What are techniques used in law to robustly incentivize people to tell the truth?

Question

In many different situations, people will have an incentive to lie in court while the other parties cannot verify whether they are lying or not.

I'm wondering whether there are techniques that can be used to (relatively robustly) incentivize people not to lie, even though what they say cannot be verified/falsified.

For example, one technique that might be used is: - We will ask you 10 questions. We know the answer to 5 of these questions, but you don't know which of the 10. If you tell us a lie in even one of the 5 cases, you will go to jail.

That person will be incentivized to tell the truth in almost all 10 cases, since statistically, if it lied in half of them they would almost certainly go to jail. In this way, we incentivize the person to tell the truth even in the cases where we can't verify the answers.

Are there methods that are used to achieve this?

Answer 1

I assume your strategy is to bury the "real" question in a set of distractors. Apart from the incentive that all good people want the truth to come out and justice to be served, there is a penalty for lying under oath: this is the crime of perjury, which can result in a prison sentence.

The strategy of testing a witness would not work (would not be allowed). "Test questions" are not allowed, and any questions have to be relevant to the matter. Opposing counsel would object and the questions would be stricken. An attorney is not allowed to threaten a witness (the test relies on a threat). Finally, perjury is not the same as getting the wrong answer, so the attorney "knowing the answer" is irrelevant. Perjury is when a person knowingly makes a literally false statement. To be jailed, it would have to be shown that the witness knew that their testimony was false.

Questioning by police is a very different matter. Unlike in court, you cannot be compelled to answer. If you say that you want an attorney, they have to stop interrogating you. Let's assume that you are under arrest, were Mirandized, and have not requested an attorney. Police have a fair amount of liberty to say untrue things, and it is probably common for them to say something like "Things will go easier on you if you just tell the truth". So there is a fundamental distinction between police interrogations and attorney (cross)-examination. That is all I will say on that, because the question of legal limits on police interrogation is one separate question, and the effectiveness of certain strategies in getting confessions is a different also complicated question (also not clearly a legal one).