Making the promise of confidentiality contingent upon a trial judge’s later evaluation of the relative importance of the patient’s interest in privacy and the evidentiary need for disclosure would eviscerate the effectiveness of the privilege. As we explained in Upjohn, if the purpose of the privilege is to be served, the participants in the confidential conversation “must be able to predict with some degree of certainty whether particular discussions will be protected. An uncertain privilege, or one which purports to be certain but results in widely varying applications by the courts, is little better than no privilege at all.”

[W]e reject respondents’ contentions that anybody who requests damages for pain and suffering has waived the psychiatric privilege because the psychiatric records might conceivably disprove the experiencing of the pain and suffering, that any claim of even . . . “garden variety” injury waives the psychotherapist-patient privilege, and that a plaintiff’s mental health is placed in issue whenever the plaintiff’s claim for unspecified damages may include[] some sort of mental injury.