What is the standard for deliberation and premeditation in a murder case?

deliberation and premeditation are proved when the trier of facts concludes not merely that the defendant harbored an intent to kill but when that intent was the result of forethought and reflection, and when careful thought and a weighing of considerations are demonstrated. A finding of deliberation and premeditation is not negated by evidence a defendant's mental condition was abnormal or his perception of reality delusional unless those conditions resulted in the failure to plan or weigh considerations for and against the proposed course of action." (People v. Stress (1988) 205 Cal.App.3d 1259, 1270.)

The evidence shows beyond a reasonable doubt that Serna intentionally killed her baby; that she engaged in planning activity before doing so; and that she weighed the consequences of her conduct, notwithstanding the evidence that her thinking was irrational, delusional, or mentally defective. It is therefore clear beyond a reasonable doubt that a rational jury would have found Serna guilty absent the error. (See People v. Gonzalez (2012) 54 Cal.4th 643, 663 [summarizing Chapman harmless error standard].) Accordingly, Serna was not prejudiced by the erroneous exclusion of mental health testimony. We conclude this claim is without merit.

B. Jury Instruction on Deliberation and Premeditation