Diaz v. United States, 602 U.S. 526

Delilah Diaz was stopped at the U.S.-Mexico border, where border officers found over 54 pounds of methamphetamine concealed in her car.
Diaz was charged with importing methamphetamine, a crime requiring proof that she "knowingly" transported the drugs.
Diaz asserted a "blind mule" defense, claiming she did not know drugs were hidden in her vehicle.
The government called Agent Flood as an expert to testify that most drug couriers know they are transporting drugs; Diaz objected, citing Federal Rule of Evidence 704(b).
The district court allowed Agent Flood to testify that "most" couriers know, but not that all do; Diaz was convicted and appealed.
The Ninth Circuit upheld Diaz’s conviction, and the Supreme Court granted certiorari on the Rule 704(b) issue.

Issue	Plaintiff's Argument	Defendant's Argument	Held
Whether expert testimony that "most couriers know"	Flood’s testimony is functionally about Diaz’s mental state	Testimony concerns a group, not Diaz specifically	Testimony about “most” is not about Diaz specifically; not a Rule 704(b) violation
violates Federal Rule of Evidence 704(b)	and therefore violates Rule 704(b).	and leaves decision to jury.

United States v. Spaulding, 293 U.S. 498 (1935) (discussing the common law ultimate-issue rule that expert witnesses could not state conclusions on issues the jury must decide)
Miranda v. Arizona, 384 U.S. 436 (1966) (providing context for Diaz’s post-arrest statements)
Morissette v. United States, 342 U.S. 246 (1952) (discussing the importance of mens rea in criminal law)
Francis v. Franklin, 471 U.S. 307 (1985) (holding burden of proof for state of mind must remain with the government)
Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993) (establishing standards for admissibility of expert testimony)
Kumho Tire Co. v. Carmichael, 526 U.S. 137 (1999) (reaffirming trial court's gatekeeping role for expert testimony)