Connecting the dots on the admissibility of expert opinions on defendant’s intent

Later this month, the U.S. Supreme Court will hear argument in Diaz v. United States, No. 23-14. Diaz will likely clarify how directly—or more to the point, how obliquely—expert witnesses in criminal cases are permitted to offer opinions about a defendant’s mens rea.

The current state of the law in this area is best illustrated by the following hypothetical: A defendant (let’s call him Walter) is charged with possessing pseudoephedrine with the intent to manufacture methamphetamine after law enforcement finds all of the ingredients necessary to make meth—pseudoephedrine, red phosphorous, lye, hydriodic acid, to name just a few—in the garage of his second home in the desert. At trial, the prosecution calls a long-time narcotics detective to offer the following four expert opinions:

Opinion No. 1: Walter is guilty of the charged crime.

Opinion No. 2: Walter possessed the pseudoephedrine with the intent to make meth.

Opinion No. 3: Meth-making organizations typically cook their product at a remote location, such that those found at a remote location with the ingredients necessary to manufacture meth are usually intending to manufacture meth.

Opinion No. 4: Manufacturing meth requires several ingredients, including pseudoephedrine, red phosphorous, lye, hydriodic acid, and others.

Which of these expert opinions is permissible, and which is impermissible?

As pertinent here, a witness qualified as an expert may offer an opinion only if that opinion “will help” or “assist” “the trier of fact.” Fed. R. Evid. (FRE) 702(a); Cal. Evid. Code (CEC) § 801(a). Although the common law did not permit helpful opinions if they dealt with any “ultimate issue” in a case, the modern rules provide that “[a]n opinion is not objectionable just because it embraces an ultimate issue.” FRE 704(a); CEC § 805.

But the rule allowing for opinions on ultimate issues has two notable exceptions:

First, an expert’s opinion cannot invade the province of the jury—that is, the opinion cannot deal with a topic reserved for the jury itself, such as the “super”-ultimate issues of whether a witness is to be believed or a criminal defendant is guilty of the charged crime(s). United States v. Lockett, 919 F. 2d 585, 590 (9th Cir. 1990); People v. Prince, 40 Cal. 4th 1179, 1227 (2007); United States v. Hill, 749 F. 3d 1250, 1258-1261 (10th Cir. 2014). This is why Opinion No. 1 is impermissible.

Second, an expert in a criminal case “must not state an opinion about whether the defendant did or did not have a mental state or condition that constitutes an element of the crime charged or of a defense.” FRE 704(b); Cal. Penal Code § 29; People v. Whitler, 171 Cal. App. 3d 337, 341 (1985). This is why Opinion No. 2 is impermissible: It is a “direct” or “explicit” opinion by an expert, in a criminal case, that Walter has a specific “mental state”—namely, the specific intent to manufacture meth that is an element of the crime charged here. United States v. Gomez, 725 F. 3d 1121, 1128 (9th Cir. 2013); United States v. Gutierrez-Farias, 294 F. 3d 657, 662-63 (5th Cir. 2002).

Like Opinion No. 2, Opinion Nos. 3 and 4 are also helpful to show the defendant’s intent to manufacture meth. If most people who possess the ingredients of meth at a remote locale do so intending to manufacture it (as per Opinion No. 3), then Walter—who also possessed all those ingredients at a remote location—probably had the same intent. If the ingredients to make meth are A, B, C, and D (as per Opinion No. 4), then a person who possesses all those ingredients probably intends to combine them into meth, much as a person who gathers the ingredients for key lime pie is probably intending to make such a pie; after all, why else would he have all those particular ingredients?

Where these two opinions differ from Opinion No. 2, however, is that they do not speak directly to Walter’s intent. Instead, they merely provide a basis from which the jury can infer that mental state. In other words, Opinion No. 2 gives the jury the complete picture regarding Walter’s intent; Opinion Nos. 3 and 4 give the jury the dots relevant to prove intent but leave it to the jury to connect those inferential dots to form the picture.

Diaz addresses whether FRE 704(b)’s bar applies when the expert’s opinion does not speak directly to a defendant’s intent and thus merely helps the jury infer that intent.

In Diaz, border officials stopped the defendant at the San Ysidro Port of Entry in Southern California. When inspecting her car, the officials heard a “crunch-like sound” and felt “resistance” when they rolled down one of the rear windows. A closer inspection revealed 28 kilograms of meth secreted in the car door compartments. In a Mirandized interview, defendant denied all knowledge of the drugs, asserting she was an unwitting courier merely driving her boyfriend’s car. At defendant’s trial for knowing importation of narcotics, prosecutors called a Homeland Security officer as an expert to offer an opinion about the modus operandi of drug trafficking organizations and how, in the expert’s experience, the “majority” of people who transport drugs across the border in such large quantities know about those drugs and are not so-called “blind mules.”

The expert opinion in Diaz is akin to Opinion No. 3. Both are opinions regarding the modus operandi of drug trafficking organizations and the typical mens rea of people involved in those organizations, from which the jury is being asked to infer the defendant’s mens rea in this case.

The courts are split on whether such opinions run afoul of FRE 704(b) and CEC § 805.

On one side of the split is the Ninth Circuit, which holds that FRE 704(b) only bars expert opinions that “‘explicit[ly]’” address the defendant’s mens rea. Gomez, 725 F. 3d at 1128; United States v. Murillo, 255 F. 3d 1169, 1178-79 (9th Cir. 2001). Under this approach, Opinion No. 3 is permissible because it merely addresses the typical mens rea of persons involved in drug trafficking organizations based on their usual modus operandi rather than explicitly addressing Walter’s mens rea. And because Opinion No. 4 speaks to Walter’s mens rea even more indirectly, it too is permissible.

On the other side of the split is most every other Circuit, all of which construe FRE 704(b) also to prohibit opinions from which a jury might infer the defendant’s mens rea, albeit to varying extents. Some Circuits seem to broadly bar any expert opinions, including opinions regarding the modus operandi of organizations, as long as it is possible to infer the defendant’s mens rea from that opinion. Gutierrez-Farias, 294 F. 3d at 662-63; United States v. Lara, 23 F. 4th 459, 475-77 (5th Cir. 2022). Other Circuits more narrowly hold that an expert opinion is admissible even if it “‘supports an inference or conclusion that the defendant did or did not have the requisite mens rea,’” as long as it does not “‘draw the ultimate’” or “‘final’” “‘inference or conclusion’” and as long as that “‘ultimate inference or conclusion does not necessarily follow from the [opinion]. ’” United States v. Watson, 260 F. 3d 301, 309 (3d Cir. 2001); United States v. Draine, 26 F. 4th 1178, 1191 (10th Cir. 2022); see People v. Killebrew, 103 Cal. App. 4th 644, 651-58 (2002) (expert may not opine that any gang member in a car with other gang members knows if they are armed). And still other Circuits ostensibly take a third approach that admits expert opinions that speak “‘in general terms about facts or circumstances from which a jury might infer’” the defendant’s mens rea unless they “point[]” too “directly to [that] mental state.” United States v. Tinsley, 62 F. 4th 376, 384 (7th Cir. 2023); United States v. Mitchell, 996 F. 2d 419, 422 (D. C. Cir. 1993). Although the last two approaches would allow Opinion No. 4 because an opinion about the ingredients of meth speaks to Walter’s intent more obliquely, e.g., United States v. Cowley, 34 F. 4th 636, 639-40 (8th Cir. 2022) (allowing expert testimony that quantity of drugs possessed by defendant is consistent with distribution and not personal use); United States v. Soler-Montalvo, 44 F. 4th 1, 14 (1st Cir. 2022), it is not entirely clear whether the courts in this camp would admit or exclude Opinion No. 3.

Diaz will hopefully connect the dots to form a more definitive line separating permissible and impermissible expert opinions.