California Code, Penal Code – PEN § 13701
These policies shall discourage, when appropriate, but not prohibit, dual arrests. Peace officers shall make reasonable efforts to identify the dominant aggressor in any incident. The dominant aggressor is the person determined to be the most significant, rather than the first, aggressor. In identifying the dominant aggressor, an officer shall consider the intent of the law to protect victims of domestic violence from continuing abuse, the threats creating fear of physical injury, the history of domestic violence between the persons involved, and whether either person acted in self-defense.