Formerly, a party never had the right to impeach, or question the credibility of, their own witnesses. The theory was that a person vouches for a witness she calls and is bound by what they say on the stand. However, after State v. Trost, a 1976 Iowa Supreme Court case, that is no longer the rule. Now the credibility of any witness may be attacked by any party, including the one who brings them to the stand.
However, the right of the Government to impeach a witness they call to the stand is not without limits. The Government can certainly impeach their own witness, including with evidence of a prior statement the witness made, which is inconsistent with what they say on the witness stand – even if that statement directly inculpates the defendant. However, the Government may NOT bring such a statement in under the guise of impeachment, when it is really trying to get in otherwise inadmissible evidence.
For example, say a person has been charged with Operating While Intoxicated. A passenger in the car later told a friend of his that the driver had “around 6-7 drinks” and was “pretty wasted” before leaving the bar to drive home. At trial, the State calls the passenger to the stand, and the passenger denies knowing or having said anything to anyone regarding the amount the driver had to drink. The State then calls the passenger’s friend to the stand and asks if the passenger said anything about how much the driver had to drink on the night in question.
When the lawyer for the driver objects, because such testimony would be extremely unfavorable for the driver, the State says the testimony about what the passenger told his friend should be allowed in for impeachment purposes – that is, to question the credibility of the passenger’s previous testimony denying he had said anything.
If the only reason the State called the passenger as a witness was to get unfavorable testimony, then impeach that testimony, that evidence must be excluded. The court must determine the real reason for such testimony – not just what the prosecutor says is the purpose for the testimony.