For the Defense - Vol. 9, Issue 3 - 6

" Most People " and Mental State: How
Diaz v. United States Allows the Prosecution
(and the Defense) to Rely on Probabilistic
Expert Testimony to Prove Mens Rea
Matthew L. Hoke
and Sarah Hyser-Staub
U
nder Federal Rule of Evidence 704(b), are an expert's
opinions about the mental state of most of a class of
people admissible to prove the mens rea of a particular
Defendant? According to the United States Supreme Court in
Diaz v. United States, 144 S. Ct. 1727 (2024), the answer is yes.
Diaz permits expert testimony about the state of mind of
" most people. "
In the summer of 2020, Delilah Diaz, an American citizen,
attempted to enter the United States from Mexico.1
Customs
and Border Patrol discovered that Ms. Diaz had " 56 packages
of methamphetamine tucked inside [her] car's door panels and
underneath the carpet in the trunk. " 2
In total, there was " just
over 54 pounds " of meth, with an " estimated retail value of
$386,550. " 3
Ultimately, Ms. Diaz " was charged with importing
methamphetamine in violation of 21 U.S.C. §§ 952 and 960. " 4
Ms. Diaz asserted " a 'blind
To
secure a conviction, the Government had to prove that Ms. Diaz
" 'knowingly' transported drugs. " 5
mule' defense " - " that she did not know that there were drugs
in the car. " 6
Before Ms. Diaz's trial, " the Government gave notice that
it would call Homeland Security Investigations Special Agent
Andrew Flood as an expert witness. " 7
Agent Flood " planned
to explain that drug traffickers 'generally do not entrust
large quantities of drugs to people who are unaware they are
transporting them.' " 8
Ms. Diaz objected that Agent Flood's
proffered testimony would violate Federal Rule of Evidence
704(b), which prohibits expert opinions about " whether [a]
defendant . . . ha[d] a mental state . . . that constitutes an
element of the crime charged. " 9
The district court allowed Agent
Agent Flood so
On appeal, she
Flood to testify as long as he testified that " most couriers, " not
all, " know [that] they are transporting drugs. " 10
testified at trial, and Ms. Diaz was convicted.11
6 For The Defense l Vol. 9, Issue 3
challenged Agent Flood's testimony under Fed. R. Evid. 704(b),
but the Ninth Circuit affirmed.12
The U.S. Supreme Court granted
certiorari.
Justice Thomas, writing for a six-justice majority, began with
the text of the rule. Fed. R. Evid. 704(a) generally provides that a
witness's " opinion is not objectionable just because it embraces
an ultimate issue. " But subsection (b) provides an exception:
In a criminal case, an expert witness 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. Those
matters are for the trier of fact alone.13
The majority observed that " [b]y its terms, Rule 704(b)'s
exception covers a narrow set of options " - " only expert
opinions in a criminal case that are about a particular person
('the defendant') and a particular ultimate issue (whether the
defendant has 'a mental state or condition' that is 'an element
of the crime charged or of a defense'). " 14
Because " Rule 704(b) applies only to opinions about the
defendant, " Agent Flood's testimony did not violate the rule
because he did not express an opinion about whether Ms. Diaz
" herself knowingly transported " meth.15
about the knowledge of most drug couriers. " 16
Rather, he " testified
Ultimately, the
majority held that " [a]n expert's conclusion that 'most people'
in a group have a particular mental state is not an opinion
about 'the defendant' and thus does not violate Rule 704(b). " 17
Justice Jackson, while joining the Court's opinion in full, wrote
a separate concurrence. She emphasized that, " as Congress
designed it, Rule 704(b) is party agnostic. " 18
In other words,
Rule 704(b) allows the prosecution and the defense " to elicit
expert testimony 'on the likelihood' that the defendant had a
For the Defense - Vol. 9, Issue 3

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