A person's handwriting may be authenticated by a lay witness -- i.e., a lay witness may identify the author of handwriting -- if the witness is personally familiar with the putative author's handwriting and the witness did not acquire such familiarity for purposes of the litigation.

United States v. Platero, 72 F.3d 806, 814-816 (10th Cir. 1995) (reversing conviction for aggravated sexual assault because the trial court's exclusion of evidence of victim's past sexual behavior with another companion violated accused's constitutional rights).The sexual history or predisposition of the alleged victim is admissible if its exclusion would violate a criminal defendant's constitutional rights.

Fed. R. Evid. 406 ("Evidence of the habit of a person or of the routine practice of an organization, whether corroborated or not and regardless of the presence of eyewitnesses, is relevant to prove that the conduct of the person or organization on a particular occasion was in conformity with the habit or routine practice.").The habit of a natural person is admissible to show the person's conduct on a particular occasion.

United States v. Withorn, 204 F.3d 790, 794 (8th Cir. 2000) (upholding trial court's admission into evidence of accused's past sexual assault where it was probative as it showed accused's propensity toward a similar type of sexual behavior).Federal Rule of Evidence 413 and comparable rules in a few States create an express exception to the prohibition against circumstantial use of character, the prohibition against the use of character to show conduct. Rules such as Rule 413 provide that in a criminal trial for sexual assault evidence of the defendant's commission of another offense or offenses of sexual assault is admissible.