Vested Interest
The third criterion of credibility is vested interest.
This refers to whether a witness has something personally at stake. Vested interest can either strengthen or weaken a person’s credibility, depending on whether they have most to gain by lying or by telling the truth.
Someone with a good reputation or a position to protect is more likely to tell the truth. If they were found to be lying, then they would lose this reputation or position, and so their vested interest strengthens their credibility. We wouldn’t expect them to put themselves at risk by giving a dishonest account.
More often, though, vested interest weakens credibility. For example, someone accused of a serious crime might have a vested interest to lie to stay of jail, and so may protest their innocence even if guilty. Similarly, a mechanic saying what is wrong with a car might have a vested interest to get more work, and so may exaggerate its defects.
In each of these cases, the vested interest of the witness might make us hesitate about accepting their testimony.