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It is well known that that lessening the punishment a witness will face might be a motivation to lie. Such witnesses trigger the need for the judge to consider giving a Vetrovec warning (named after the case where the principle originated).

Such a warning is needed for certain "unsavoury or disreputable" witnesses when they provide inculpatory reference. Various factors go into deciding whether such a warning is required, including whether the witness received benefits for cooperation such that there is a risk he is lying to the court.

1. Did the witness have any motive to lie or mislead the court?

2. Did the witness receive benefits for cooperation such that there is a risk he is lying to the court?

3. Did the witness have a long criminal history?

4. Did the witness have a history of lying to or manipulating the police?

5. Was the witness an accomplice with knowledge of the circumstances such that it would be easy to falsely implicate the accused?

6. Did the witness have access to disclosure or other information that may explain his evidence or how it was given?

7. Did the witness minimize his own wrongdoing?

8. Did the witness exhibit selective memory?

9. Did important evidence emerge only after the witness exhausted his memory or after prodding by police?

10. Did new information emerge for the first time at trial despite many previous statements?

11. Was the witness evasive?

12. Was the witness testimony inconsistent with external evidence, particularly objective evidence that has been accepted?

13. Did the witness provide prior inconsistent statements?

14. Was the witness internally consistent: did the witness evidence change in testifying?

15. Upon the application of common sense, is the witness evidence impossible, improbable or unlikely?

The warning should include:

(1) drawing the attention of the jury to the testimonial evidence requiring special scrutiny; (2) explaining why this evidence is subject to special scrutiny; (3) cautioning the jury that it is dangerous to convict on unconfirmed evidence of this sort, though the jury is entitled to do so if satisfied that the evidence is true; and (4) that the jury, in determining the veracity of the suspect evidence, should look for evidence from another source tending to show that the untrustworthy witness is telling the truth as to the guilt of the accused.

canada

Yes; defence has no ability to offer a deal.

You also ask:

Doesn't this give an unfair advantage to the prosecution over the defence?

It is well known that that lessening the punishment a witness will face might be a motivation to lie. Such witnesses trigger the need for the judge to consider giving a Vetrovec warning (named after the case where the principle originated).

Such a warning is needed for certain "unsavoury or disreputable" witnesses when they provide inculpatory reference. Various factors go into deciding whether such a warning is required, including whether the witness received benefits for cooperation such that there is a risk he is lying to the court.

1. Did the witness have any motive to lie or mislead the court?

2. Did the witness receive benefits for cooperation such that there is a risk he is lying to the court?

3. Did the witness have a long criminal history?

4. Did the witness have a history of lying to or manipulating the police?

5. Was the witness an accomplice with knowledge of the circumstances such that it would be easy to falsely implicate the accused?

6. Did the witness have access to disclosure or other information that may explain his evidence or how it was given?

7. Did the witness minimize his own wrongdoing?

8. Did the witness exhibit selective memory?

9. Did important evidence emerge only after the witness exhausted his memory or after prodding by police?

10. Did new information emerge for the first time at trial despite many previous statements?

11. Was the witness evasive?

12. Was the witness testimony inconsistent with external evidence, particularly objective evidence that has been accepted?

13. Did the witness provide prior inconsistent statements?

14. Was the witness internally consistent: did the witness evidence change in testifying?

15. Upon the application of common sense, is the witness evidence impossible, improbable or unlikely?

The warning should include:

(1) drawing the attention of the jury to the testimonial evidence requiring special scrutiny; (2) explaining why this evidence is subject to special scrutiny; (3) cautioning the jury that it is dangerous to convict on unconfirmed evidence of this sort, though the jury is entitled to do so if satisfied that the evidence is true; and (4) that the jury, in determining the veracity of the suspect evidence, should look for evidence from another source tending to show that the untrustworthy witness is telling the truth as to the guilt of the accused.