Law Regarding Evidence of Battered women's syndrome: Battered women's syndrome (BWS) has been defined to mean "'"a series of common characteristics that appear in women who are abused physically and psychologically over an extended period of time by the dominant male figure in their lives." ' " (People v. Humphrey (1996) 13 Cal.4th 1073, 1083-1084.) "Expert BWS testimony is relevant to explain that it is common for people who have been physically and mentally abused to act in ways that may be difficult for a layperson to understand. " (People v. Riggs (2008) 44 Cal.4th 248, 293.) Evidence Code section 1107, subdivision (a) permits expert testimony on BWS: "In a criminal action, expert testimony is admissible by either the prosecution or the defense regarding intimate partner battering and its effects, including the nature and effect of physical, emotional, or mental abuse on the beliefs, perceptions, or behavior of victims of domestic violence, except when offered against a criminal defendant to prove the occurrence of the act or acts of abuse which form the basis of the criminal charge." Expert testimony on BWS may also be relevant under the more general provision of Evidence Code section 801, subdivision (a) if the testimony is "related to a subject that is sufficiently beyond common experience that the opinion of an expert would assist the trier of facts." (See People v. Brown (2004) 33 Cal.4th 892, 895 expert testimony on behavior of domestic violence victims may be relevant under Evidence Code section 801 when there has been only one incident of domestic violence.)