Regardless of the type of case, motive to falsely testify of a primary witness is almost always of some relevance. The recent case of Nappi v. Yelich, from the Tenth highlights that.
The Sixth Amendment’s confrontation right, which applies equally to defendants in state prosecutions, “means more than being allowed to confront the witness physically.” Davis v. Alaska, 415 U.S. 308, 315 (1974). It includes a right of cross-examination, which provides “the principle means by which the believability of a witness and the truth of his [or her] testimony are tested.” Id. at 316; see also Pennsylvania v. Ritchie, 480 U.S. 39, 52 (1987) . . .. To be sure, a trial judge has discretion to limit or preclude inquiry into collateral, repetitive, or “unduly harassing” subjects. Davis, 415 U.S. at 316. But this discretion has limits and “the exposure of a witness’ motivation in testifying is a proper and important function of the constitutionally protected right of cross‐examination.” Id. at 316‐17.
The state court’s conclusion that cross‐examination of the state’s main witness’ motive for testifying was a collateral matter was contrary to clearly established Supreme Court precedent. See Delaware v. Van Arsdall, 475 U.S. 673, 679 (1989) (ruling that preventing cross‐examination on a subject the “jury might reasonably have found furnished the witness a motive for favoring the prosecution in his testimony” violated the defendant’s Confrontation Clause right); Brinson v. Walker, 547 F.3d 387, 392 (2d Cir. 2008) .
And for the sexual assault case:
In Olden v. Kentucky, defendants were prosecuted for, among other things, rape; the defense was consent and that the victim had a motive to lie to conceal her extramarital relationship. 488 U.S. 227, 228‐30 (1988) (per curiam). The state court prevented defendant from exposing this alleged motive to lie, but the Supreme Court concluded that this ruling “failed to accord proper weight to petitioner’s Sixth Amendment right to be confronted with the witnesses against him.” Id. at 231 (internal quotation marks omitted). Here, as in Olden, the testimony of a witness whose motive was at issue was “central, indeed crucial, to the prosecution’s case.” Id. at 233. “If the purpose of cross‐examination is to explore more than general credibility, the subject of inquiry is not collateral.” Dunbar v. Harris, 612 F.2d 690, 693 (2d Cir. 1979).