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I have another question inspired from watching Law and Order.

Often, a defense attorney will give notice of a defense, and both the defense lawyer and prosecutor will be in the Judges chambers making their arguments, with the judge deciding if they allow the defense or not.

Why does a judge have this power? What is the basis in law?

Why is a defense attorney not free to use whatever defense they feel best serves their clients?

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A judge has the authority to determine what law applies to a case and to instruct the jury accordingly, and also has the authority to determine which evidence is admissible.

Presentation of a defense, in practice, involves presentation of evidence supporting a legal theory.

In order to be admissible in evidence in an evidentiary hearing or trial, the evidence must be relevant to a legal theory that is in some way connected to the evidence. If no reasonable juror could make a ruling establishing that a legally recognized defense was established based upon the proposed evidence (especially if the proposed evidence is prejudicial to the prosecution case on the basis of reasoning that is not a legally valid defense) it can be excluded.

For example, evidence in support of the theory that the defendant murdered the victim because the murder victim raped the defendant's sister six years ago, might very well sway a jury to acquit the defendant. So a defense attorney might want to make this argument. But, this is not a legally recognized justification for murder, so evidence in support of this defense would be excluded as irrelevant by the judge.

In federal court, and in states with rules of evidence based upon the federal rules of evidence, the primary legal authority behind this is Rule of Evidence 402:

Rule 402. General Admissibility of Relevant Evidence

Relevant evidence is admissible unless any of the following provides otherwise:

the United States Constitution; a federal statute; these rules; or other rules prescribed by the Supreme Court.

Irrelevant evidence is not admissible.

Some states also have procedural notice requirements for certain kinds of affirmative defenses.

For example, if someone is arguing an alibi, a notice of an intent to present this defense must be provided by the defense a certain number of days before trial, so that the prosecution can develop the very different in kind type of evidence needed to rebut that defense, rather than having someone acquitted due to surprise when rebuttal evidence exists but the prosecution doesn't know in advance to locate the necessary witnesses and evidence to rebut this kind of defense.

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    Ah, very clear answer, thank you! So essentially it comes down to allowing or disallowing the evidence, since the defense will stem from that?
    – Jake Rankin
    May 17, 2020 at 16:31
  • Is there (I honestly don't know) also some notion that it's the judge's responsibility to oversee that the lawyer actually is representing their client's best interests? As in, it's going to save the whole legal system a bunch of trouble in a later "ineffective assistance of counsel" proceeding if the judge, here, can serve as a second set of eyes to ensure that someone's not just mailing it in? Or is that not part of a judge's portfolio?
    – nitsua60
    May 18, 2020 at 0:15
  • In the first case, that would be an unwise judge, mainly because the defendant cannot be prevented from testifying. If the jury gets one whiff there was evidence the defense wanted to present but couldn't, it's over.
    – Joshua
    May 18, 2020 at 2:30
  • "defendant murdered the victim because the murder victim raped the defendant's sister" - on L&O in chambers, they're usually arguing about whether someone's testimony is relevant as evidence, not the whole 'defense' (unless it is, and it always is because: drama). And then if 'oops', "the jury will disregard" and if it's bad enough then "mistrial" which while in chambers they were warned would happen should they pursue that line of questioning.
    – Mazura
    May 18, 2020 at 12:47
  • A major problem with the notion of "relevance" is that there is no particular set of rules juries are supposed to follow when assessing witness credibility, and thus often no way of determining whether a piece of evidence might reasonably affect such assessments. If the defendant doesn't have particular evidence of innocence, the prosecution witness would have an unusually string motive to make a false accusation, evidence of such motives may not be directly relevant to the factual matter of the defendant's guilt, but could substantially increase the level of reasonable doubt.
    – supercat
    May 18, 2020 at 19:33

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