The final criterion of credibility is neutrality.

Neutral witnesses are witnesses who are likely to be objective, to reach a conclusion based on the evidence without being swayed by personal prejudice. The opposite of neutrality is bias. Someone is biased if they are pre-disposed to reach a certain conclusion.

The simplest way in which bias tends to arise is when people have good or bad relationships with other people involved. Witnesses may distort the truth in order to stand up for friends, relatives or colleagues, assuming that they get on with them. Witnesses might also lie in order to get enemies into trouble.

The best witness will therefore be a neutral witness, one without prejudice, unconnected to the other people involved in a dispute.

Students often confuse vested interest and neutrality. Vested interest refers to whether the witness personally has something at stake, whether they stand to gain or lose anything depending on how an event is interpreted. Witnesses may have no vested interest in an incident whatsoever, nothing personally at stake, but still suffer from bias, e.g. if they know others who are involved.