What is the legal test for instructing a jury on a lesser included crime?

"'"'"[I]t is settled that in criminal cases, even in the absence of a request, the trial court must instruct on the general principles of law relevant to the issues raised by the evidence. [Citations.] The general principles of law governing the case are those principles closely and openly connected with the facts before the court, and which are necessary for the jury's understanding of the case." [Citation.] That obligation has been held to include giving instructions on lesser included offenses when the evidence raises a question as to whether all of the elements of the charged offense were present [citation], but not when there is no evidence that the offense was less than that charged. [Citations.]'"' (People v. Valdez (2004) 32 Cal.4th 73, 115 [8 Cal.Rptr.3d 271, 82 P.3d 296] (Valdez).)" (People v. Banks (2014) 59 Cal.4th 1113, 1159 (Banks).)

"The rule that juries must be instructed on lesser included offenses '"prevents either party, whether by design or inadvertence, from forcing an all-or-nothing choice between conviction of the stated offense on the one hand, or complete acquittal on the other. Hence, the rule encourages a verdict, within the charge chosen by the prosecution, that is neither 'harsher [n]or more lenient than the evidence merits.'"' (People v. Smith (2013) 57 Cal.4th 232, 239-240 [159 Cal.Rptr.3d 57, 303 P.3d 368] (Smith).) Because instructing on lesser included offenses serves to increase the accuracy of verdicts, the requirement to do so applies '"even when as a matter of trial tactics a defendant not only fails to request the instruction but expressly objects to its being given." [Citation.]' (Valdez, supra, 32 Cal.4th at p. 115.)" (Banks, supra, 59 Cal.4th at pp. 1159-1160.)

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