Cobarrubio v. State
In Cobarrubio v. State, 675 S.W.2d 749, 751-752 (Tex. Crim. App. 1983), the Court held that, when the issue is raised by the evidence, a sudden passion instruction must be included in the portion of the jury charge that instructs on the offense of murder, so that the jury will be required to acquit the defendant of murder if it has a reasonable doubt about sudden passion.
Because the absence of sudden passion was considered to be an element of murder, the jury would necessarily be required to find unanimously that the defendant did not act under sudden passion in order to convict a defendant in a case where the existence of sudden passion was raised by the evidence.
In a later decision, the Court recognized as a legitimate possibility that a hung jury might result from disagreement about the sudden passion issue. Ruiz v. State, 753 S.W.2d 681, 687 (Tex. Crim. App. 1988).