Justice Thomas continued in relevant part: Expert testimony 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). Federal Rule of Evidence 704(a) sets out a general rule that “an opinion is not objectionable just because it embraces an ultimate issue.” The provision at issue, Rule 704(b), is an exception to that general rule. Rule 704 departed from the once-prevailing common-law practice that a witness could not state their conclusions on any ultimate issue, i.e., issues that the jury must resolve to decide the case. See United States v. Spaulding, 293 U. S. 498, 506. When adopted in 1975, Rule 704 had no exceptions: All ultimate-issue opinions were permitted. Years later, Rule 704(b) was adopted to proscribe only expert opinions in a criminal case that are about whether a defendant has “a mental state or condition” that is “an element of the crime charged or of a defense.”
In this case, Agent Flood did not express an opinion about whether Diaz herself knowingly transported methamphetamine. Instead, he testified about the knowledge of most drug couriers. That opinion does not necessarily describe Diaz’s mental state. Because Agent Flood did not express an opinion about whether Diaz herself knowingly transported methamphetamine, his testimony did not violate Rule 704(b).
The Court’s analysis begs several questions. One concerns the agent’s bases of knowledge underlying his opinion that most drug couriers know they are carrying drugs. It is difficult to imagine how one could poll all drug couriers and guarantee accurate answers that could be used to reach the agent’s conclusion. While the Government could counter that this opinion is common sense, that counter weighs in favor of not admitting the opinion evidence at issue. That is because opinion evidence is only supposed to be admitted if it is necessary to assist the jury. Questions of common sense do not require opinion evidence.
