Massachusetts General Laws Part III. Courts, Judicial Officers and Proceedings in Civil Cases (Ch. 211-262) Ch. 231, § 94

In an action for libel, the defendant may allege and prove in mitigation of damages that the plaintiff already has brought action for or recovered damages for, or has received or has agreed to receive compensation in respect of, substantially the same libel as that for which such action was brought. In an action for libel or slander, he may introduce in evidence, in mitigation of damages and in rebuttal of evidence of actual malice, acts of the plaintiff which create a reasonable suspicion that the matters charged against him by the defendant are true.