The purpose of impeachment, a primary tool of cross-examination, is to discredit the testimony of a witness in a court case. McCormick on Evidence § 33 lays out five main ways to challenge a witness’s credibility.
The first, what McCormick terms “self-contradiction,” is to introduce evidence that the witness has said something different in the past. This is what is called a prior inconsistent statement in Washington. The rules for introducing such a statement in D.C. can be found here.
The second mode of impeachment is to introduce evidence of bias on the part of the witness. Such bias – some basis for suggesting that witness might have shaded his/her testimony – could be based, for example, on emotional or pecuniary interests. The witness, for example, might be motivated to provide favorable testimony for a family member or friend.
The third mode of impeachment would be to challenge the witness’s “character”; that is, the person’s honesty, truthfulness, integrity, and morality. After all, the witness might be simply lying.
The fourth mode is to question the accuracy of the witness’s testimony based on the witness’s ability to have observed, remembered, or recounted the matters at issue. The passage of time, for example, may have clouded the witness’s ability to recall events accurately. Or the witness could be challenged on his/her ability to have seen or heard events.
The fifth and final method would be to introduce evidence that directly contradicts the witness’s testimony. A surveillance recording in an assault case, for example, might show that it was the complainant, not the defendant, who initiated the physical altercation. Alternatively, an audio recording could show that the complainant had threatened or provoked the defendant, thereby suggesting that the defendant was acting in self-defense.
One of the most popular methods for impeaching a witness is referred to as the “three C’s”: confirm, credit and confront. (The method is also referred to as “repeat, buildup, impeach”).
First you confirm that the witness has testified to a specific fact. This prevents the witness from later backtracking or claiming confusion. Second you credit the witness’ statement from a prior occasion. You might note, for example, that the prior statement was also made under oath. Finally, you confront the witness with the text of that prior statement.