Why are some witnesses barred from testifying even when their testimony is relevant?

In the context of evidence law, witnesses are sometimes barred from testifying or permitted to withhold information even when their testimony is relevant because the legal system prioritizes certain ethical and social interests over the absolute search for facts.123

The exclusion of relevant testimony generally arises from two distinct legal concepts:

• Incapacity: In some jurisdictions, certain individuals may be deemed legally incompetent to testify. While modern trends favor utilizing all potential witnesses to establish the truth, some systems still exclude individuals based on specific relationships to the parties involved or, in some cases, mental state.1
• Privilege: This protection allows witnesses to refuse to provide evidence or prevents them from being forced to testify. These privileges are designed to protect fundamental values, such as the right against self-incrimination, the sanctity of marriage, or the confidentiality of professional relationships, including those between attorneys and clients, clergymen and believers, and sometimes physicians or journalists.23

By protecting these interests, the law acknowledges that forcing individuals to choose between their personal or professional integrity and the truth can create conflicts that the legal system chooses to avoid.2