After every trial, regardless of the outcome, one side will always analyse the results and ask themselves if another strategy or approach would have been better. In the context of the Jodi Arias trial, I have heard a few commentators mention that she would have been off not arguing self defense, but had instead tried something different like diminished capacity. For trial watchers who also happen to watch a lot of TV procedural shows, they will know diminished capacity is the favorite defense strategy of fictional TV criminal defense lawyers. So what exactly is the defense of "diminished capacity"?
Briefly, it is "evidence that the defendant suffered from a mental disease or defect that would prove that the defendant did not have a state of mind that is an element of the defense" (Model Penal Code 4.02(1)). The important point here is that diminished capacity is the same thing as the so called "insanity defense", or in Arizona "not guilty except insane", but is a lesser version of it. Diminished capacity is a "mental illness that falls short of that required to invoke the defense of irresponsibility, but that may be put in evidence as tending to show that the defendant lacked the specific mens rea required for commission of offense charged." In layman's terms, diminished capacity means the defendant knew right from wrong, and thus the insanity defense would not apply, but the defendant did not have the required intent to commit the crime because of a mental disease.
In the case of Jodi Arias, if her defense team had argued diminished capacity instead of self-defense, the argument would have been she was not criminally insane when she killed Travis Alexander, but instead, she did not have the specific intent to kill Mr. Alexander as a result of mental illness, and thus, at the very least was not guilty of first degree murder.
In contrast to diminished capacity, Arizona uses the M'Naghten test for the "guilty except insane" defense:
A person is not responsible for criminal conduct by reason of insanity if at the time of such conduct the person was suffering from such a mental disease or defect as not to know the nature and quality of the act or, if such person did know, that such person did not know that what he was doing was wrong. (State v. Ramos)
While guilty except insane wouldn't work, diminished capacity seems like a plausible argument, at least according to Monday morning quarterbacks criticizing her defense team. So why wasn't that the defense she actually used in her case? The answer is very simple: because neither she, nor any defendant for that matter in Arizona state court, is allowed to use it. In State v. Mott, the Arizona Supreme Court held the defendant was not entitled to present evidence of "battered women syndrome" or diminished capacity because the Arizona legislature had did not include it as a justification to criminal conduct. And thus, even if Ms. Arias would have had a better result with that defense than self-defense, it was not available at all.