Missouri Revised Statutes Title XXXVII. Criminal Procedure § 546.260. Defendant may testify in own behalf--spouse may testify for husband or wife--spouse may testify against husband or wife, when

1. No person shall be incompetent to testify as a witness in any criminal cause or prosecution by reason of being the person on trial or examination, or by reason of being the husband or wife of the accused, but any such facts may be shown for the purpose of affecting the credibility of such witness; provided, that no person on trial or examination, nor wife or husband of such person, shall be required to testify, but any such person may testify at his or her option either on behalf of or against the defendant, and shall be liable to cross-examination, as to any matter referred to in his examination in chief, and may be contradicted and impeached as any other witness in the case; provided, that in no case shall husband or wife, when testifying under the provisions of this section, be permitted to disclose confidential communications had or made between them in the relation of such husband and wife.

2. Notwithstanding subsection 1 of this section or any other provision of law to the contrary, in any criminal prosecution under chapter 565, 566 or 568, involving an alleged victim under the age of eighteen, a spouse shall be a competent witness against a defendant spouse, and no spousal privilege as set forth in subsection 1 of this section or any other provision of law shall exist.