Expert Testimony Ruling Expands Evidence Options for Both Sides

The US Supreme Court’s June 20 decision in the drug courier case, Diaz v. United States, effectively nullified a rule prohibiting expert evidence about a defendant’s state of mind, which could unduly sway jurors to find an accused guilty.

The Diaz case involved the prosecution of a drug courier who said she didn’t know she was transporting drugs. Delilah Diaz claimed her boyfriend planted them in the doors of the car she was driving.

A police officer testified against her, as an expert investigator with decades of experience in drug cases. He testified that most couriers know what they are carrying, in part because drug cartels for many reasons don’t want to entrust valuable drugs to ignorant couriers.

Federal Rule of Evidence 704(b) forbids expert testimony “about whether a defendant did or did not have a mental state” (here, knowledge) that is an element of the crime. Congress enacted that rule in reaction to John Hinckley’s acquittal on grounds of insanity for attempting to assassinate President Ronald Reagan and severely wounding Reagan’s press secretary. Congress was concerned that the jury was overly persuaded by an expert’s testimony as to Hinckley’s state of mind at the time of the crime.

Diaz contended in the Supreme Court that this police expert testimony against her amounted to the testimony forbidden by Rule 704(b). The prosecution disagreed. The testimony was about the state of mind of drug couriers generally, not directly about defendant’s state of mind in this case, and the testimony didn’t say all couriers know, just most, the prosecution argued. Thus, the prosecution said, it wasn’t, as the rule requires, “about the defendant” nor did it say, as the rule also requires, that she “did have” the mental state required, knowledge.

The Supreme Court—in an opinion by Justice Clarence Thomas—agreed with the prosecution. If the police expert had addressed defendant’s knowledge, rather than the class of couriers generally, or had said all couriers know, then the Supreme Court said 704(b) would probably have been violated.

The Supreme Court didn’t decide whether an opinion saying “defendant herself probably knew” would be banned. Such a statement would be directly about the defendant herself, but would still leave open whether she did know, since only the word “probably” is used. What if the testimony said “99% of couriers know?” Whether that would be admissible was left undecided.

The high court will probably eventually have to decide at some point if the phraseologies “defendant probably knew” and “99% of couriers know” are permitted or are too close to the forbidden form. There will be other phrases that will arise that will need to be tested. The history of these prosecutions makes it inevitable, because drug courier cases and variations of this kind of testimony are extremely frequent.

The federal circuits have been split in their approach, some condemning on a broad basis any expression by an expert witness that they deem to be the “functional equivalent” of an opinion forbidden by the rule, while others have prohibited only opinions that “defendant had the knowledge.”

Justice Neil Gorsuch dissented in Diaz, siding with the defendant, joined by Justices Sonia Sotomayor and Elena Kagan.

Gorsuch’s main concern seems to be that it is doubtful any police “expert” could be an expert about the inside of someone’s mind. He is doubly suspicious because a police “expert” has a substantial bias, and the so-called experience underlying their opinion is in the nature of self-fulfilling prophecy.

But this very genuine and enormous concern is better addressed under another applicable evidence rule not addressed by the Supreme Court: Federal Rule of Evidence 702, which requires helpfulness, qualifications, and support for expert testimony. Gorsuch does say—having been defeated in this case—that he hopes in the future that other rules like Rule 702 (as interpreted by Daubert and Kumho) and Federal Rule of Evidence 403 (allowing exclusion for misleadingness and prejudice) will provide guardrails.

Justice Ketanji Brown Jackson concurred with the majority opinion. She cites extensively the amicus brief filed by law professors including this author, expressing concerns if the Supreme Court had gone the other way. That would have imperiled a whole range of other potentially valuable expert evidence where characteristics of a class help determine the mental state of a defendant.

Some of this evidence favors defendants, such as battered woman syndrome in a self-defense or duress case, or psychiatric testimony about what states of mind people with schizophrenia or other mental disease ordinarily have, in insanity defense cases.

The latter was what Congress was trying to encourage with Rule 704(b) rather than direct opinions on a defendant’s legal insanity. Even expert testimony that certain drug couriers ordinarily don’t know of drugs’ presence would be prohibited. In Diaz, the defense itself offered expert testimony that hidden drugs in cars aren’t ordinarily detectable. That also would have been banned.

The kind of testimony the prosecution offered in Diaz comes up in legions of drug courier cases where various circumlocutions have been employed to avoid Rule 704(b). Some have been successful, some not. The circuit courts have had varying degrees of tolerance.

The Supreme Court has now decreed that most of these tactics will avoid 704(b) proscription, and prosecution of drug couriers will consequently be easier. But, as indicated above, some might still be banned and the high court may yet challenge such testimony under other rules.

But by and large, Rule 704(b) has been reduced almost to a nullity because cleverly advised experts will find it easy to fashion phraseology to get around it.

The case is Diaz v. United States, US, No. 23-14, decided 6/20/24.

This article does not necessarily reflect the opinion of Bloomberg Industry Group, Inc., the publisher of Bloomberg Law and Bloomberg Tax, or its owners.

Author Information

Paul F. Rothstein is professor at Georgetown Law and is author of the books “Evidence: Cases, Materials and Problems” and “Evidence in a Nutshell: Federal Rules of Evidence.” He filed an amici curiae along with other evidence professors supporting the US.

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