The Supreme Court ruled Thursday that prosecutors can get expert witnesses to testify about the general state of mind of someone in a similar situation to a defendant, effectively giving prosecutors more leeway in pursuing criminal cases.

Federal court rules prohibit expert witnesses from stating opinions about a defendant’s mental state when it’s an element of the alleged crime. But the justices said in a 6-3 ruling that general testimony about people in a similar situation doesn’t violate that rule.

“An 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),” Justice Clarence Thomas wrote for the majority.

The case stems from an incident at the U.S.-Mexico border, where Customs and Border Protection officers arrested Delilah Diaz as she entered the country with more than 54 pounds of methamphetamine in her car doors and trunk.

Diaz said it was her boyfriend’s car and claimed she had no idea she was carrying drugs — though she admitted she only had seen him a few times, didn’t know his phone number and couldn’t say where he lived.

At trial, prosecutors wanted to put a government investigator on the stand to testify that Diaz must have known about the drugs because cartels never send drug couriers through the border unaware. Diaz objected, saying that amounted to testimony about her state of mind, which is illegal under the federal rules of evidence.

A court agreed, but said the investigator, as an expert witness, could testify that couriers “in most circumstances” know about the drugs because cartels don’t send them through unaware.

After being convicted, Diaz admitted she did in fact know about the drugs and had smuggled before.

Thursday’s ruling spawned a curious breakdown among the court members.

Justice Ketanji Brown Jackson, a Biden appointee, joined five Republican appointees in ruling for the prosecutors.

But she wrote her own opinion to say the same standard applies to the defense. They, too, can use expert witnesses to testify about the general state of mind of someone in a similar situation.

Justice Neil M. Gorsuch, a Trump appointee, led the three dissenting justices, who complained that the ruling stacks the deck against defendants in the many cases where knowledge of a crime is a key part of the burden the government faces.

“Prosecutors can now put an expert on the stand — someone who apparently has the convenient ability to read minds — and let him hold forth on what ‘most’ people like the defendant think when they commit a legally proscribed act,” Justice Gorsuch wrote.

• Stephen Dinan can be reached at sdinan@washingtontimes.com.