federal common-law privilege |
| Any case filed in California state court | California privilege law (CEC) |
Proposition 8 Never Touches Privileges
California’s Proposition 8 is famous for relaxing hearsay rules, impeachment rules, and other evidentiary doctrines in criminal cases. It’s tempting to assume it also loosens privileges — it doesn’t. Privileges are an express exception to Prop 8. Whatever else Prop 8 changes, attorney-client, physician-patient, psychotherapist-patient, spousal, clergy-penitent, counselor-victim, and newsperson’s shield protections apply in California criminal cases exactly as they do in civil cases.
| Doctrine | Affected by Prop 8 in Criminal Cases? |
|---|---|
| Hearsay exclusions | Yes — many CEC-specific hearsay bars are abrogated |
| Character evidence limits | Yes, in several respects |
| Privileges (all types) | No — expressly preserved |
| Secondary evidence rule | Yes |
| Confrontation Clause | No effect — federal constitutional floor remains |
Worked Example: The Diversity Negligence Suit
A plaintiff sues a defendant for negligence in federal court in California, jurisdiction based solely on diversity. The defendant subpoenas the plaintiff’s treating physician.
There’s no federal physician-patient privilege as a general matter. But because this is a diversity case and California substantive law supplies the negligence rule of decision, FRE 501 directs the federal court to apply California’s physician-patient privilege (CEC § 990 et seq.). The plaintiff may assert it — subject to the patient-litigant exception, which applies only if the plaintiff has put their physical or mental condition at issue in the litigation.
Change the facts slightly: same physician, same subpoena, but the lawsuit is a federal civil rights claim rather than a diversity negligence suit. Now federal common-law privilege governs instead, and the outcome could differ.
Common Mistakes Bar Examiners Test
- Applying federal common-law privilege by default in a diversity case, instead of checking FRE 501 and routing to state law.
- Assuming Proposition 8 loosens or eliminates privileges in criminal cases — it explicitly does not.
- Failing to check confidentiality carefully, assuming any conversation with a privileged person is automatically protected even when third parties were present.
- Misidentifying the privilege-holder, which determines who can assert or waive the privilege — the wrong party asserting it loses the objection.
Frequently Asked Questions
Does federal law have its own physician-patient privilege?
No general one. When a diversity case turns on state-law claims, FRE 501 sends the court to state privilege law instead — in California, that means the CEC governs.
Does Proposition 8 loosen any privilege rules in criminal cases?
No. Prop 8 explicitly preserves all privilege rules; it changed hearsay, impeachment, and other evidentiary doctrines, but privileges apply in criminal cases exactly as in civil cases.
What destroys the confidentiality element of a privilege?
The presence of a third party not necessary to the relationship’s purpose usually destroys confidentiality. An unknown eavesdropper who overhears without the holder’s knowledge typically does not.
Key Takeaways
- Every privilege requires a confidential communication, within a protected relationship, for the purpose the privilege protects.
- FRE 501 applies federal common-law privilege generally, but routes to state privilege law in diversity cases under the Erie doctrine.
- California federal courts hearing diversity cases apply California’s CEC privilege rules, not a nonexistent federal equivalent.
- Proposition 8 never affects privileges, even though it reshapes hearsay and other evidence rules in California criminal cases.
- Always identify the privilege-holder first — the wrong party asserting the privilege loses the objection regardless of the underlying merits.
This article is educational and is not legal advice. Consult a licensed California attorney about your situation.
Related guides
- California-only evidence privileges
- California evidence essay approach
- attorney-client privilege in California